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| author | Jakub Jelinek <jakub@redhat.com> | 2024-03-07 10:02:00 +0100 |
|---|---|---|
| committer | Jakub Jelinek <jakub@redhat.com> | 2024-03-07 10:02:00 +0100 |
| commit | c655c8d8d845b36c59babb2413ce7aa3584dbeda (patch) | |
| tree | 0d1b1c6672f7b0ab79770ea1a5871eb16d76a0af | |
| parent | e1bd0f293d8407d4e8149fbafd470612323dc938 (diff) | |
| download | gcc-c655c8d8d845b36c59babb2413ce7aa3584dbeda.zip gcc-c655c8d8d845b36c59babb2413ce7aa3584dbeda.tar.gz gcc-c655c8d8d845b36c59babb2413ce7aa3584dbeda.tar.bz2 | |
expand: Fix UB in choose_mult_variant [PR105533]
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.
2024-03-07 Jakub Jelinek <jakub@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.
| -rw-r--r-- | gcc/expmed.cc | 14 | ||||
| -rw-r--r-- | gcc/testsuite/gcc.dg/pr105533.c | 9 |
2 files changed, 18 insertions, 5 deletions
diff --git a/gcc/expmed.cc b/gcc/expmed.cc index 5916d6e..4ec035e 100644 --- a/gcc/expmed.cc +++ b/gcc/expmed.cc @@ -3285,11 +3285,15 @@ choose_mult_variant (machine_mode mode, HOST_WIDE_INT val, 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); } diff --git a/gcc/testsuite/gcc.dg/pr105533.c b/gcc/testsuite/gcc.dg/pr105533.c new file mode 100644 index 0000000..912685e --- /dev/null +++ b/gcc/testsuite/gcc.dg/pr105533.c @@ -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); +} |