* [RFC PATCH] powerpc: use __builtin_trap() in BUG/WARN macros.
@ 2019-08-17 18:37 Christophe Leroy
2019-08-18 12:20 ` Segher Boessenkool
0 siblings, 1 reply; 2+ messages in thread
From: Christophe Leroy @ 2019-08-17 18:37 UTC (permalink / raw)
To: Benjamin Herrenschmidt, Paul Mackerras, Michael Ellerman,
Segher Boessenkool
Cc: linuxppc-dev, linux-kernel
The below exemples of use of WARN_ON() show that the result
is sub-optimal in regard of the capabilities of powerpc.
void test_warn1(unsigned long long a)
{
WARN_ON(a);
}
void test_warn2(unsigned long a)
{
WARN_ON(a);
}
void test_warn3(unsigned long a, unsigned long b)
{
WARN_ON(a < b);
}
void test_warn4(unsigned long a, unsigned long b)
{
WARN_ON(!a);
}
void test_warn5(unsigned long a, unsigned long b)
{
WARN_ON(!a && b);
}
00000000 <test_warn1>:
0: 7c 64 23 78 or r4,r3,r4
4: 31 24 ff ff addic r9,r4,-1
8: 7c 89 21 10 subfe r4,r9,r4
c: 0f 04 00 00 twnei r4,0
10: 4e 80 00 20 blr
00000014 <test_warn2>:
14: 31 23 ff ff addic r9,r3,-1
18: 7c 69 19 10 subfe r3,r9,r3
1c: 0f 03 00 00 twnei r3,0
20: 4e 80 00 20 blr
00000024 <test_warn3>:
24: 7c 84 18 10 subfc r4,r4,r3
28: 7d 29 49 10 subfe r9,r9,r9
2c: 7d 29 00 d0 neg r9,r9
30: 0f 09 00 00 twnei r9,0
34: 4e 80 00 20 blr
00000038 <test_warn4>:
38: 7c 63 00 34 cntlzw r3,r3
3c: 54 63 d9 7e rlwinm r3,r3,27,5,31
40: 0f 03 00 00 twnei r3,0
44: 4e 80 00 20 blr
00000048 <test_warn5>:
48: 2f 83 00 00 cmpwi cr7,r3,0
4c: 39 20 00 00 li r9,0
50: 41 9e 00 0c beq cr7,5c <test_warn5+0x14>
54: 7c 84 00 34 cntlzw r4,r4
58: 54 89 d9 7e rlwinm r9,r4,27,5,31
5c: 0f 09 00 00 twnei r9,0
60: 4e 80 00 20 blr
RELOCATION RECORDS FOR [__bug_table]:
OFFSET TYPE VALUE
00000000 R_PPC_ADDR32 .text+0x0000000c
0000000c R_PPC_ADDR32 .text+0x0000001c
00000018 R_PPC_ADDR32 .text+0x00000030
00000018 R_PPC_ADDR32 .text+0x00000030
00000024 R_PPC_ADDR32 .text+0x00000040
00000030 R_PPC_ADDR32 .text+0x0000005c
Using __builtin_trap() instead of inline assembly of twnei/tdnei
provides a far better result:
00000000 <test_warn1>:
0: 7c 64 23 78 or r4,r3,r4
4: 0f 04 00 00 twnei r4,0
8: 4e 80 00 20 blr
0000000c <test_warn2>:
c: 0f 03 00 00 twnei r3,0
10: 4e 80 00 20 blr
00000014 <test_warn3>:
14: 7c 43 20 08 twllt r3,r4
18: 4e 80 00 20 blr
0000001c <test_warn4>:
1c: 0c 83 00 00 tweqi r3,0
20: 4e 80 00 20 blr
00000024 <test_warn5>:
24: 2f 83 00 00 cmpwi cr7,r3,0
28: 41 9e 00 08 beq cr7,30 <test_warn5+0xc>
2c: 0c 84 00 00 tweqi r4,0
30: 4e 80 00 20 blr
RELOCATION RECORDS FOR [__bug_table]:
OFFSET TYPE VALUE
00000000 R_PPC_ADDR32 .text+0x00000004
0000000c R_PPC_ADDR32 .text+0x0000000c
00000018 R_PPC_ADDR32 .text+0x00000014
00000024 R_PPC_ADDR32 .text+0x0000001c
00000030 R_PPC_ADDR32 .text+0x0000002c
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
---
arch/powerpc/include/asm/bug.h | 24 ++++++++++++++----------
1 file changed, 14 insertions(+), 10 deletions(-)
diff --git a/arch/powerpc/include/asm/bug.h b/arch/powerpc/include/asm/bug.h
index fed7e6241349..1a37c8d30b78 100644
--- a/arch/powerpc/include/asm/bug.h
+++ b/arch/powerpc/include/asm/bug.h
@@ -44,14 +44,14 @@
#ifdef CONFIG_DEBUG_BUGVERBOSE
#define _EMIT_BUG_ENTRY \
".section __bug_table,\"aw\"\n" \
- "2:\t" PPC_LONG "1b, %0\n" \
+ "2:\t" PPC_LONG "1b - 4, %0\n" \
"\t.short %1, %2\n" \
".org 2b+%3\n" \
".previous\n"
#else
#define _EMIT_BUG_ENTRY \
".section __bug_table,\"aw\"\n" \
- "2:\t" PPC_LONG "1b\n" \
+ "2:\t" PPC_LONG "1b - 4\n" \
"\t.short %2\n" \
".org 2b+%3\n" \
".previous\n"
@@ -64,8 +64,9 @@
*/
#define BUG() do { \
+ __builtin_trap(); \
__asm__ __volatile__( \
- "1: twi 31,0,0\n" \
+ "1:\n" \
_EMIT_BUG_ENTRY \
: : "i" (__FILE__), "i" (__LINE__), \
"i" (0), "i" (sizeof(struct bug_entry))); \
@@ -77,18 +78,20 @@
if (x) \
BUG(); \
} else { \
+ if (x) \
+ __builtin_trap(); \
__asm__ __volatile__( \
- "1: "PPC_TLNEI" %4,0\n" \
+ "1:\n" \
_EMIT_BUG_ENTRY \
: : "i" (__FILE__), "i" (__LINE__), "i" (0), \
- "i" (sizeof(struct bug_entry)), \
- "r" ((__force long)(x))); \
+ "i" (sizeof(struct bug_entry))); \
} \
} while (0)
#define __WARN_FLAGS(flags) do { \
+ __builtin_trap(); \
__asm__ __volatile__( \
- "1: twi 31,0,0\n" \
+ "1:\n" \
_EMIT_BUG_ENTRY \
: : "i" (__FILE__), "i" (__LINE__), \
"i" (BUGFLAG_WARNING|(flags)), \
@@ -101,13 +104,14 @@
if (__ret_warn_on) \
__WARN(); \
} else { \
+ if (__ret_warn_on) \
+ __builtin_trap(); \
__asm__ __volatile__( \
- "1: "PPC_TLNEI" %4,0\n" \
+ "1:\n" \
_EMIT_BUG_ENTRY \
: : "i" (__FILE__), "i" (__LINE__), \
"i" (BUGFLAG_WARNING|BUGFLAG_TAINT(TAINT_WARN)),\
- "i" (sizeof(struct bug_entry)), \
- "r" (__ret_warn_on)); \
+ "i" (sizeof(struct bug_entry))); \
} \
unlikely(__ret_warn_on); \
})
--
2.17.1
^ permalink raw reply related [flat|nested] 2+ messages in thread
* Re: [RFC PATCH] powerpc: use __builtin_trap() in BUG/WARN macros.
2019-08-17 18:37 [RFC PATCH] powerpc: use __builtin_trap() in BUG/WARN macros Christophe Leroy
@ 2019-08-18 12:20 ` Segher Boessenkool
0 siblings, 0 replies; 2+ messages in thread
From: Segher Boessenkool @ 2019-08-18 12:20 UTC (permalink / raw)
To: Christophe Leroy; +Cc: linuxppc-dev, Paul Mackerras, linux-kernel
Hi Christophe,
On Sat, Aug 17, 2019 at 06:37:50PM +0000, Christophe Leroy wrote:
> #define BUG() do { \
> + __builtin_trap(); \
GCC will optimise away all code after this, it knows it is unreachable.
But you want to keep that BUG_ENTRY stuff I think?
The same will happen with a BUG_ON if the compiler can prove your
condition is always true.
> __asm__ __volatile__( \
> - "1: twi 31,0,0\n" \
> + "1:\n" \
> _EMIT_BUG_ENTRY \
> : : "i" (__FILE__), "i" (__LINE__), \
> "i" (0), "i" (sizeof(struct bug_entry))); \
(GCC wil generate a different trap btw; what is called "trap" as extended
mnemonic, that is, "tw 31,0,0", not the same thing as "twi", if that
matters for the exception handler).
Segher
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2019-08-17 18:37 [RFC PATCH] powerpc: use __builtin_trap() in BUG/WARN macros Christophe Leroy
2019-08-18 12:20 ` Segher Boessenkool
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