Re: A question about redudant load elimination

[Ye Joey <joey dot ye dot cc at gmail dot com>]

From tree dump we can see that there are two assignments from x, one
to unsigned and one to signed. I guess that's the reason. Apparently
there is room to improve though.

  int prephitmp.8;
  int * D.2027;
  unsigned int D.2026;
  unsigned int x.1;
  int x.0;

  # BLOCK 2 freq:10000
  # PRED: ENTRY [100.0%]  (fallthru,exec)
  x.0_1 = x;
  x.1_2 = (unsigned int) x.0_1;  // unsigned move
  D.2026_3 = x.1_2 * 4;
  D.2027_5 = a_4(D) + D.2026_3;
  *D.2027_5 = 1;
  prephitmp.8_6 = x;         // signed move

On Mon, Nov 14, 2011 at 4:01 PM, Jiangning Liu <jiangning.liu@arm.com> wrote:
> Hi,
>
> For this test case,
>
> int x;
> extern void f(void);
>
> void g(int *a)
> {
> ? ? ? ?a[x] = 1;
> ? ? ? ?if (x == 100)
> ? ? ? ? ? ? ? ?f();
> ? ? ? ?a[x] = 2;
> }
>
> For trunk, the x86 assembly code is like below,
>
> ? ? ? ?movl ? ?x, %eax
> ? ? ? ?movl ? ?16(%esp), %ebx
> ? ? ? ?movl ? ?$1, (%ebx,%eax,4)
> ? ? ? ?movl ? ?x, %eax ? // Is this a redundant one?
> ? ? ? ?cmpl ? ?$100, %eax
> ? ? ? ?je ? ? ?.L4
> ? ? ? ?movl ? ?$2, (%ebx,%eax,4)
> ? ? ? ?addl ? ?$8, %esp
> ? ? ? ?.cfi_remember_state
> ? ? ? ?.cfi_def_cfa_offset 8
> ? ? ? ?popl ? ?%ebx
> ? ? ? ?.cfi_restore 3
> ? ? ? ?.cfi_def_cfa_offset 4
> ? ? ? ?ret
> ? ? ? ?.p2align 4,,7
> ? ? ? ?.p2align 3
> .L4:
> ? ? ? ?.cfi_restore_state
> ? ? ? ?call ? ?f
> ? ? ? ?movl ? ?x, %eax
> ? ? ? ?movl ? ?$2, (%ebx,%eax,4)
> ? ? ? ?addl ? ?$8, %esp
> ? ? ? ?.cfi_def_cfa_offset 8
> ? ? ? ?popl ? ?%ebx
> ? ? ? ?.cfi_restore 3
> ? ? ? ?.cfi_def_cfa_offset 4
> ? ? ? ?Ret
>
> Is the 2nd "movl x, %eax" is a redundant one for single thread programming
> model? If yes, can this be optimized away?
>
> Thanks,
> -Jiangning
>
>
>
>