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?
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
