Conditional sibcalls?

[Konstantin Vladimirov]

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