Re: [PATCH v1] [RFC] Improve folding for comparisons with zero in tree-ssa-forwprop.

[Jeff Law <jeffreyalaw@gmail.com>]

On 3/16/23 09:27, Manolis Tsamis wrote:
> For this C testcase:
> 
> void g();
> void f(unsigned int *a)
> {
>    if (++*a == 1)
>      g();
> }
> 
> GCC will currently emit a comparison with 1 by using the value
> of *a after the increment. This can be improved by comparing
> against 0 and using the value before the increment. As a result
> there is a potentially shorter dependancy chain (no need to wait
> for the result of +1) and on targets with compare zero instructions
> the generated code is one instruction shorter.
> 
> Example from Aarch64:
> 
> Before
>          ldr     w1, [x0]
>          add     w1, w1, 1
>          str     w1, [x0]
>          cmp     w1, 1
>          beq     .L4
>          ret
> 
> After
>          ldr     w1, [x0]
>          add     w2, w1, 1
>          str     w2, [x0]
>          cbz     w1, .L4
>          ret
> 
> gcc/ChangeLog:
> 
>          * tree-ssa-forwprop.cc (combine_cond_expr_cond):
>          (forward_propagate_into_comparison_1): Optimize
>          for zero comparisons.
Deferring to gcc-14.  Though I'm generally supportive of normalizing to 
a comparison against zero when we safely can :-)

jeff