https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126369
Bug ID: 126369
Summary: VLA versioning can generate unreachable code
Product: gcc
Version: 16.0
Status: UNCONFIRMED
Keywords: missed-optimization
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: tnfchris at gcc dot gnu.org
Blocks: 53947, 115130
Target Milestone: ---
Target: aarch64*
The following loop
char b[100];
int c(int a) {
unsigned d = 0;
for (; d < a; ++d)
{
if (b[0] + b[d + 1])
return 0;
}
return 1;
}
when compiled with -march=armv9-a -mautovec-preference=sve-only -Ofast
generates a runtime alignment check.
https://godbolt.org/z/bs18h4zWb
adrp x4, .LANCHOR0
add x5, x4, :lo12:.LANCHOR0
ldrb w6, [x4, #:lo12:.LANCHOR0]
cmp w0, 2
bls .L2
add x1, x5, 1
cntw x2
orr x1, x1, x2
sub x3, x2, #1
cmp x2, 4096
and x1, x1, x3
ccmp x1, 0, 0, ls
bne .L2
sub x5, x5, x2
add w3, w0, 1
mov z30.h, w6
add x5, x5, 1
mov w1, 0
ptrue p6.b, all
however this is moot since the accesses of b inside the loop will always be
misaligned and so versioning for alignment will never succeed.
The dr for this access is
Statement _4 = b[_3];
is executed at most 98 (bounded by 98) + 1 times in loop 1.
Loops range found for d_8: [irange] unsigned int [0, 99] and calculated range
:[irange] unsigned int [0, 100]
Registering value_relation (_3 > d_8) (bb3) at _3 = d_8 + 1;
success.
base_address: &b
offset from base address: 0
constant offset from base address: 1
step: 1
base alignment: 8
base misalignment: 0
offset alignment: 128
step alignment: 1
base_object: b
Access function 0: {1, +, 1}<nw>_1
so it knows that access is misaligned by a byte.
However vect_compute_data_ref_alignment is very conservative for VLA. It sets
misalignment to unknown and then bails out
unsigned HOST_WIDE_INT vect_align_c;
if (!vector_alignment.is_constant (&vect_align_c))
return;
and so we lose the information that the load is misaligned and can never reach
alignment.
vect_compute_data_ref_alignment should be enhanced to support this.
Referenced Bugs:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53947
[Bug 53947] [meta-bug] vectorizer missed-optimizations
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115130
[Bug 115130] [meta-bug] early break vectorization
