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* [PATCH] Only add -fno-var-tracking-assignments workaround for old GCC versions.
       [not found] <CAP-5=fUT-1-CR-KMMsrpzgw9b3nBooeY05=YU9XKa5enO9SK+A@mail.gmail.com>
@ 2020-10-14 11:01 ` Mark Wielaard
  2020-10-14 15:17   ` Andi Kleen
  2020-10-17 12:01   ` [PATCH V2] " Mark Wielaard
  0 siblings, 2 replies; 5+ messages in thread
From: Mark Wielaard @ 2020-10-14 11:01 UTC (permalink / raw)
  To: linux-kernel, Masahiro Yamada, Michal Marek, linux-kbuild
  Cc: Ian Rogers, Mark Wielaard, linux-toolchains, Andi Kleen,
	Nick Desaulniers, Segher Boessenkool, Florian Weimer

Some old GCC versions between 4.5.0 and 4.9.1 might miscompile code
with -fvar-tracking-assingments (which is enabled by default with -g -O2).
commit 2062afb4f added -fno-var-tracking-assignments unconditionally to
work around this. But newer versions of GCC no longer have this bug, so
only add it for versions of GCC before 5.0.

Signed-off-by: Mark Wielaard <mark@klomp.org>
Acked-by: Ian Rogers <irogers@google.com>
Cc: linux-toolchains@vger.kernel.org
Cc: Andi Kleen <andi@firstfloor.org>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Segher Boessenkool <segher@kernel.crashing.org>
Cc: Florian Weimer <fw@deneb.enyo.de>
---
 Makefile | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/Makefile b/Makefile
index 51540b291738..8477fee5f309 100644
--- a/Makefile
+++ b/Makefile
@@ -813,7 +813,9 @@ KBUILD_CFLAGS	+= -ftrivial-auto-var-init=zero
 KBUILD_CFLAGS	+= -enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang
 endif
 
-DEBUG_CFLAGS	:= $(call cc-option, -fno-var-tracking-assignments)
+# Workaround https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61801
+# for old versions of GCC.
+DEBUG_CFLAGS	:= $(call cc-ifversion, -lt, 0500, $(call cc-option, -fno-var-tracking-assignments))
 
 ifdef CONFIG_DEBUG_INFO
 ifdef CONFIG_DEBUG_INFO_SPLIT
-- 
2.18.4


^ permalink raw reply	[flat|nested] 5+ messages in thread

* Re: [PATCH] Only add -fno-var-tracking-assignments workaround for old GCC versions.
  2020-10-14 11:01 ` [PATCH] Only add -fno-var-tracking-assignments workaround for old GCC versions Mark Wielaard
@ 2020-10-14 15:17   ` Andi Kleen
  2020-10-17 12:01   ` [PATCH V2] " Mark Wielaard
  1 sibling, 0 replies; 5+ messages in thread
From: Andi Kleen @ 2020-10-14 15:17 UTC (permalink / raw)
  To: Mark Wielaard
  Cc: linux-kernel, Masahiro Yamada, Michal Marek, linux-kbuild,
	Ian Rogers, linux-toolchains, Andi Kleen, Nick Desaulniers,
	Segher Boessenkool, Florian Weimer

On Wed, Oct 14, 2020 at 01:01:32PM +0200, Mark Wielaard wrote:
> Some old GCC versions between 4.5.0 and 4.9.1 might miscompile code
> with -fvar-tracking-assingments (which is enabled by default with -g -O2).
> commit 2062afb4f added -fno-var-tracking-assignments unconditionally to
> work around this. But newer versions of GCC no longer have this bug, so
> only add it for versions of GCC before 5.0.

Add

... This allows various tools such as a perf probe or gdb debuggers
or systemtap to resolve variable locations using dwarf locations in 
more code.
> 
> Signed-off-by: Mark Wielaard <mark@klomp.org>
> Acked-by: Ian Rogers <irogers@google.com>
> Cc: linux-toolchains@vger.kernel.org
> Cc: Andi Kleen <andi@firstfloor.org>
> Cc: Nick Desaulniers <ndesaulniers@google.com>
> Cc: Segher Boessenkool <segher@kernel.crashing.org>
> Cc: Florian Weimer <fw@deneb.enyo.de>

Reviewed-by: Andi Kleen <ak@linux.intel.com>

-Andi

^ permalink raw reply	[flat|nested] 5+ messages in thread

* [PATCH V2] Only add -fno-var-tracking-assignments workaround for old GCC versions.
  2020-10-14 11:01 ` [PATCH] Only add -fno-var-tracking-assignments workaround for old GCC versions Mark Wielaard
  2020-10-14 15:17   ` Andi Kleen
@ 2020-10-17 12:01   ` Mark Wielaard
  2020-10-19 19:30     ` Nick Desaulniers
  2020-10-20 15:27     ` Masahiro Yamada
  1 sibling, 2 replies; 5+ messages in thread
From: Mark Wielaard @ 2020-10-17 12:01 UTC (permalink / raw)
  To: linux-kernel, Masahiro Yamada, Michal Marek, linux-kbuild
  Cc: Ian Rogers, Andi Kleen, Mark Wielaard, linux-toolchains,
	Nick Desaulniers, Segher Boessenkool, Florian Weimer,
	Sedat Dilek

Some old GCC versions between 4.5.0 and 4.9.1 might miscompile code
with -fvar-tracking-assingments (which is enabled by default with -g -O2).
commit 2062afb4f added -fno-var-tracking-assignments unconditionally to
work around this. But newer versions of GCC no longer have this bug, so
only add it for versions of GCC before 5.0. This allows various tools
such as a perf probe or gdb debuggers or systemtap to resolve variable
locations using dwarf locations in more code.

Changes in V2:
- Update commit message explaining purpose.
- Explicitly mention GCC version in comment.
- Wrap workaround in ifdef CONFIG_CC_IS_GCC

Signed-off-by: Mark Wielaard <mark@klomp.org>
Acked-by: Ian Rogers <irogers@google.com>
Reviewed-by: Andi Kleen <andi@firstfloor.org>
Cc: linux-toolchains@vger.kernel.org
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Segher Boessenkool <segher@kernel.crashing.org>
Cc: Florian Weimer <fw@deneb.enyo.de>
Cc: Sedat Dilek <sedat.dilek@gmail.com>
---
 Makefile | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/Makefile b/Makefile
index 51540b291738..964754b4cedf 100644
--- a/Makefile
+++ b/Makefile
@@ -813,7 +813,11 @@ KBUILD_CFLAGS	+= -ftrivial-auto-var-init=zero
 KBUILD_CFLAGS	+= -enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang
 endif
 
-DEBUG_CFLAGS	:= $(call cc-option, -fno-var-tracking-assignments)
+# Workaround for GCC versions < 5.0
+# https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61801
+ifdef CONFIG_CC_IS_GCC
+DEBUG_CFLAGS	:= $(call cc-ifversion, -lt, 0500, $(call cc-option, -fno-var-tracking-assignments))
+endif
 
 ifdef CONFIG_DEBUG_INFO
 ifdef CONFIG_DEBUG_INFO_SPLIT
-- 
2.18.4


^ permalink raw reply	[flat|nested] 5+ messages in thread

* Re: [PATCH V2] Only add -fno-var-tracking-assignments workaround for old GCC versions.
  2020-10-17 12:01   ` [PATCH V2] " Mark Wielaard
@ 2020-10-19 19:30     ` Nick Desaulniers
  2020-10-20 15:27     ` Masahiro Yamada
  1 sibling, 0 replies; 5+ messages in thread
From: Nick Desaulniers @ 2020-10-19 19:30 UTC (permalink / raw)
  To: Mark Wielaard
  Cc: LKML, Masahiro Yamada, Michal Marek, Linux Kbuild mailing list,
	Ian Rogers, Andi Kleen, linux-toolchains, Segher Boessenkool,
	Florian Weimer, Sedat Dilek

On Sat, Oct 17, 2020 at 5:02 AM Mark Wielaard <mark@klomp.org> wrote:
>
> Some old GCC versions between 4.5.0 and 4.9.1 might miscompile code
> with -fvar-tracking-assingments (which is enabled by default with -g -O2).
> commit 2062afb4f added -fno-var-tracking-assignments unconditionally to
> work around this. But newer versions of GCC no longer have this bug, so
> only add it for versions of GCC before 5.0. This allows various tools
> such as a perf probe or gdb debuggers or systemtap to resolve variable
> locations using dwarf locations in more code.
>
> Changes in V2:
> - Update commit message explaining purpose.
> - Explicitly mention GCC version in comment.
> - Wrap workaround in ifdef CONFIG_CC_IS_GCC
>
> Signed-off-by: Mark Wielaard <mark@klomp.org>
> Acked-by: Ian Rogers <irogers@google.com>
> Reviewed-by: Andi Kleen <andi@firstfloor.org>
> Cc: linux-toolchains@vger.kernel.org
> Cc: Nick Desaulniers <ndesaulniers@google.com>
> Cc: Segher Boessenkool <segher@kernel.crashing.org>
> Cc: Florian Weimer <fw@deneb.enyo.de>
> Cc: Sedat Dilek <sedat.dilek@gmail.com>
> ---
>  Makefile | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/Makefile b/Makefile
> index 51540b291738..964754b4cedf 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -813,7 +813,11 @@ KBUILD_CFLAGS      += -ftrivial-auto-var-init=zero
>  KBUILD_CFLAGS  += -enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang
>  endif
>
> -DEBUG_CFLAGS   := $(call cc-option, -fno-var-tracking-assignments)
> +# Workaround for GCC versions < 5.0
> +# https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61801
> +ifdef CONFIG_CC_IS_GCC
> +DEBUG_CFLAGS   := $(call cc-ifversion, -lt, 0500, $(call cc-option, -fno-var-tracking-assignments))

Thanks for adding the comment. That will help us find+remove this when
the kernel's minimum supported version of GCC advances to gcc-5.  The
current minimum supported version of GCC according to
Documentation/process/changes.rst is gcc-4.9 (so anything older is
irrelevant, and we drop support for it).  If gcc 4.9 supports
`-fno-var-tracking-assignments` (it looks like it does:
https://godbolt.org/z/oa53f5), then we should drop the `cc-option`
call, which will save us a compiler invocation for each invocation of
`make`.

> +endif
>
>  ifdef CONFIG_DEBUG_INFO
>  ifdef CONFIG_DEBUG_INFO_SPLIT
> --
> 2.18.4
>


-- 
Thanks,
~Nick Desaulniers

^ permalink raw reply	[flat|nested] 5+ messages in thread

* Re: [PATCH V2] Only add -fno-var-tracking-assignments workaround for old GCC versions.
  2020-10-17 12:01   ` [PATCH V2] " Mark Wielaard
  2020-10-19 19:30     ` Nick Desaulniers
@ 2020-10-20 15:27     ` Masahiro Yamada
  1 sibling, 0 replies; 5+ messages in thread
From: Masahiro Yamada @ 2020-10-20 15:27 UTC (permalink / raw)
  To: Mark Wielaard
  Cc: Linux Kernel Mailing List, Michal Marek,
	Linux Kbuild mailing list, Ian Rogers, Andi Kleen,
	linux-toolchains, Nick Desaulniers, Segher Boessenkool,
	Florian Weimer, Sedat Dilek

On Sat, Oct 17, 2020 at 9:02 PM Mark Wielaard <mark@klomp.org> wrote:
>
> Some old GCC versions between 4.5.0 and 4.9.1 might miscompile code
> with -fvar-tracking-assingments (which is enabled by default with -g -O2).
> commit 2062afb4f added -fno-var-tracking-assignments unconditionally to
> work around this. But newer versions of GCC no longer have this bug, so
> only add it for versions of GCC before 5.0. This allows various tools
> such as a perf probe or gdb debuggers or systemtap to resolve variable
> locations using dwarf locations in more code.
>
> Changes in V2:
> - Update commit message explaining purpose.
> - Explicitly mention GCC version in comment.
> - Wrap workaround in ifdef CONFIG_CC_IS_GCC
>
> Signed-off-by: Mark Wielaard <mark@klomp.org>
> Acked-by: Ian Rogers <irogers@google.com>
> Reviewed-by: Andi Kleen <andi@firstfloor.org>
> Cc: linux-toolchains@vger.kernel.org
> Cc: Nick Desaulniers <ndesaulniers@google.com>
> Cc: Segher Boessenkool <segher@kernel.crashing.org>
> Cc: Florian Weimer <fw@deneb.enyo.de>
> Cc: Sedat Dilek <sedat.dilek@gmail.com>
> ---

Applied to linux-kbuild. Thanks.



>  Makefile | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/Makefile b/Makefile
> index 51540b291738..964754b4cedf 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -813,7 +813,11 @@ KBUILD_CFLAGS      += -ftrivial-auto-var-init=zero
>  KBUILD_CFLAGS  += -enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang
>  endif
>
> -DEBUG_CFLAGS   := $(call cc-option, -fno-var-tracking-assignments)
> +# Workaround for GCC versions < 5.0
> +# https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61801
> +ifdef CONFIG_CC_IS_GCC
> +DEBUG_CFLAGS   := $(call cc-ifversion, -lt, 0500, $(call cc-option, -fno-var-tracking-assignments))
> +endif
>
>  ifdef CONFIG_DEBUG_INFO
>  ifdef CONFIG_DEBUG_INFO_SPLIT
> --
> 2.18.4
>


-- 
Best Regards
Masahiro Yamada

^ permalink raw reply	[flat|nested] 5+ messages in thread

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     [not found] <CAP-5=fUT-1-CR-KMMsrpzgw9b3nBooeY05=YU9XKa5enO9SK+A@mail.gmail.com>
2020-10-14 11:01 ` [PATCH] Only add -fno-var-tracking-assignments workaround for old GCC versions Mark Wielaard
2020-10-14 15:17   ` Andi Kleen
2020-10-17 12:01   ` [PATCH V2] " Mark Wielaard
2020-10-19 19:30     ` Nick Desaulniers
2020-10-20 15:27     ` Masahiro Yamada

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