[Bug tree-optimization/111764] [11/12/13/14 Regression] Wrong code at -O3 on x86_64-linux-gnu
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111764 --- Comment #7 from CVS Commits --- The master branch has been updated by Richard Biener : https://gcc.gnu.org/g:05f98310b54da95e468d799f4a910174320cccbb commit r14-4588-g05f98310b54da95e468d799f4a910174320cccbb Author: Richard Biener Date: Thu Oct 12 09:09:46 2023 +0200 tree-optimization/111764 - wrong reduction vectorization The following removes a misguided attempt to allow x + x in a reduction path, also allowing x * x which isn't valid. x + x actually never arrives this way but instead is canonicalized to 2 * x. This makes reduction path handling consistent with how we handle the single-stmt reduction case. PR tree-optimization/111764 * tree-vect-loop.cc (check_reduction_path): Remove the attempt to allow x + x via special-casing of assigns. * gcc.dg/vect/pr111764.c: New testcase.
[Bug tree-optimization/111764] [11/12/13/14 Regression] Wrong code at -O3 on x86_64-linux-gnu
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111764
--- Comment #6 from Richard Biener ---
Shorter testcase, fails at -O2 -ftree-vectorize
short b;
int main()
{
b = 2;
for (int a = 1; a <= 9; a++)
b = b * b;
if (b != 0)
__builtin_abort ();
}
it "works" with unsigned short b because the non-path reduction detection
doesn't allow two uses of 'b':
t.c:5:21: note: Analyze phi: b_lsm.6_2 = PHI
t.c:5:21: missed: reduction used in loop.
t.c:5:21: missed: Unknown def-use cycle pattern.
but the path based reduction detection has
/* We want to allow x + x but not x < 1 ? x : 2. */
if (is_gimple_assign (op_use_stmt)
&& gimple_assign_rhs_code (op_use_stmt) == COND_EXPR)
which also allows x * x to slip through. Even when fixing the earlier
issue of the initial value we end up with
vect_b.13_27 = { 2, 1, 1, 1, 1, 1, 1, 1 };
vect_powmult_5.14_28 = vect_b.13_27 * vect_b.13_27;
allowing x + x is only OK because it's 2 * x and we will actually never
see it as x + x.
[Bug tree-optimization/111764] [11/12/13/14 Regression] Wrong code at -O3 on x86_64-linux-gnu
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111764 --- Comment #5 from Richard Biener --- Hmm, that's not enough, the issue is this is detected as reduction at all.
[Bug tree-optimization/111764] [11/12/13/14 Regression] Wrong code at -O3 on x86_64-linux-gnu
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111764
--- Comment #4 from Richard Biener ---
/* Try to simplify the vector initialization by applying an
adjustment after the reduction has been performed. */
if (!reduc_info->reused_accumulator
&& STMT_VINFO_DEF_TYPE (stmt_info) == vect_reduction_def
&& !operand_equal_p (neutral_op, initial_def))
{
STMT_VINFO_REDUC_EPILOGUE_ADJUSTMENT (reduc_info)
= initial_def;
initial_def = neutral_op;
}
maybe just allow PLUS/MINUS_EXPR here. Maybe it's all premature ...
[Bug tree-optimization/111764] [11/12/13/14 Regression] Wrong code at -O3 on x86_64-linux-gnu
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111764 Richard Biener changed: What|Removed |Added Status|NEW |ASSIGNED Assignee|unassigned at gcc dot gnu.org |rguenth at gcc dot gnu.org --- Comment #3 from Richard Biener --- OK, so the vectorizer transforms b = 2; for (;;) b = b * b; to b = 1; for (;;) b = b * b; b *= 2; which is obviously wrong(TM).
[Bug tree-optimization/111764] [11/12/13/14 Regression] Wrong code at -O3 on x86_64-linux-gnu
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111764 --- Comment #2 from Andrew Pinski --- (In reply to Andrew Pinski from comment #1) > Note r11-5965-g04bff1bbfc1 just exposed the issue, you can reproduce the > issue before that with `-fno-vector-cost-model` added and it worked with > that option in GCC 9.5.0 but was broken in GCC 10.1.0. Sorry I mean `-fno-vect-cost-model`.
[Bug tree-optimization/111764] [11/12/13/14 Regression] Wrong code at -O3 on x86_64-linux-gnu
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111764 Andrew Pinski changed: What|Removed |Added Status|UNCONFIRMED |NEW Last reconfirmed||2023-10-10 Target Milestone|--- |11.5 Summary|Wrong code at -O3 on|[11/12/13/14 Regression] |x86_64-linux-gnu since |Wrong code at -O3 on |r11-5965-g04bff1bbfc1 |x86_64-linux-gnu Ever confirmed|0 |1 Known to work||10.5.0 Keywords||needs-bisection Known to fail||11.1.0, 12.1.0, 13.1.0 --- Comment #1 from Andrew Pinski --- Note r11-5965-g04bff1bbfc1 just exposed the issue, you can reproduce the issue before that with `-fno-vector-cost-model` added and it worked with that option in GCC 9.5.0 but was broken in GCC 10.1.0. Confirmed.
