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    Subject[037/143] Fix gcc 4.5.1 miscompiling drivers/char/i8k.c (again)
    2.6.32-longterm review patch.  If anyone has any objections, please let us know.


    From: Jim Bos <>

    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 <>
    Acked-by: Jakub Jelinek <>
    Cc: Andi Kleen <>
    Cc: Andreas Schwab <>
    Signed-off-by: Linus Torvalds <>
    Signed-off-by: Greg Kroah-Hartman <>

    drivers/char/i8k.c | 8 ++++----
    1 file changed, 4 insertions(+), 4 deletions(-)

    --- a/drivers/char/i8k.c
    +++ b/drivers/char/i8k.c
    @@ -119,7 +119,7 @@ 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"
    @@ -141,11 +141,11 @@ static int i8k_smm(struct smm_regs *regs
    "shrl $8,%%eax\n\t"
    "andl $1,%%eax\n"
    - :"=a"(rc), "+m" (*regs)
    + :"=a"(rc)
    : "a"(regs)
    : "%ebx", "%ecx", "%edx", "%esi", "%edi", "memory");
    - 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 @@ static int i8k_smm(struct smm_regs *regs
    "shrl $8,%%eax\n\t"
    "andl $1,%%eax\n"
    - :"=a"(rc), "+m" (*regs)
    + :"=a"(rc)
    : "a"(regs)
    : "%ebx", "%ecx", "%edx", "%esi", "%edi", "memory");

     \ /
      Last update: 2011-05-06 03:01    [W:0.025 / U:19.032 seconds]
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