On Thu, Feb 21, 2013 at 03:57:05PM +0400, Konstantin Vladimirov wrote:
> Hi,
>
> Discovered this optimization possibilty on private backend, but can
> easily reproduce on x86
>
> Consider code, say test.c:
>
> static __attribute__((noinline)) unsigned int*
> proxy1( unsigned int* codeBuffer, unsigned int oper, unsigned int a, unsigned
> in
> {
> return codeBuffer;
> }
>
> static __attribute__((noinline)) unsigned int*
> proxy2( unsigned int* codeBuffer, unsigned int oper, unsigned int a, unsigned
> in
> {
> return codeBuffer;
> }
>
> __attribute__((noinline)) unsigned int*
> myFunc( unsigned int* codeBuffer, unsigned int oper)
> {
> if( (oper & 0xF) == 14)
> {
> return proxy1( codeBuffer, oper, 0x22, 0x2102400b);
> }
> else
> {
> return proxy2( codeBuffer, oper, 0x22, 0x1102400b);
> }
> }
>
> With ~/x86-toolchain-4.7.2/bin/gcc -O2 -fomit-frame-pointer -S test.c,
> gcc yields:
>
> myFunc:
> .LFB2:
> .cfi_startproc
> andl $15, %esi
> cmpl $14, %esi
> je .L6
> jmp proxy2.isra.1
> .p2align 4,,10
> .p2align 3
> .L6:
> jmp proxy1.isra.0
>
> Which can be simplified to:
>
> myFunc:
> .LFB2:
> .cfi_startproc
> andl $15, %esi
> cmpl $14, %esi
> je proxy2.isra.1 // <--- conditional sibling call here
> .p2align 4,,10
> .p2align 3
> jmp proxy1.isra.0
Apart from the je/jne thinko you mentioned, the .p2align directives
are completely useless in this case.
Regards,
Gabriel
