Re: A case where PHI-OPT pessimizes the code

[Steven Bosscher]

On Mon, Apr 23, 2012 at 2:27 PM, Richard Guenther
<richard.guent...@gmail.com> wrote:
> int foo (_Bool b)
> {
>  if (b)
>    return 1;
>  else
>    return 0;
> }

Indeed PHI-OPT performs the transformation on this code, too. But the
resulting code on powerpc64 is fine:

[stevenb@gcc1-power7 gcc]$ cat t.c.149t.optimized

;; Function foo (foo, funcdef_no=0, decl_uid=1996, cgraph_uid=0)

foo (_Bool b)
{
  int D.2006;

<bb 2>:
  D.2006_4 = (int) b_2(D);
  return D.2006_4;

}


[stevenb@gcc1-power7 gcc]$ cat t.s
        .file   "t.c"
        .section        ".toc","aw"
        .section        ".text"
        .align 2
        .p2align 4,,15
        .globl foo
        .section        ".opd","aw"
        .align 3
foo:
        .quad   .L.foo,.TOC.@tocbase,0
        .previous
        .type   foo, @function
.L.foo:
        blr
        .long 0
        .byte 0,0,0,0,0,0,0,0
        .size   foo,.-.L.foo
        .ident  "GCC: (GNU) 4.8.0 20120418 (experimental) [trunk
revision 186580]"
[stevenb@gcc1-power7 gcc]$


However, this C test case shows the problem:

[stevenb@gcc1-power7 gcc]$ head -n 24 t.c
#define ONEUL (1UL)

int
foo (long unsigned int a)
{
  _Bool b;
  long unsigned int cst, csui;

  if (a > 27) goto return_zero;
/*cst = 217579583UL;*/
  cst = (ONEUL <<  0) | (ONEUL <<  1) | (ONEUL <<  2) | (ONEUL <<  3)
| (ONEUL <<  4) |
        (ONEUL <<  5) | (ONEUL << 19) | (ONEUL << 20) | (ONEUL << 21)
| (ONEUL << 22) |
        (ONEUL << 23) | (ONEUL << 26) | (ONEUL << 27);
  csui = (ONEUL << a);
  b = ((csui & cst) != 0);
  if (b)
    return 1;
  else
    return 0;

return_zero:
   return 0;
}

[stevenb@gcc1-power7 gcc]$ ./cc1 -quiet -O2 -fdump-tree-all t.c
[stevenb@gcc1-power7 gcc]$ cat t.s
        .file   "t.c"
        .section        ".toc","aw"
        .section        ".text"
        .align 2
        .p2align 4,,15
        .globl foo
        .section        ".opd","aw"
        .align 3
foo:
        .quad   .L.foo,.TOC.@tocbase,0
        .previous
        .type   foo, @function
.L.foo:
        cmpldi 7,3,27
        bgt 7,.L3
        li 10,1
        lis 9,0xcf8
        sld 3,10,3
        ori 9,9,63
        and. 10,3,9
        mfcr 9
        rlwinm 9,9,3,1
        xori 3,9,1
        blr
        .p2align 4,,15
.L3:
.L2:
        li 3,0
        blr
        .long 0
        .byte 0,0,0,0,0,0,0,0
        .size   foo,.-.L.foo
        .ident  "GCC: (GNU) 4.8.0 20120418 (experimental) [trunk
revision 186580]"


I will file a PR for this later today, maybe after trying on a few
other targets to see if this is a middle-end problem or a target
issue.

Ciao!
Steven