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* [PATCH v2 1/2] compiler-gcc: Allow arch-specific override of unreachable
@ 2016-05-27 15:45 Paul Burton
  2016-05-27 15:45 ` Paul Burton
  2016-05-27 15:45 ` [PATCH v2 2/2] MIPS: Workaround GCC __builtin_unreachable reordering bug Paul Burton
  0 siblings, 2 replies; 4+ messages in thread
From: Paul Burton @ 2016-05-27 15:45 UTC (permalink / raw)
  To: linux-mips; +Cc: Ralf Baechle, linux-arch, Paul Burton, # v4 . 4+

Include an arch-specific asm/compiler-gcc.h and allow for it to define a
custom version of unreachable(), which is needed to workaround a bug in
current versions of GCC for the MIPS architecture. This patch includes
an effectively empty asm-generic implementation of asm/compiler-gcc.h &
leaves the architecture specifics to a further patch.

Marked for stable as far back as v4.4 such that the MIPS patch depending
upon it can be backported too.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: <stable@vger.kernel.org> # v4.4+
---
Changes in v2:
- Add generic-y entries to arch Kbuild files. Oops!
---
 arch/alpha/include/asm/Kbuild      | 1 +
 arch/arc/include/asm/Kbuild        | 1 +
 arch/arm/include/asm/Kbuild        | 1 +
 arch/arm64/include/asm/Kbuild      | 1 +
 arch/avr32/include/asm/Kbuild      | 1 +
 arch/blackfin/include/asm/Kbuild   | 1 +
 arch/c6x/include/asm/Kbuild        | 1 +
 arch/cris/include/asm/Kbuild       | 1 +
 arch/frv/include/asm/Kbuild        | 1 +
 arch/h8300/include/asm/Kbuild      | 1 +
 arch/hexagon/include/asm/Kbuild    | 1 +
 arch/ia64/include/asm/Kbuild       | 1 +
 arch/m32r/include/asm/Kbuild       | 1 +
 arch/m68k/include/asm/Kbuild       | 1 +
 arch/metag/include/asm/Kbuild      | 1 +
 arch/microblaze/include/asm/Kbuild | 1 +
 arch/mips/include/asm/Kbuild       | 1 +
 arch/mn10300/include/asm/Kbuild    | 1 +
 arch/nios2/include/asm/Kbuild      | 1 +
 arch/openrisc/include/asm/Kbuild   | 1 +
 arch/parisc/include/asm/Kbuild     | 1 +
 arch/powerpc/include/asm/Kbuild    | 1 +
 arch/s390/include/asm/Kbuild       | 1 +
 arch/score/include/asm/Kbuild      | 1 +
 arch/sh/include/asm/Kbuild         | 1 +
 arch/sparc/include/asm/Kbuild      | 1 +
 arch/tile/include/asm/Kbuild       | 1 +
 arch/um/include/asm/Kbuild         | 1 +
 arch/unicore32/include/asm/Kbuild  | 1 +
 arch/x86/include/asm/Kbuild        | 1 +
 arch/xtensa/include/asm/Kbuild     | 1 +
 include/asm-generic/compiler-gcc.h | 8 ++++++++
 include/linux/compiler-gcc.h       | 7 ++++++-
 33 files changed, 45 insertions(+), 1 deletion(-)
 create mode 100644 include/asm-generic/compiler-gcc.h

diff --git a/arch/alpha/include/asm/Kbuild b/arch/alpha/include/asm/Kbuild
index ffd9cf5..94b962d 100644
--- a/arch/alpha/include/asm/Kbuild
+++ b/arch/alpha/include/asm/Kbuild
@@ -1,6 +1,7 @@
 
 
 generic-y += clkdev.h
+generic-y += compiler-gcc.h
 generic-y += cputime.h
 generic-y += exec.h
 generic-y += irq_work.h
diff --git a/arch/arc/include/asm/Kbuild b/arch/arc/include/asm/Kbuild
index 0b10ef2..ea3febf 100644
--- a/arch/arc/include/asm/Kbuild
+++ b/arch/arc/include/asm/Kbuild
@@ -2,6 +2,7 @@ generic-y += auxvec.h
 generic-y += bitsperlong.h
 generic-y += bugs.h
 generic-y += clkdev.h
+generic-y += compiler-gcc.h
 generic-y += cputime.h
 generic-y += device.h
 generic-y += div64.h
diff --git a/arch/arm/include/asm/Kbuild b/arch/arm/include/asm/Kbuild
index 55e0e3e..071fb46 100644
--- a/arch/arm/include/asm/Kbuild
+++ b/arch/arm/include/asm/Kbuild
@@ -2,6 +2,7 @@
 
 generic-y += bitsperlong.h
 generic-y += clkdev.h
+generic-y += compiler-gcc.h
 generic-y += cputime.h
 generic-y += current.h
 generic-y += early_ioremap.h
diff --git a/arch/arm64/include/asm/Kbuild b/arch/arm64/include/asm/Kbuild
index cff532a..4dc925a 100644
--- a/arch/arm64/include/asm/Kbuild
+++ b/arch/arm64/include/asm/Kbuild
@@ -2,6 +2,7 @@ generic-y += bug.h
 generic-y += bugs.h
 generic-y += checksum.h
 generic-y += clkdev.h
+generic-y += compiler-gcc.h
 generic-y += cputime.h
 generic-y += current.h
 generic-y += delay.h
diff --git a/arch/avr32/include/asm/Kbuild b/arch/avr32/include/asm/Kbuild
index 241b9b9..de29503 100644
--- a/arch/avr32/include/asm/Kbuild
+++ b/arch/avr32/include/asm/Kbuild
@@ -1,5 +1,6 @@
 
 generic-y += clkdev.h
+generic-y += compiler-gcc.h
 generic-y += cputime.h
 generic-y += delay.h
 generic-y += device.h
diff --git a/arch/blackfin/include/asm/Kbuild b/arch/blackfin/include/asm/Kbuild
index 91d49c0..a53e643 100644
--- a/arch/blackfin/include/asm/Kbuild
+++ b/arch/blackfin/include/asm/Kbuild
@@ -2,6 +2,7 @@
 generic-y += auxvec.h
 generic-y += bitsperlong.h
 generic-y += bugs.h
+generic-y += compiler-gcc.h
 generic-y += cputime.h
 generic-y += current.h
 generic-y += device.h
diff --git a/arch/c6x/include/asm/Kbuild b/arch/c6x/include/asm/Kbuild
index 64465e7..64f709b 100644
--- a/arch/c6x/include/asm/Kbuild
+++ b/arch/c6x/include/asm/Kbuild
@@ -5,6 +5,7 @@ generic-y += barrier.h
 generic-y += bitsperlong.h
 generic-y += bugs.h
 generic-y += clkdev.h
+generic-y += compiler-gcc.h
 generic-y += cputime.h
 generic-y += current.h
 generic-y += device.h
diff --git a/arch/cris/include/asm/Kbuild b/arch/cris/include/asm/Kbuild
index 1778805..d52d996 100644
--- a/arch/cris/include/asm/Kbuild
+++ b/arch/cris/include/asm/Kbuild
@@ -4,6 +4,7 @@ generic-y += barrier.h
 generic-y += bitsperlong.h
 generic-y += clkdev.h
 generic-y += cmpxchg.h
+generic-y += compiler-gcc.h
 generic-y += cputime.h
 generic-y += device.h
 generic-y += div64.h
diff --git a/arch/frv/include/asm/Kbuild b/arch/frv/include/asm/Kbuild
index 1fa084c..213d33d 100644
--- a/arch/frv/include/asm/Kbuild
+++ b/arch/frv/include/asm/Kbuild
@@ -1,5 +1,6 @@
 
 generic-y += clkdev.h
+generic-y += compiler-gcc.h
 generic-y += cputime.h
 generic-y += exec.h
 generic-y += irq_work.h
diff --git a/arch/h8300/include/asm/Kbuild b/arch/h8300/include/asm/Kbuild
index 373cb23..2b2cad6 100644
--- a/arch/h8300/include/asm/Kbuild
+++ b/arch/h8300/include/asm/Kbuild
@@ -5,6 +5,7 @@ generic-y += bugs.h
 generic-y += cacheflush.h
 generic-y += checksum.h
 generic-y += clkdev.h
+generic-y += compiler-gcc.h
 generic-y += cputime.h
 generic-y += current.h
 generic-y += delay.h
diff --git a/arch/hexagon/include/asm/Kbuild b/arch/hexagon/include/asm/Kbuild
index db8ddab..66c3a759 100644
--- a/arch/hexagon/include/asm/Kbuild
+++ b/arch/hexagon/include/asm/Kbuild
@@ -6,6 +6,7 @@ generic-y += barrier.h
 generic-y += bug.h
 generic-y += bugs.h
 generic-y += clkdev.h
+generic-y += compiler-gcc.h
 generic-y += cputime.h
 generic-y += current.h
 generic-y += device.h
diff --git a/arch/ia64/include/asm/Kbuild b/arch/ia64/include/asm/Kbuild
index 502a91d..cc6a991 100644
--- a/arch/ia64/include/asm/Kbuild
+++ b/arch/ia64/include/asm/Kbuild
@@ -1,5 +1,6 @@
 
 generic-y += clkdev.h
+generic-y += compiler-gcc.h
 generic-y += exec.h
 generic-y += irq_work.h
 generic-y += kvm_para.h
diff --git a/arch/m32r/include/asm/Kbuild b/arch/m32r/include/asm/Kbuild
index 860e440..e06a38f 100644
--- a/arch/m32r/include/asm/Kbuild
+++ b/arch/m32r/include/asm/Kbuild
@@ -1,5 +1,6 @@
 
 generic-y += clkdev.h
+generic-y += compiler-gcc.h
 generic-y += cputime.h
 generic-y += exec.h
 generic-y += irq_work.h
diff --git a/arch/m68k/include/asm/Kbuild b/arch/m68k/include/asm/Kbuild
index eb85bd9..2624c10 100644
--- a/arch/m68k/include/asm/Kbuild
+++ b/arch/m68k/include/asm/Kbuild
@@ -1,6 +1,7 @@
 generic-y += barrier.h
 generic-y += bitsperlong.h
 generic-y += clkdev.h
+generic-y += compiler-gcc.h
 generic-y += cputime.h
 generic-y += device.h
 generic-y += emergency-restart.h
diff --git a/arch/metag/include/asm/Kbuild b/arch/metag/include/asm/Kbuild
index 29acb89d..aae290e 100644
--- a/arch/metag/include/asm/Kbuild
+++ b/arch/metag/include/asm/Kbuild
@@ -2,6 +2,7 @@ generic-y += auxvec.h
 generic-y += bitsperlong.h
 generic-y += bugs.h
 generic-y += clkdev.h
+generic-y += compiler-gcc.h
 generic-y += cputime.h
 generic-y += current.h
 generic-y += device.h
diff --git a/arch/microblaze/include/asm/Kbuild b/arch/microblaze/include/asm/Kbuild
index b0ae88c..4e91bad 100644
--- a/arch/microblaze/include/asm/Kbuild
+++ b/arch/microblaze/include/asm/Kbuild
@@ -1,6 +1,7 @@
 
 generic-y += barrier.h
 generic-y += clkdev.h
+generic-y += compiler-gcc.h
 generic-y += cputime.h
 generic-y += device.h
 generic-y += exec.h
diff --git a/arch/mips/include/asm/Kbuild b/arch/mips/include/asm/Kbuild
index c7fe4d0..9ccb519 100644
--- a/arch/mips/include/asm/Kbuild
+++ b/arch/mips/include/asm/Kbuild
@@ -1,5 +1,6 @@
 # MIPS headers
 generic-(CONFIG_GENERIC_CSUM) += checksum.h
+generic-y += compiler-gcc.h
 generic-y += cputime.h
 generic-y += current.h
 generic-y += dma-contiguous.h
diff --git a/arch/mn10300/include/asm/Kbuild b/arch/mn10300/include/asm/Kbuild
index 1c8dd0f..9bcf3f5 100644
--- a/arch/mn10300/include/asm/Kbuild
+++ b/arch/mn10300/include/asm/Kbuild
@@ -1,6 +1,7 @@
 
 generic-y += barrier.h
 generic-y += clkdev.h
+generic-y += compiler-gcc.h
 generic-y += cputime.h
 generic-y += exec.h
 generic-y += irq_work.h
diff --git a/arch/nios2/include/asm/Kbuild b/arch/nios2/include/asm/Kbuild
index d63330e..16fd572 100644
--- a/arch/nios2/include/asm/Kbuild
+++ b/arch/nios2/include/asm/Kbuild
@@ -6,6 +6,7 @@ generic-y += bitsperlong.h
 generic-y += bug.h
 generic-y += bugs.h
 generic-y += clkdev.h
+generic-y += compiler-gcc.h
 generic-y += cputime.h
 generic-y += current.h
 generic-y += device.h
diff --git a/arch/openrisc/include/asm/Kbuild b/arch/openrisc/include/asm/Kbuild
index 2832f03..27be320 100644
--- a/arch/openrisc/include/asm/Kbuild
+++ b/arch/openrisc/include/asm/Kbuild
@@ -12,6 +12,7 @@ generic-y += checksum.h
 generic-y += clkdev.h
 generic-y += cmpxchg-local.h
 generic-y += cmpxchg.h
+generic-y += compiler-gcc.h
 generic-y += cputime.h
 generic-y += current.h
 generic-y += device.h
diff --git a/arch/parisc/include/asm/Kbuild b/arch/parisc/include/asm/Kbuild
index f9b3a81..4f2cdd1 100644
--- a/arch/parisc/include/asm/Kbuild
+++ b/arch/parisc/include/asm/Kbuild
@@ -2,6 +2,7 @@
 generic-y += auxvec.h
 generic-y += barrier.h
 generic-y += clkdev.h
+generic-y += compiler-gcc.h
 generic-y += cputime.h
 generic-y += device.h
 generic-y += div64.h
diff --git a/arch/powerpc/include/asm/Kbuild b/arch/powerpc/include/asm/Kbuild
index ab9f4e0..23116ca 100644
--- a/arch/powerpc/include/asm/Kbuild
+++ b/arch/powerpc/include/asm/Kbuild
@@ -1,4 +1,5 @@
 generic-y += clkdev.h
+generic-y += compiler-gcc.h
 generic-y += div64.h
 generic-y += irq_regs.h
 generic-y += irq_work.h
diff --git a/arch/s390/include/asm/Kbuild b/arch/s390/include/asm/Kbuild
index 9043d2e..22e1921 100644
--- a/arch/s390/include/asm/Kbuild
+++ b/arch/s390/include/asm/Kbuild
@@ -1,6 +1,7 @@
 
 
 generic-y += clkdev.h
+generic-y += compiler-gcc.h
 generic-y += irq_work.h
 generic-y += mcs_spinlock.h
 generic-y += mm-arch-hooks.h
diff --git a/arch/score/include/asm/Kbuild b/arch/score/include/asm/Kbuild
index a05218f..b2277b6 100644
--- a/arch/score/include/asm/Kbuild
+++ b/arch/score/include/asm/Kbuild
@@ -4,6 +4,7 @@ header-y +=
 
 generic-y += barrier.h
 generic-y += clkdev.h
+generic-y += compiler-gcc.h
 generic-y += cputime.h
 generic-y += irq_work.h
 generic-y += mcs_spinlock.h
diff --git a/arch/sh/include/asm/Kbuild b/arch/sh/include/asm/Kbuild
index a319745..9c15552 100644
--- a/arch/sh/include/asm/Kbuild
+++ b/arch/sh/include/asm/Kbuild
@@ -1,6 +1,7 @@
 
 generic-y += bitsperlong.h
 generic-y += clkdev.h
+generic-y += compiler-gcc.h
 generic-y += cputime.h
 generic-y += current.h
 generic-y += delay.h
diff --git a/arch/sparc/include/asm/Kbuild b/arch/sparc/include/asm/Kbuild
index e9286188..d36f1ca 100644
--- a/arch/sparc/include/asm/Kbuild
+++ b/arch/sparc/include/asm/Kbuild
@@ -2,6 +2,7 @@
 
 
 generic-y += clkdev.h
+generic-y += compiler-gcc.h
 generic-y += cputime.h
 generic-y += div64.h
 generic-y += emergency-restart.h
diff --git a/arch/tile/include/asm/Kbuild b/arch/tile/include/asm/Kbuild
index ba35c41..969d85c 100644
--- a/arch/tile/include/asm/Kbuild
+++ b/arch/tile/include/asm/Kbuild
@@ -4,6 +4,7 @@ header-y += ../arch/
 generic-y += bug.h
 generic-y += bugs.h
 generic-y += clkdev.h
+generic-y += compiler-gcc.h
 generic-y += cputime.h
 generic-y += div64.h
 generic-y += emergency-restart.h
diff --git a/arch/um/include/asm/Kbuild b/arch/um/include/asm/Kbuild
index 904f3eb..dcae7de 100644
--- a/arch/um/include/asm/Kbuild
+++ b/arch/um/include/asm/Kbuild
@@ -1,6 +1,7 @@
 generic-y += barrier.h
 generic-y += bug.h
 generic-y += clkdev.h
+generic-y += compiler-gcc.h
 generic-y += cputime.h
 generic-y += current.h
 generic-y += delay.h
diff --git a/arch/unicore32/include/asm/Kbuild b/arch/unicore32/include/asm/Kbuild
index 256c45b..d24f77b 100644
--- a/arch/unicore32/include/asm/Kbuild
+++ b/arch/unicore32/include/asm/Kbuild
@@ -4,6 +4,7 @@ generic-y += auxvec.h
 generic-y += bitsperlong.h
 generic-y += bugs.h
 generic-y += clkdev.h
+generic-y += compiler-gcc.h
 generic-y += cputime.h
 generic-y += current.h
 generic-y += device.h
diff --git a/arch/x86/include/asm/Kbuild b/arch/x86/include/asm/Kbuild
index aeac434..5b8fe8b 100644
--- a/arch/x86/include/asm/Kbuild
+++ b/arch/x86/include/asm/Kbuild
@@ -5,6 +5,7 @@ genhdr-y += unistd_64.h
 genhdr-y += unistd_x32.h
 
 generic-y += clkdev.h
+generic-y += compiler-gcc.h
 generic-y += cputime.h
 generic-y += dma-contiguous.h
 generic-y += early_ioremap.h
diff --git a/arch/xtensa/include/asm/Kbuild b/arch/xtensa/include/asm/Kbuild
index b56855a..2320040 100644
--- a/arch/xtensa/include/asm/Kbuild
+++ b/arch/xtensa/include/asm/Kbuild
@@ -1,6 +1,7 @@
 generic-y += bitsperlong.h
 generic-y += bug.h
 generic-y += clkdev.h
+generic-y += compiler-gcc.h
 generic-y += cputime.h
 generic-y += div64.h
 generic-y += emergency-restart.h
diff --git a/include/asm-generic/compiler-gcc.h b/include/asm-generic/compiler-gcc.h
new file mode 100644
index 0000000..e8ba230
--- /dev/null
+++ b/include/asm-generic/compiler-gcc.h
@@ -0,0 +1,8 @@
+#ifndef __LINUX_COMPILER_H
+#error "Please don't include <asm/compiler.gcc.h> directly, include <linux/compiler.h> instead."
+#endif
+
+/*
+ * We have nothing architecture-specific to do here, simply leave everything to
+ * the generic linux/compiler-gcc.h.
+ */
diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
index 3d5202e..e556cb8 100644
--- a/include/linux/compiler-gcc.h
+++ b/include/linux/compiler-gcc.h
@@ -9,6 +9,9 @@
 		     + __GNUC_MINOR__ * 100	\
 		     + __GNUC_PATCHLEVEL__)
 
+/* Allow architectures to override some definitions where necessary */
+#include <asm/compiler-gcc.h>
+
 /* Optimization barrier */
 
 /* The "volatile" is due to gcc bugs */
@@ -196,7 +199,9 @@
  * this in the preprocessor, but we can live with this because they're
  * unreleased.  Really, we need to have autoconf for the kernel.
  */
-#define unreachable() __builtin_unreachable()
+#ifndef unreachable
+# define unreachable() __builtin_unreachable()
+#endif
 
 /* Mark a function definition as prohibited from being cloned. */
 #define __noclone	__attribute__((__noclone__, __optimize__("no-tracer")))
-- 
2.8.3

^ permalink raw reply related	[flat|nested] 4+ messages in thread
* [PATCH v2 1/2] compiler-gcc: Allow arch-specific override of unreachable
  2016-05-27 15:45 [PATCH v2 1/2] compiler-gcc: Allow arch-specific override of unreachable Paul Burton
@ 2016-05-27 15:45 ` Paul Burton
  2016-05-27 15:45 ` [PATCH v2 2/2] MIPS: Workaround GCC __builtin_unreachable reordering bug Paul Burton
  1 sibling, 0 replies; 4+ messages in thread
From: Paul Burton @ 2016-05-27 15:45 UTC (permalink / raw)
  To: linux-mips; +Cc: Ralf Baechle, linux-arch, Paul Burton, # v4 . 4+

Include an arch-specific asm/compiler-gcc.h and allow for it to define a
custom version of unreachable(), which is needed to workaround a bug in
current versions of GCC for the MIPS architecture. This patch includes
an effectively empty asm-generic implementation of asm/compiler-gcc.h &
leaves the architecture specifics to a further patch.

Marked for stable as far back as v4.4 such that the MIPS patch depending
upon it can be backported too.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: <stable@vger.kernel.org> # v4.4+
---
Changes in v2:
- Add generic-y entries to arch Kbuild files. Oops!
---
 arch/alpha/include/asm/Kbuild      | 1 +
 arch/arc/include/asm/Kbuild        | 1 +
 arch/arm/include/asm/Kbuild        | 1 +
 arch/arm64/include/asm/Kbuild      | 1 +
 arch/avr32/include/asm/Kbuild      | 1 +
 arch/blackfin/include/asm/Kbuild   | 1 +
 arch/c6x/include/asm/Kbuild        | 1 +
 arch/cris/include/asm/Kbuild       | 1 +
 arch/frv/include/asm/Kbuild        | 1 +
 arch/h8300/include/asm/Kbuild      | 1 +
 arch/hexagon/include/asm/Kbuild    | 1 +
 arch/ia64/include/asm/Kbuild       | 1 +
 arch/m32r/include/asm/Kbuild       | 1 +
 arch/m68k/include/asm/Kbuild       | 1 +
 arch/metag/include/asm/Kbuild      | 1 +
 arch/microblaze/include/asm/Kbuild | 1 +
 arch/mips/include/asm/Kbuild       | 1 +
 arch/mn10300/include/asm/Kbuild    | 1 +
 arch/nios2/include/asm/Kbuild      | 1 +
 arch/openrisc/include/asm/Kbuild   | 1 +
 arch/parisc/include/asm/Kbuild     | 1 +
 arch/powerpc/include/asm/Kbuild    | 1 +
 arch/s390/include/asm/Kbuild       | 1 +
 arch/score/include/asm/Kbuild      | 1 +
 arch/sh/include/asm/Kbuild         | 1 +
 arch/sparc/include/asm/Kbuild      | 1 +
 arch/tile/include/asm/Kbuild       | 1 +
 arch/um/include/asm/Kbuild         | 1 +
 arch/unicore32/include/asm/Kbuild  | 1 +
 arch/x86/include/asm/Kbuild        | 1 +
 arch/xtensa/include/asm/Kbuild     | 1 +
 include/asm-generic/compiler-gcc.h | 8 ++++++++
 include/linux/compiler-gcc.h       | 7 ++++++-
 33 files changed, 45 insertions(+), 1 deletion(-)
 create mode 100644 include/asm-generic/compiler-gcc.h

diff --git a/arch/alpha/include/asm/Kbuild b/arch/alpha/include/asm/Kbuild
index ffd9cf5..94b962d 100644
--- a/arch/alpha/include/asm/Kbuild
+++ b/arch/alpha/include/asm/Kbuild
@@ -1,6 +1,7 @@
 
 
 generic-y += clkdev.h
+generic-y += compiler-gcc.h
 generic-y += cputime.h
 generic-y += exec.h
 generic-y += irq_work.h
diff --git a/arch/arc/include/asm/Kbuild b/arch/arc/include/asm/Kbuild
index 0b10ef2..ea3febf 100644
--- a/arch/arc/include/asm/Kbuild
+++ b/arch/arc/include/asm/Kbuild
@@ -2,6 +2,7 @@ generic-y += auxvec.h
 generic-y += bitsperlong.h
 generic-y += bugs.h
 generic-y += clkdev.h
+generic-y += compiler-gcc.h
 generic-y += cputime.h
 generic-y += device.h
 generic-y += div64.h
diff --git a/arch/arm/include/asm/Kbuild b/arch/arm/include/asm/Kbuild
index 55e0e3e..071fb46 100644
--- a/arch/arm/include/asm/Kbuild
+++ b/arch/arm/include/asm/Kbuild
@@ -2,6 +2,7 @@
 
 generic-y += bitsperlong.h
 generic-y += clkdev.h
+generic-y += compiler-gcc.h
 generic-y += cputime.h
 generic-y += current.h
 generic-y += early_ioremap.h
diff --git a/arch/arm64/include/asm/Kbuild b/arch/arm64/include/asm/Kbuild
index cff532a..4dc925a 100644
--- a/arch/arm64/include/asm/Kbuild
+++ b/arch/arm64/include/asm/Kbuild
@@ -2,6 +2,7 @@ generic-y += bug.h
 generic-y += bugs.h
 generic-y += checksum.h
 generic-y += clkdev.h
+generic-y += compiler-gcc.h
 generic-y += cputime.h
 generic-y += current.h
 generic-y += delay.h
diff --git a/arch/avr32/include/asm/Kbuild b/arch/avr32/include/asm/Kbuild
index 241b9b9..de29503 100644
--- a/arch/avr32/include/asm/Kbuild
+++ b/arch/avr32/include/asm/Kbuild
@@ -1,5 +1,6 @@
 
 generic-y += clkdev.h
+generic-y += compiler-gcc.h
 generic-y += cputime.h
 generic-y += delay.h
 generic-y += device.h
diff --git a/arch/blackfin/include/asm/Kbuild b/arch/blackfin/include/asm/Kbuild
index 91d49c0..a53e643 100644
--- a/arch/blackfin/include/asm/Kbuild
+++ b/arch/blackfin/include/asm/Kbuild
@@ -2,6 +2,7 @@
 generic-y += auxvec.h
 generic-y += bitsperlong.h
 generic-y += bugs.h
+generic-y += compiler-gcc.h
 generic-y += cputime.h
 generic-y += current.h
 generic-y += device.h
diff --git a/arch/c6x/include/asm/Kbuild b/arch/c6x/include/asm/Kbuild
index 64465e7..64f709b 100644
--- a/arch/c6x/include/asm/Kbuild
+++ b/arch/c6x/include/asm/Kbuild
@@ -5,6 +5,7 @@ generic-y += barrier.h
 generic-y += bitsperlong.h
 generic-y += bugs.h
 generic-y += clkdev.h
+generic-y += compiler-gcc.h
 generic-y += cputime.h
 generic-y += current.h
 generic-y += device.h
diff --git a/arch/cris/include/asm/Kbuild b/arch/cris/include/asm/Kbuild
index 1778805..d52d996 100644
--- a/arch/cris/include/asm/Kbuild
+++ b/arch/cris/include/asm/Kbuild
@@ -4,6 +4,7 @@ generic-y += barrier.h
 generic-y += bitsperlong.h
 generic-y += clkdev.h
 generic-y += cmpxchg.h
+generic-y += compiler-gcc.h
 generic-y += cputime.h
 generic-y += device.h
 generic-y += div64.h
diff --git a/arch/frv/include/asm/Kbuild b/arch/frv/include/asm/Kbuild
index 1fa084c..213d33d 100644
--- a/arch/frv/include/asm/Kbuild
+++ b/arch/frv/include/asm/Kbuild
@@ -1,5 +1,6 @@
 
 generic-y += clkdev.h
+generic-y += compiler-gcc.h
 generic-y += cputime.h
 generic-y += exec.h
 generic-y += irq_work.h
diff --git a/arch/h8300/include/asm/Kbuild b/arch/h8300/include/asm/Kbuild
index 373cb23..2b2cad6 100644
--- a/arch/h8300/include/asm/Kbuild
+++ b/arch/h8300/include/asm/Kbuild
@@ -5,6 +5,7 @@ generic-y += bugs.h
 generic-y += cacheflush.h
 generic-y += checksum.h
 generic-y += clkdev.h
+generic-y += compiler-gcc.h
 generic-y += cputime.h
 generic-y += current.h
 generic-y += delay.h
diff --git a/arch/hexagon/include/asm/Kbuild b/arch/hexagon/include/asm/Kbuild
index db8ddab..66c3a759 100644
--- a/arch/hexagon/include/asm/Kbuild
+++ b/arch/hexagon/include/asm/Kbuild
@@ -6,6 +6,7 @@ generic-y += barrier.h
 generic-y += bug.h
 generic-y += bugs.h
 generic-y += clkdev.h
+generic-y += compiler-gcc.h
 generic-y += cputime.h
 generic-y += current.h
 generic-y += device.h
diff --git a/arch/ia64/include/asm/Kbuild b/arch/ia64/include/asm/Kbuild
index 502a91d..cc6a991 100644
--- a/arch/ia64/include/asm/Kbuild
+++ b/arch/ia64/include/asm/Kbuild
@@ -1,5 +1,6 @@
 
 generic-y += clkdev.h
+generic-y += compiler-gcc.h
 generic-y += exec.h
 generic-y += irq_work.h
 generic-y += kvm_para.h
diff --git a/arch/m32r/include/asm/Kbuild b/arch/m32r/include/asm/Kbuild
index 860e440..e06a38f 100644
--- a/arch/m32r/include/asm/Kbuild
+++ b/arch/m32r/include/asm/Kbuild
@@ -1,5 +1,6 @@
 
 generic-y += clkdev.h
+generic-y += compiler-gcc.h
 generic-y += cputime.h
 generic-y += exec.h
 generic-y += irq_work.h
diff --git a/arch/m68k/include/asm/Kbuild b/arch/m68k/include/asm/Kbuild
index eb85bd9..2624c10 100644
--- a/arch/m68k/include/asm/Kbuild
+++ b/arch/m68k/include/asm/Kbuild
@@ -1,6 +1,7 @@
 generic-y += barrier.h
 generic-y += bitsperlong.h
 generic-y += clkdev.h
+generic-y += compiler-gcc.h
 generic-y += cputime.h
 generic-y += device.h
 generic-y += emergency-restart.h
diff --git a/arch/metag/include/asm/Kbuild b/arch/metag/include/asm/Kbuild
index 29acb89d..aae290e 100644
--- a/arch/metag/include/asm/Kbuild
+++ b/arch/metag/include/asm/Kbuild
@@ -2,6 +2,7 @@ generic-y += auxvec.h
 generic-y += bitsperlong.h
 generic-y += bugs.h
 generic-y += clkdev.h
+generic-y += compiler-gcc.h
 generic-y += cputime.h
 generic-y += current.h
 generic-y += device.h
diff --git a/arch/microblaze/include/asm/Kbuild b/arch/microblaze/include/asm/Kbuild
index b0ae88c..4e91bad 100644
--- a/arch/microblaze/include/asm/Kbuild
+++ b/arch/microblaze/include/asm/Kbuild
@@ -1,6 +1,7 @@
 
 generic-y += barrier.h
 generic-y += clkdev.h
+generic-y += compiler-gcc.h
 generic-y += cputime.h
 generic-y += device.h
 generic-y += exec.h
diff --git a/arch/mips/include/asm/Kbuild b/arch/mips/include/asm/Kbuild
index c7fe4d0..9ccb519 100644
--- a/arch/mips/include/asm/Kbuild
+++ b/arch/mips/include/asm/Kbuild
@@ -1,5 +1,6 @@
 # MIPS headers
 generic-(CONFIG_GENERIC_CSUM) += checksum.h
+generic-y += compiler-gcc.h
 generic-y += cputime.h
 generic-y += current.h
 generic-y += dma-contiguous.h
diff --git a/arch/mn10300/include/asm/Kbuild b/arch/mn10300/include/asm/Kbuild
index 1c8dd0f..9bcf3f5 100644
--- a/arch/mn10300/include/asm/Kbuild
+++ b/arch/mn10300/include/asm/Kbuild
@@ -1,6 +1,7 @@
 
 generic-y += barrier.h
 generic-y += clkdev.h
+generic-y += compiler-gcc.h
 generic-y += cputime.h
 generic-y += exec.h
 generic-y += irq_work.h
diff --git a/arch/nios2/include/asm/Kbuild b/arch/nios2/include/asm/Kbuild
index d63330e..16fd572 100644
--- a/arch/nios2/include/asm/Kbuild
+++ b/arch/nios2/include/asm/Kbuild
@@ -6,6 +6,7 @@ generic-y += bitsperlong.h
 generic-y += bug.h
 generic-y += bugs.h
 generic-y += clkdev.h
+generic-y += compiler-gcc.h
 generic-y += cputime.h
 generic-y += current.h
 generic-y += device.h
diff --git a/arch/openrisc/include/asm/Kbuild b/arch/openrisc/include/asm/Kbuild
index 2832f03..27be320 100644
--- a/arch/openrisc/include/asm/Kbuild
+++ b/arch/openrisc/include/asm/Kbuild
@@ -12,6 +12,7 @@ generic-y += checksum.h
 generic-y += clkdev.h
 generic-y += cmpxchg-local.h
 generic-y += cmpxchg.h
+generic-y += compiler-gcc.h
 generic-y += cputime.h
 generic-y += current.h
 generic-y += device.h
diff --git a/arch/parisc/include/asm/Kbuild b/arch/parisc/include/asm/Kbuild
index f9b3a81..4f2cdd1 100644
--- a/arch/parisc/include/asm/Kbuild
+++ b/arch/parisc/include/asm/Kbuild
@@ -2,6 +2,7 @@
 generic-y += auxvec.h
 generic-y += barrier.h
 generic-y += clkdev.h
+generic-y += compiler-gcc.h
 generic-y += cputime.h
 generic-y += device.h
 generic-y += div64.h
diff --git a/arch/powerpc/include/asm/Kbuild b/arch/powerpc/include/asm/Kbuild
index ab9f4e0..23116ca 100644
--- a/arch/powerpc/include/asm/Kbuild
+++ b/arch/powerpc/include/asm/Kbuild
@@ -1,4 +1,5 @@
 generic-y += clkdev.h
+generic-y += compiler-gcc.h
 generic-y += div64.h
 generic-y += irq_regs.h
 generic-y += irq_work.h
diff --git a/arch/s390/include/asm/Kbuild b/arch/s390/include/asm/Kbuild
index 9043d2e..22e1921 100644
--- a/arch/s390/include/asm/Kbuild
+++ b/arch/s390/include/asm/Kbuild
@@ -1,6 +1,7 @@
 
 
 generic-y += clkdev.h
+generic-y += compiler-gcc.h
 generic-y += irq_work.h
 generic-y += mcs_spinlock.h
 generic-y += mm-arch-hooks.h
diff --git a/arch/score/include/asm/Kbuild b/arch/score/include/asm/Kbuild
index a05218f..b2277b6 100644
--- a/arch/score/include/asm/Kbuild
+++ b/arch/score/include/asm/Kbuild
@@ -4,6 +4,7 @@ header-y +=
 
 generic-y += barrier.h
 generic-y += clkdev.h
+generic-y += compiler-gcc.h
 generic-y += cputime.h
 generic-y += irq_work.h
 generic-y += mcs_spinlock.h
diff --git a/arch/sh/include/asm/Kbuild b/arch/sh/include/asm/Kbuild
index a319745..9c15552 100644
--- a/arch/sh/include/asm/Kbuild
+++ b/arch/sh/include/asm/Kbuild
@@ -1,6 +1,7 @@
 
 generic-y += bitsperlong.h
 generic-y += clkdev.h
+generic-y += compiler-gcc.h
 generic-y += cputime.h
 generic-y += current.h
 generic-y += delay.h
diff --git a/arch/sparc/include/asm/Kbuild b/arch/sparc/include/asm/Kbuild
index e9286188..d36f1ca 100644
--- a/arch/sparc/include/asm/Kbuild
+++ b/arch/sparc/include/asm/Kbuild
@@ -2,6 +2,7 @@
 
 
 generic-y += clkdev.h
+generic-y += compiler-gcc.h
 generic-y += cputime.h
 generic-y += div64.h
 generic-y += emergency-restart.h
diff --git a/arch/tile/include/asm/Kbuild b/arch/tile/include/asm/Kbuild
index ba35c41..969d85c 100644
--- a/arch/tile/include/asm/Kbuild
+++ b/arch/tile/include/asm/Kbuild
@@ -4,6 +4,7 @@ header-y += ../arch/
 generic-y += bug.h
 generic-y += bugs.h
 generic-y += clkdev.h
+generic-y += compiler-gcc.h
 generic-y += cputime.h
 generic-y += div64.h
 generic-y += emergency-restart.h
diff --git a/arch/um/include/asm/Kbuild b/arch/um/include/asm/Kbuild
index 904f3eb..dcae7de 100644
--- a/arch/um/include/asm/Kbuild
+++ b/arch/um/include/asm/Kbuild
@@ -1,6 +1,7 @@
 generic-y += barrier.h
 generic-y += bug.h
 generic-y += clkdev.h
+generic-y += compiler-gcc.h
 generic-y += cputime.h
 generic-y += current.h
 generic-y += delay.h
diff --git a/arch/unicore32/include/asm/Kbuild b/arch/unicore32/include/asm/Kbuild
index 256c45b..d24f77b 100644
--- a/arch/unicore32/include/asm/Kbuild
+++ b/arch/unicore32/include/asm/Kbuild
@@ -4,6 +4,7 @@ generic-y += auxvec.h
 generic-y += bitsperlong.h
 generic-y += bugs.h
 generic-y += clkdev.h
+generic-y += compiler-gcc.h
 generic-y += cputime.h
 generic-y += current.h
 generic-y += device.h
diff --git a/arch/x86/include/asm/Kbuild b/arch/x86/include/asm/Kbuild
index aeac434..5b8fe8b 100644
--- a/arch/x86/include/asm/Kbuild
+++ b/arch/x86/include/asm/Kbuild
@@ -5,6 +5,7 @@ genhdr-y += unistd_64.h
 genhdr-y += unistd_x32.h
 
 generic-y += clkdev.h
+generic-y += compiler-gcc.h
 generic-y += cputime.h
 generic-y += dma-contiguous.h
 generic-y += early_ioremap.h
diff --git a/arch/xtensa/include/asm/Kbuild b/arch/xtensa/include/asm/Kbuild
index b56855a..2320040 100644
--- a/arch/xtensa/include/asm/Kbuild
+++ b/arch/xtensa/include/asm/Kbuild
@@ -1,6 +1,7 @@
 generic-y += bitsperlong.h
 generic-y += bug.h
 generic-y += clkdev.h
+generic-y += compiler-gcc.h
 generic-y += cputime.h
 generic-y += div64.h
 generic-y += emergency-restart.h
diff --git a/include/asm-generic/compiler-gcc.h b/include/asm-generic/compiler-gcc.h
new file mode 100644
index 0000000..e8ba230
--- /dev/null
+++ b/include/asm-generic/compiler-gcc.h
@@ -0,0 +1,8 @@
+#ifndef __LINUX_COMPILER_H
+#error "Please don't include <asm/compiler.gcc.h> directly, include <linux/compiler.h> instead."
+#endif
+
+/*
+ * We have nothing architecture-specific to do here, simply leave everything to
+ * the generic linux/compiler-gcc.h.
+ */
diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
index 3d5202e..e556cb8 100644
--- a/include/linux/compiler-gcc.h
+++ b/include/linux/compiler-gcc.h
@@ -9,6 +9,9 @@
 		     + __GNUC_MINOR__ * 100	\
 		     + __GNUC_PATCHLEVEL__)
 
+/* Allow architectures to override some definitions where necessary */
+#include <asm/compiler-gcc.h>
+
 /* Optimization barrier */
 
 /* The "volatile" is due to gcc bugs */
@@ -196,7 +199,9 @@
  * this in the preprocessor, but we can live with this because they're
  * unreleased.  Really, we need to have autoconf for the kernel.
  */
-#define unreachable() __builtin_unreachable()
+#ifndef unreachable
+# define unreachable() __builtin_unreachable()
+#endif
 
 /* Mark a function definition as prohibited from being cloned. */
 #define __noclone	__attribute__((__noclone__, __optimize__("no-tracer")))
-- 
2.8.3


^ permalink raw reply related	[flat|nested] 4+ messages in thread
* [PATCH v2 2/2] MIPS: Workaround GCC __builtin_unreachable reordering bug
  2016-05-27 15:45 [PATCH v2 1/2] compiler-gcc: Allow arch-specific override of unreachable Paul Burton
  2016-05-27 15:45 ` Paul Burton
@ 2016-05-27 15:45 ` Paul Burton
  2016-05-27 15:45   ` Paul Burton
  1 sibling, 1 reply; 4+ messages in thread
From: Paul Burton @ 2016-05-27 15:45 UTC (permalink / raw)
  To: linux-mips
  Cc: Ralf Baechle, linux-arch, Paul Burton, # v4 . 4+,
	Matthew Fortune, Robert Suchanek

Current versions of GCC for the MIPS architecture suffer from a bug
which can lead to instructions from beyond an unreachable statement
being incorrectly reordered into earlier branch delay slots if the
unreachable statement is the only content of a case in a switch
statement. This can lead to seemingly random behaviour, such as invalid
memory accesses from incorrectly reordered loads or stores.

See this potential GCC fix for details:

    https://gcc.gnu.org/ml/gcc-patches/2015-09/msg00360.html

Work around this bug by placing a volatile asm statement, which GCC is
prevented from reordering past, prior to the __builtin_unreachable call.

This bug affects at least a maltasmvp_defconfig kernel built from the
v4.4 tag using GCC 4.9.2 (from a Codescape SDK 2015.06-05 toolchain),
with the result being an address exception taken after log messages
about the L1 caches (during probe of the L2 cache):

    Initmem setup node 0 [mem 0x0000000080000000-0x000000009fffffff]
    VPE topology {2,2} total 4
    Primary instruction cache 64kB, VIPT, 4-way, linesize 32 bytes.
    Primary data cache 64kB, 4-way, PIPT, no aliases, linesize 32 bytes
    <AdEL exception here>

This is early enough that the kernel exception vectors are not in use,
so any further output depends upon the bootloader. This is reproducible
in QEMU where no further output occurs - ie. the system hangs here.
Given the affected v4.4 configuration this patch is marked for stable
backport to v4.4, however I cannot test every configuration so it's
entirely possible that this bug hits other platforms in earlier kernel
versions. Given the nature of the bug it may potentially be hit with
differing symptoms.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: <stable@vger.kernel.org> # v4.4+
Cc: Matthew Fortune <matthew.fortune@imgtec.com>
Cc: Robert Suchanek <robert.suchanek@imgtec.com>
---
This is an alternative approach to this earlier patch which seems to
have been rejected:

    https://patchwork.linux-mips.org/patch/12556/
    https://marc.info/?l=linux-mips&m=145555921408274&w=2

Changes in v2:
- Remove generic-y entry.
---
 arch/mips/include/asm/Kbuild         |  1 -
 arch/mips/include/asm/compiler-gcc.h | 29 +++++++++++++++++++++++++++++
 2 files changed, 29 insertions(+), 1 deletion(-)
 create mode 100644 arch/mips/include/asm/compiler-gcc.h

diff --git a/arch/mips/include/asm/Kbuild b/arch/mips/include/asm/Kbuild
index 9ccb519..c7fe4d0 100644
--- a/arch/mips/include/asm/Kbuild
+++ b/arch/mips/include/asm/Kbuild
@@ -1,6 +1,5 @@
 # MIPS headers
 generic-(CONFIG_GENERIC_CSUM) += checksum.h
-generic-y += compiler-gcc.h
 generic-y += cputime.h
 generic-y += current.h
 generic-y += dma-contiguous.h
diff --git a/arch/mips/include/asm/compiler-gcc.h b/arch/mips/include/asm/compiler-gcc.h
new file mode 100644
index 0000000..565be9b
--- /dev/null
+++ b/arch/mips/include/asm/compiler-gcc.h
@@ -0,0 +1,29 @@
+/*
+ * With GCC v4.5 onwards can use __builtin_unreachable to indicate to the
+ * compiler that a particular code path will never be hit. This allows it to be
+ * optimised out of the generated binary.
+ *
+ * Unfortunately GCC from at least v4.9.2 to current head of tree as of May
+ * 2016 suffer from a bug that can lead to instructions from beyond an
+ * unreachable statement being incorrectly reordered into earlier delay slots
+ * if the unreachable statement is the only content of a case in a switch
+ * statement. This can lead to seemingly random behaviour, such as invalid
+ * memory accesses from incorrectly reordered loads or stores. See this
+ * potential GCC fix for details:
+ *
+ *   https://gcc.gnu.org/ml/gcc-patches/2015-09/msg00360.html
+ *
+ * We work around this by placing a volatile asm statement, which GCC is
+ * prevented from reordering past, prior to the __builtin_unreachable call. The
+ * .insn statement is required to ensure that any branches to the statement,
+ * which sadly must be kept due to the asm statement, are known to be branches
+ * to code and satisfy linker requirements for microMIPS kernels.
+ */
+#if GCC_VERSION >= 40500
+# define unreachable() do {			\
+	asm volatile(".insn");			\
+	__builtin_unreachable();		\
+} while (0)
+#endif
+
+#include <asm-generic/compiler-gcc.h>
-- 
2.8.3

^ permalink raw reply related	[flat|nested] 4+ messages in thread
* [PATCH v2 2/2] MIPS: Workaround GCC __builtin_unreachable reordering bug
  2016-05-27 15:45 ` [PATCH v2 2/2] MIPS: Workaround GCC __builtin_unreachable reordering bug Paul Burton
@ 2016-05-27 15:45   ` Paul Burton
  0 siblings, 0 replies; 4+ messages in thread
From: Paul Burton @ 2016-05-27 15:45 UTC (permalink / raw)
  To: linux-mips
  Cc: Ralf Baechle, linux-arch, Paul Burton, # v4 . 4+,
	Matthew Fortune, Robert Suchanek

Current versions of GCC for the MIPS architecture suffer from a bug
which can lead to instructions from beyond an unreachable statement
being incorrectly reordered into earlier branch delay slots if the
unreachable statement is the only content of a case in a switch
statement. This can lead to seemingly random behaviour, such as invalid
memory accesses from incorrectly reordered loads or stores.

See this potential GCC fix for details:

    https://gcc.gnu.org/ml/gcc-patches/2015-09/msg00360.html

Work around this bug by placing a volatile asm statement, which GCC is
prevented from reordering past, prior to the __builtin_unreachable call.

This bug affects at least a maltasmvp_defconfig kernel built from the
v4.4 tag using GCC 4.9.2 (from a Codescape SDK 2015.06-05 toolchain),
with the result being an address exception taken after log messages
about the L1 caches (during probe of the L2 cache):

    Initmem setup node 0 [mem 0x0000000080000000-0x000000009fffffff]
    VPE topology {2,2} total 4
    Primary instruction cache 64kB, VIPT, 4-way, linesize 32 bytes.
    Primary data cache 64kB, 4-way, PIPT, no aliases, linesize 32 bytes
    <AdEL exception here>

This is early enough that the kernel exception vectors are not in use,
so any further output depends upon the bootloader. This is reproducible
in QEMU where no further output occurs - ie. the system hangs here.
Given the affected v4.4 configuration this patch is marked for stable
backport to v4.4, however I cannot test every configuration so it's
entirely possible that this bug hits other platforms in earlier kernel
versions. Given the nature of the bug it may potentially be hit with
differing symptoms.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: <stable@vger.kernel.org> # v4.4+
Cc: Matthew Fortune <matthew.fortune@imgtec.com>
Cc: Robert Suchanek <robert.suchanek@imgtec.com>
---
This is an alternative approach to this earlier patch which seems to
have been rejected:

    https://patchwork.linux-mips.org/patch/12556/
    https://marc.info/?l=linux-mips&m=145555921408274&w=2

Changes in v2:
- Remove generic-y entry.
---
 arch/mips/include/asm/Kbuild         |  1 -
 arch/mips/include/asm/compiler-gcc.h | 29 +++++++++++++++++++++++++++++
 2 files changed, 29 insertions(+), 1 deletion(-)
 create mode 100644 arch/mips/include/asm/compiler-gcc.h

diff --git a/arch/mips/include/asm/Kbuild b/arch/mips/include/asm/Kbuild
index 9ccb519..c7fe4d0 100644
--- a/arch/mips/include/asm/Kbuild
+++ b/arch/mips/include/asm/Kbuild
@@ -1,6 +1,5 @@
 # MIPS headers
 generic-(CONFIG_GENERIC_CSUM) += checksum.h
-generic-y += compiler-gcc.h
 generic-y += cputime.h
 generic-y += current.h
 generic-y += dma-contiguous.h
diff --git a/arch/mips/include/asm/compiler-gcc.h b/arch/mips/include/asm/compiler-gcc.h
new file mode 100644
index 0000000..565be9b
--- /dev/null
+++ b/arch/mips/include/asm/compiler-gcc.h
@@ -0,0 +1,29 @@
+/*
+ * With GCC v4.5 onwards can use __builtin_unreachable to indicate to the
+ * compiler that a particular code path will never be hit. This allows it to be
+ * optimised out of the generated binary.
+ *
+ * Unfortunately GCC from at least v4.9.2 to current head of tree as of May
+ * 2016 suffer from a bug that can lead to instructions from beyond an
+ * unreachable statement being incorrectly reordered into earlier delay slots
+ * if the unreachable statement is the only content of a case in a switch
+ * statement. This can lead to seemingly random behaviour, such as invalid
+ * memory accesses from incorrectly reordered loads or stores. See this
+ * potential GCC fix for details:
+ *
+ *   https://gcc.gnu.org/ml/gcc-patches/2015-09/msg00360.html
+ *
+ * We work around this by placing a volatile asm statement, which GCC is
+ * prevented from reordering past, prior to the __builtin_unreachable call. The
+ * .insn statement is required to ensure that any branches to the statement,
+ * which sadly must be kept due to the asm statement, are known to be branches
+ * to code and satisfy linker requirements for microMIPS kernels.
+ */
+#if GCC_VERSION >= 40500
+# define unreachable() do {			\
+	asm volatile(".insn");			\
+	__builtin_unreachable();		\
+} while (0)
+#endif
+
+#include <asm-generic/compiler-gcc.h>
-- 
2.8.3


^ permalink raw reply related	[flat|nested] 4+ messages in thread
end of thread, other threads:[~2016-05-27 15:46 UTC | newest]

Thread overview: 4+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2016-05-27 15:45 [PATCH v2 1/2] compiler-gcc: Allow arch-specific override of unreachable Paul Burton
2016-05-27 15:45 ` Paul Burton
2016-05-27 15:45 ` [PATCH v2 2/2] MIPS: Workaround GCC __builtin_unreachable reordering bug Paul Burton
2016-05-27 15:45   ` Paul Burton

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