On Thu, 7 Mar 2024, Jakub Jelinek wrote:
> Hi!
>
> As documented in the function comment, choose_mult_variant attempts to
> compute costs of 3 different cases, val, -val and val - 1.
> The -val case is actually only done if val fits into host int, so there
> should be no overflow, but the val - 1 case is done unconditionally.
> val is shwi (but inside of synth_mult already uhwi), so when val is
> HOST_WIDE_INT_MIN, val - 1 invokes UB. The following patch fixes that
> by using val - HOST_WIDE_INT_1U, but I'm not really convinced it would
> DTRT for > 64-bit modes, so I've guarded it as well. Though, arch
> would need to have really strange costs that something that could be
> expressed as x << 63 would be better expressed as (x * 0x7fffffffffffffff) + 1
> In the long term, I think we should just rewrite
> choose_mult_variant/synth_mult etc. to work on wide_int.
>
> Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
OK.
> 2024-03-07 Jakub Jelinek <ja...@redhat.com>
>
> PR middle-end/105533
> * expmed.cc (choose_mult_variant): Only try the val - 1 variant
> if val is not HOST_WIDE_INT_MIN or if mode has exactly
> HOST_BITS_PER_WIDE_INT precision. Avoid triggering UB while computing
> val - 1.
>
> * gcc.dg/pr105533.c: New test.
>
> --- gcc/expmed.cc.jj 2024-01-03 11:51:27.327789537 +0100
> +++ gcc/expmed.cc 2024-03-06 16:01:27.898693433 +0100
> @@ -3285,11 +3285,15 @@ choose_mult_variant (machine_mode mode,
> limit.latency = mult_cost - op_cost;
> }
>
> - synth_mult (&alg2, val - 1, &limit, mode);
> - alg2.cost.cost += op_cost;
> - alg2.cost.latency += op_cost;
> - if (CHEAPER_MULT_COST (&alg2.cost, &alg->cost))
> - *alg = alg2, *variant = add_variant;
> + if (val != HOST_WIDE_INT_MIN
> + || GET_MODE_UNIT_PRECISION (mode) == HOST_BITS_PER_WIDE_INT)
> + {
> + synth_mult (&alg2, val - HOST_WIDE_INT_1U, &limit, mode);
> + alg2.cost.cost += op_cost;
> + alg2.cost.latency += op_cost;
> + if (CHEAPER_MULT_COST (&alg2.cost, &alg->cost))
> + *alg = alg2, *variant = add_variant;
> + }
>
> return MULT_COST_LESS (&alg->cost, mult_cost);
> }
> --- gcc/testsuite/gcc.dg/pr105533.c.jj 2024-03-06 16:03:50.347757251
> +0100
> +++ gcc/testsuite/gcc.dg/pr105533.c 2024-03-06 16:03:26.226084751 +0100
> @@ -0,0 +1,9 @@
> +/* PR middle-end/105533 */
> +/* { dg-do compile } */
> +/* { dg-options "-O2" } */
> +
> +long long
> +foo (long long x, long long y)
> +{
> + return ((x < 0) & (y != 0)) * (-__LONG_LONG_MAX__ - 1);
> +}
>
> Jakub
>
>
--
Richard Biener <rguent...@suse.de>
SUSE Software Solutions Germany GmbH,
Frankenstrasse 146, 90461 Nuernberg, Germany;
GF: Ivo Totev, Andrew McDonald, Werner Knoblich; (HRB 36809, AG Nuernberg)
