https://bugs.llvm.org/show_bug.cgi?id=52419
Bug ID: 52419
Summary: [AArch64] Incorrect optimization to sqrdmlah
Product: libraries
Version: trunk
Hardware: Other
OS: Linux
Status: NEW
Severity: normal
Priority: P
Component: Backend: AArch64
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected],
[email protected], [email protected],
[email protected]
Currently sqrdmulh followed by an sqadd can be optimized into sqrdmlah when the
rdm target feature is provided which is incorrect for a small number of values.
#include <arm_neon.h>
#include <stdio.h>
int main(int argc, char **argv) {
int32x2_t a = {static_cast<int32_t>(0x80000000),
static_cast<int32_t>(0x80000000)};
int32x2_t b = {static_cast<int32_t>(0x80000000),
static_cast<int32_t>(0x80000000)};
int32x2_t c = {static_cast<int32_t>(0x80000000),
static_cast<int32_t>(0x80000000)};
auto val = vqadd_s32(a, vqrdmulh_s32(b, c));
printf("%d,%d\n", vget_lane_s32(val, 0), vget_lane_s32(val, 1));
return 0;
}
When compiling that with clang++ -march=armv8a+rdm -O2 test.cpp it would
optimize it into sqrdmlah and output: 0,0.
However, when compiling with clang++ -O2 test.cpp to get the sqrdmulh sqadd it
would actually be -1,-1
This also appears to be how the vqrdmlah intrinsics are currently implemented.
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