https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95962
Bug ID: 95962
Summary: Inefficient code for simple arm_neon.h iota operation
Product: gcc
Version: 11.0
Status: UNCONFIRMED
Keywords: missed-optimization
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: rsandifo at gcc dot gnu.org
Blocks: 95958
Target Milestone: ---
Target: aarch64*-*-*
For:
#include <arm_neon.h>
int32x4_t
foo (void)
{
int32_t array[] = { 0, 1, 2, 3 };
return vld1q_s32 (array);
}
we produce:
foo:
.LFB4217:
.cfi_startproc
sub sp, sp, #16
.cfi_def_cfa_offset 16
mov x0, 2
mov x1, 4294967296
movk x0, 0x3, lsl 32
stp x1, x0, [sp]
ldr q0, [sp]
add sp, sp, 16
.cfi_def_cfa_offset 0
ret
In contrast, clang produces essentially perfect code:
adrp x8, .LCPI0_0
ldr q0, [x8, :lo12:.LCPI0_0]
ret
I think the problem is a combination of two things:
- __builtin_aarch64_ld1v4si & co. are treated as general
functions rather than pure functions, so in principle
it could write to the given address. This stops us
promoting the array to a constant.
- The loads could be reduced to native gimple-level
operations, at least on little-endian targets.
IMO this a bug rather than an enhancement. Intrinsics only
exist to optimise code, and what GCC is doing falls short
of what users should reasonably expect.
Referenced Bugs:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95958
[Bug 95958] [meta-bug] Inefficient arm_neon.h code for AArch64
