https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110780
Bug ID: 110780
Summary: aarch64 NEON redundant displaced ld3
Product: gcc
Version: 14.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: nate at thatsmathematics dot com
Target Milestone: ---
Compile the following with gcc 14.0.0 20230723 on aarch64 with -O3:
#include <stdint.h>
void CSI2toBE12(uint8_t* pCSI2, uint8_t* pBE, uint8_t* pCSI2LineEnd)
{
while (pCSI2 < pCSI2LineEnd) {
pBE[0] = pCSI2[0];
pBE[1] = ((pCSI2[2] & 0xf) << 4) | (pCSI2[1] >> 4);
pBE[2] = ((pCSI2[1] & 0xf) << 4) | (pCSI2[2] >> 4);
pCSI2 += 3;
pBE += 3;
}
}
Godbolt: https://godbolt.org/z/WshTPKzY5
In the inner loop (.L5 of the godbolt asm) we have
ld3 {v25.16b - v27.16b}, [x3]
add x6, x3, 1
// no intervening stores
ld3 {v25.16b - v27.16b}, [x6]
The second load is redundant. v25, v26 are the same as what was already in
v26, v27 respectively. The value loaded into v27 is new but it is not used in
the subsequent code.
This might also account for some extra later complexity, because it means that
the last 48 bytes of the input can't be handled by this loop (or else the
second load would be out of bounds by one byte) and so must be handled
specially.
