On 11/15/2010 08:51 PM, Jakub Jelinek wrote:
> On Mon, Nov 15, 2010 at 11:21:30AM -0800, Linus Torvalds wrote:
>> On Mon, Nov 15, 2010 at 11:12 AM, Jakub Jelinek <ja...@redhat.com> wrote:
>>>
>>> Ah, the problem is that memory_identifier_string is only initialized in
>>> ipa-reference.c's initialization, so it can be (and is in this case) NULL in
>>> ipa-pure-const.c.
>>
>> Ok. And I guess you can verify that all versions of gcc do this
>> correctly for "asm volatile"?
>
> Yes, reading 4.1/4.2/4.3/4.4/4.5/4.6 code ipa-pure-const.c handles
> asm volatile correctly, in each case the function is no longer assumed to be
> pure or const in the discovery (of course, user can still say the
> function is const or pure). 4.0 and earlier didn't have ipa-pure-const.c.
>
> Using the simplified
>
> extern void abort (void);
>
> __attribute__((noinline)) int
> foo (int *p)
> {
> int r;
> asm ("movl $6, (%1)\n\txorl %0, %0" : "=r" (r) : "r" (p) : "memory");
> return r;
> }
>
> int
> main (void)
> {
> int p = 8;
> if ((foo (&p) ? : p) != 6)
> abort ();
> return 0;
> }
>
> testcase shows that in 4.1/4.2/4.3/4.4 this is miscompiled only when using
> -fno-ipa-reference, in 4.5 it is miscompiled always when optimizing
> unless -fno-ipa-pure-const (as 4.5 added local-pure-const pass which is run
> before ipa-reference) and in 4.6 this has been fixed by Honza when
> doing ipa cleanups.
>
>> Because since we'll have to work around this problem in the kernel, I
>> suspect the simplest solution is to remove the "+m" that causes
>> register pressure problems, and then use "asm volatile" to work around
>> the const-function bug.
>
> Yes.
>
> Jakub
>
Linus,
In case you didn't already fixed this, here's the follow-up patch.
---
The fix to work around the gcc miscompiling i8k.c to add "+m (*regs)"
caused register pressure problems. Changing the 'asm' statement to
'asm volatile' instead should prevent that and works around the gcc
bug as well.
Signed-off-by: Jim Bos <jim...@xs4all.nl>
--- linux/drivers/char/i8k.c.ORIG 2010-11-15 21:04:19.000000000 +0100
+++ linux/drivers/char/i8k.c 2010-11-15 21:02:32.000000000 +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
