Conditional sibcalls?

[Konstantin Vladimirov]

Hi,

Sorry, mistyped. Please read `jne` instead of `je` in handwritten
"optimized" assembler.

---
With best regards, Konstantin

On Thu, Feb 21, 2013 at 3:57 PM, Konstantin Vladimirov
<konstantin.vladimirov@gmail.com> 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
>
> I want to somehow introduce conditional sibling calls in my backend.
> What approach would you advise as the best?
>
> Target GCC is 4.7.2 (with some patches from 4.7.3 trunk).
>
> ---
> With best regards, Konstantin