[Bug target/51244] SH Target: Inefficient conditional branch

[oleg.endo@t-online.de]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51244

--- Comment #6 from Oleg Endo <oleg.endo@t-online.de> 2011-12-28 15:59:35 UTC ---
(In reply to comment #3)
> Created attachment 26191 [details]
> Proposed patch to improve some of the issues.
> 
> The attached patch removes the useless sequence and still allows the -1
> constant to be CSE-ed for such cases as the example function above.
> 
> I haven't ran all tests on it yet, but CSiBE shows average code size reduction
> of approx. -0.1% for -m4* with some code size increases in some files.

Some of the code size increases are caused by the ifcvt.c pass which tries to
transform sequences like:

int test_func_6 (int a, int b, int c)
{
  if (a == 16)
    c = 0;
  return b + c;
}

into branch-free code like:
        mov     r4,r0   ! 45    movsi_ie/2    [length = 2]
        cmp/eq  #16,r0  ! 9     cmpeqsi_t/2    [length = 2]
        mov     #-1,r0  ! 34    movsi_ie/3    [length = 2]
        negc    r0,r0   ! 38    *negc    [length = 2]
        neg     r0,r0   ! 36    negsi2    [length = 2]
        and     r6,r0   ! 37    *andsi3_compact/2    [length = 2]
        rts             ! 48    *return_i    [length = 2]
        add     r5,r0   ! 14    *addsi3_compact    [length = 2]

instead of the more compact (and on SH4 most likely better):
        mov    r4,r0   ! 41    movsi_ie/2    [length = 2]
        cmp/eq    #16,r0  ! 9    cmpeqsi_t/2    [length = 2]
        bf    0f      ! 34    *movsicc_t_true/2    [length = 4]
        mov    #0,r6
0:
        add    r5,r6   ! 14    *addsi3_compact    [length = 2]
        rts             ! 44    *return_i    [length = 2]
        mov    r6,r0   ! 19    movsi_ie/2    [length = 2]

This particular case is handled in noce_try_store_flag_mask, which does the
transformation if BRANCH_COST >= 2, which is true for -m4.  I guess before the
patch ifcvt didn't realize that this transformation can be applied.

I've tried setting BRANCH_COST to 1, which avoids this transformation but
increases overall code size a bit.