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Fix gcc 4.5.1 miscompiling drivers/char/i8k.c (again)

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The fix in commit 6b4e81db25 ("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>
This commit is contained in:
Jim Bos 2010-11-15 21:22:37 +01:00 committed by Linus Torvalds
parent 620751a259
commit 22d3243de8
1 changed files with 4 additions and 4 deletions
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8
drivers/char/i8k.c
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@ -120,7 +120,7 @@ static int i8k_smm(struct smm_regs *regs)
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"
@ -142,11 +142,11 @@ static int i8k_smm(struct smm_regs *regs)
"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"
@ -168,7 +168,7 @@ static int i8k_smm(struct smm_regs *regs)
"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