[PATCH 86/91] Fix gcc 4.5.1 miscompiling drivers/char/i8k.c (again)

From: Willy Tarreau
Date: Sun Feb 05 2012 - 17:32:15 EST

2.6.27-longterm review patch. If anyone has any objections, please let us know.


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@xxxxxxxxx>
Acked-by: Jakub Jelinek <jakub@xxxxxxxxxx>
Cc: Andi Kleen <andi@xxxxxxxxxxxxxx>
Cc: Andreas Schwab <schwab@xxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxx>
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 @@
"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 @@
"shrl $8,%%eax\n\t"
"andl $1,%%eax\n"
- :"=a"(rc), "+m" (*regs)
+ :"=a"(rc)
: "a"(regs)
: "%ebx", "%ecx", "%edx", "%esi", "%edi", "memory");

To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/