On Thu, 7 May 2020, Jakub Jelinek wrote:
> Hi!
>
> The ffs expanders on several targets (x86, ia64, aarch64 at least)
> emit a conditional move or similar code to handle the case when the
> argument is 0, which makes the code longer.
> If we know from VRP that the argument will not be zero, we can (if the
> target has also an ctz expander) just use ctz which is undefined at zero
> and thus the expander doesn't need to deal with that.
>
> Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
can you use direct_internal_fn_supported_p (IFN_CTZ, type,
OPTIMIZE_FOR_SPEED)?
> 2020-05-06 Jakub Jelinek <ja...@redhat.com>
>
> PR tree-optimization/94956
> * match.pd (FFS): Optimize __builtin_ffs* of non-zero argument into
> __builtin_ctz* + 1 if ctz_optab has an expander.
>
> * gcc.target/i386/pr94956.c: New test.
>
> --- gcc/match.pd.jj 2020-05-06 15:03:51.618058839 +0200
> +++ gcc/match.pd 2020-05-06 15:48:23.658858289 +0200
> @@ -5990,6 +5990,16 @@ (define_operator_list COND_TERNARY
> (convert (IFN_POPCOUNT:type @0)))))
> #endif
>
> +/* __builtin_ffs needs to deal on many targets with the possible zero
> + argument. If we know the argument is always non-zero, __builtin_ctz + 1
> + should lead to better code. */
> +(simplify
> + (FFS tree_expr_nonzero_p@0)
> + (if (INTEGRAL_TYPE_P (TREE_TYPE (@0))
> + && (optab_handler (ctz_optab, TYPE_MODE (TREE_TYPE (@0)))
> + != CODE_FOR_nothing))
> + (plus (CTZ:type @0) { build_one_cst (type); })))
> +
> /* Simplify:
>
> a = a1 op a2
> --- gcc/testsuite/gcc.target/i386/pr94956.c.jj 2020-05-06
> 16:35:47.085876237 +0200
> +++ gcc/testsuite/gcc.target/i386/pr94956.c 2020-05-06 16:39:52.927140038
> +0200
> @@ -0,0 +1,28 @@
> +/* PR tree-optimization/94956 */
> +/* { dg-do compile } */
> +/* { dg-options "-O2" } */
> +/* { dg-final { scan-assembler-not "\tcmovne\t" } } */
> +/* { dg-final { scan-assembler-not "\tsete\t" } } */
> +
> +int
> +foo (unsigned x)
> +{
> + if (x == 0) __builtin_unreachable ();
> + return __builtin_ffs (x) - 1;
> +}
> +
> +int
> +bar (unsigned long x)
> +{
> + if (x == 0) __builtin_unreachable ();
> + return __builtin_ffsl (x) - 1;
> +}
> +
> +#ifdef __x86_64__
> +int
> +baz (unsigned long long x)
> +{
> + if (x == 0) __builtin_unreachable ();
> + return __builtin_ffsll (x) - 1;
> +}
> +#endif
>
>
> Jakub
>
>
--
Richard Biener <rguent...@suse.de>
SUSE Software Solutions Germany GmbH, Maxfeldstrasse 5, 90409 Nuernberg,
Germany; GF: Felix Imendörffer; HRB 36809 (AG Nuernberg)
