https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107717
Bug ID: 107717
Summary: [13 Regression] ICEs expanding permutes after
g:dc95e1e9702f2f6367bbc108c8d01169be1b66d2
Product: gcc
Version: 13.0
Status: UNCONFIRMED
Keywords: ice-on-valid-code
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: tnfchris at gcc dot gnu.org
Target Milestone: ---
Target: aarch64*
After
commit dc95e1e9702f2f6367bbc108c8d01169be1b66d2 (origin/trunk, origin/master,
origin/HEAD)
Author: Hongyu Wang <hongyu.w...@intel.com>
Date: Mon Jan 17 13:01:51 2022 +0800
Optimize VEC_PERM_EXPR with same permutation index and operation
The sequence
c1 = VEC_PERM_EXPR (a, a, mask)
c2 = VEC_PERM_EXPR (b, b, mask)
c3 = c1 op c2
can be optimized to
c = a op b
c3 = VEC_PERM_EXPR (c, c, mask)
for all integer vector operation, and float operation with
full permutation.
gcc/ChangeLog:
PR target/98167
* match.pd: New perm + vector op patterns for int and fp vector.
gcc/testsuite/ChangeLog:
PR target/98167
* gcc.target/i386/pr98167.c: New test.
We see various ICEs, an example is
void foo(int n, char *restrict out, char *restrict in) {
for (int i=n; i-->0; ) {
out[i] += in[i];
}
}
compiled with
aarch64-none-linux-gnu -O3 -march=armv8-a+sve2
The problem is that the match.pd pattern as written causes the permute to
switch from a single register permute to a two register one.
The reason is that when the folded result is expanded in SSA form
vec_perm (op @0 @1) (op @0 @1)
the result of applying op twice results in two distinct SSA names. This fails
because expand_vec_perm_const now tries to use a two operand expansion because
there's no easy way to tell that these two operands are the same.
If it happens early enough we can CSE the operands, but when this happens after
vec_lower it generated something the target does not support.
I tried getting expand_vec_perm_const to recognize that they are the same, but
that's quite hard.
It's best to prevent the generation of the two SSA names to begin with, or add
an additional rule for match.pd that's able to CSE this.
I'm making this issue because I don't know which approach upstream would like
so it's easier to ask first.
