| Date | Sun, 05 Feb 2012 23:11:15 +0100 | From | Willy Tarreau <> | Subject | [PATCH 86/91] Fix gcc 4.5.1 miscompiling drivers/char/i8k.c (again) |
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2.6.27-longterm review patch. If anyone has any objections, please let us know.
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commit 22d3243de86bc92d874abb7c5b185d5c47aba323 upstream.
The fix in commit 6b4e81db2552 ("i8k: Tell gcc that *regs gets clobbered") to work around the gcc miscompiling i8k.c to add "+m (*regs)" caused register pressure problems and a build failure.
Changing the 'asm' statement to 'asm volatile' instead should prevent that and works around the gcc bug as well, so we can remove the "+m".
[ Background on the gcc bug: a memory clobber fails to mark the function the asm resides in as non-pure (aka "__attribute__((const))"), so if the function does nothing else that triggers the non-pure logic, gcc will think that that function has no side effects at all. As a result, callers will be mis-compiled.
Adding the "+m" made gcc see that it's not a pure function, and so does "asm volatile". The problem was never really the need to mark "*regs" as changed, since the memory clobber did that part - the problem was just a bug in the gcc "pure" function analysis - Linus ]
Signed-off-by: Jim Bos <jim876@xs4all.nl> Acked-by: Jakub Jelinek <jakub@redhat.com> Cc: Andi Kleen <andi@firstfloor.org> Cc: Andreas Schwab <schwab@linux-m68k.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de> --- drivers/char/i8k.c | 8 ++++---- 1 files changed, 4 insertions(+), 4 deletions(-)
Index: longterm-2.6.27/drivers/char/i8k.c =================================================================== --- longterm-2.6.27.orig/drivers/char/i8k.c 2012-02-05 22:34:47.002916726 +0100 +++ longterm-2.6.27/drivers/char/i8k.c 2012-02-05 22:34:47.136914619 +0100 @@ -119,7 +119,7 @@ int eax = regs->eax; #if defined(CONFIG_X86_64) - asm("pushq %%rax\n\t" + asm volatile("pushq %%rax\n\t" "movl 0(%%rax),%%edx\n\t" "pushq %%rdx\n\t" "movl 4(%%rax),%%ebx\n\t" @@ -141,11 +141,11 @@ "lahf\n\t" "shrl $8,%%eax\n\t" "andl $1,%%eax\n" - :"=a"(rc), "+m" (*regs) + :"=a"(rc) : "a"(regs) : "%ebx", "%ecx", "%edx", "%esi", "%edi", "memory"); #else - asm("pushl %%eax\n\t" + asm volatile("pushl %%eax\n\t" "movl 0(%%eax),%%edx\n\t" "push %%edx\n\t" "movl 4(%%eax),%%ebx\n\t" @@ -167,7 +167,7 @@ "lahf\n\t" "shrl $8,%%eax\n\t" "andl $1,%%eax\n" - :"=a"(rc), "+m" (*regs) + :"=a"(rc) : "a"(regs) : "%ebx", "%ecx", "%edx", "%esi", "%edi", "memory"); #endif
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