On 25-02-15 18:26, Patrick Baggett wrote:
>>
>>
>> In general things don't get optimized across function calls, except in
>> case of inlinable functions.
>>
>> And for compiler attributes it's the opposite,__attribute__((const)) and
>> __attribute((pure)) can be used to indicate some kind of safety to optimize
>> across functions.
>>
>> https://gcc.gnu.org/onlinedocs/gcc/Function-Attributes.html
>>
>> This is true, but LTO increases the compiler's ability to make these sorts
> of optimizations across function calls and even C source file boundaries
> without you needing to explicitly mark functions as such.
Even if pthread_mutex_lock was completely inlined there would still be a asm
volatile(("" ::: "memory")) in there acting as a complete memory barrier to the
compiler.
Create a function called dummy.c, abusing the fact that gcc can't handle
pointers well so it won't get reduced to a constant return value:
int x;
int *px = &x;
int main() {
if (*px == 1)
return 1;
asm volatile("" ::: "memory");
if (*px == 1)
return 1;
return -1;
}
Now compile with gcc test.c -O3 -fwhole-program, and run objdump -d a.out:
400400: 83 3d 49 0c 20 00 01 cmpl $0x1,0x200c49(%rip) #
601050 <x>
400407: 74 09 je 400412 <main+0x12>
400409: 83 3d 40 0c 20 00 01 cmpl $0x1,0x200c40(%rip) #
601050 <x>
400410: 75 06 jne 400418 <main+0x18>
400412: b8 01 00 00 00 mov $0x1,%eax
400417: c3 retq
400418: 83 c8 ff or $0xffffffff,%eax
40041b: c3 retq
Hey my second check didn't get compiled away.. magic.
And to show that a random function call does the same, replace the barrier with
random():
0000000000400440 <main>:
400440: 83 3d 09 0c 20 00 01 cmpl $0x1,0x200c09(%rip) #
601050 <x>
400447: 74 1b je 400464 <main+0x24>
400449: 50 push %rax
40044a: 31 c0 xor %eax,%eax
40044c: e8 df ff ff ff callq 400430 <random@plt>
400451: 83 3d f8 0b 20 00 01 cmpl $0x1,0x200bf8(%rip) #
601050 <x>
400458: b8 01 00 00 00 mov $0x1,%eax
40045d: 75 0b jne 40046a <main+0x2a>
40045f: 48 83 c4 08 add $0x8,%rsp
400463: c3 retq
400464: b8 01 00 00 00 mov $0x1,%eax
400469: c3 retq
40046a: 83 c8 ff or $0xffffffff,%eax
40046d: eb f0 jmp 40045f <main+0x1f>
And just to be thorough, showing what happens without function call or barrier:
0000000000400400 <main>:
400400: 8b 05 4a 0c 20 00 mov 0x200c4a(%rip),%eax #
601050 <x>
400406: ba ff ff ff ff mov $0xffffffff,%edx
40040b: 83 f8 01 cmp $0x1,%eax
40040e: 0f 45 c2 cmovne %edx,%eax
400411: c3 retq
~Maarten
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