https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111502
Bug ID: 111502
Summary: Suboptimal unaligned 2/4-byte memcpy on strict-align
targets
Product: gcc
Version: 13.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: lasse.collin at tukaani dot org
Target Milestone: ---
I was playing with RISC-V GCC 12.2.0 from Arch Linux. I noticed
inefficient-looking assembly output in code that uses memcpy to access 32-bit
unaligned integers. I tried Godbolt with 16/32-bit integers and seems that the
same weirdness happens with RV32 & RV64 with GCC 13.2.0 and trunk, and also on
a few other targets. (Clang's output looks OK.)
For a little endian target:
#include <stdint.h>
#include <string.h>
uint32_t bytes16(const uint8_t *b)
{
return (uint32_t)b[0]
| ((uint32_t)b[1] << 8);
}
uint32_t copy16(const uint8_t *b)
{
uint16_t v;
memcpy(&v, b, sizeof(v));
return v;
}
riscv64-linux-gnu-gcc -march=rv64gc -O2 -mtune=size
bytes16:
lhu a0,0(a0)
ret
copy16:
lhu a0,0(a0)
ret
That looks good because -mno-strict-align is the default.
After omitting -mtune=size, unaligned access isn't used (the output is the same
as with -mstrict-align):
riscv64-linux-gnu-gcc -march=rv64gc -O2
bytes16:
lbu a5,1(a0)
lbu a0,0(a0)
slli a5,a5,8
or a0,a5,a0
ret
copy16:
lbu a4,0(a0)
lbu a5,1(a0)
addi sp,sp,-16
sb a4,14(sp)
sb a5,15(sp)
lhu a0,14(sp)
addi sp,sp,16
jr ra
bytes16 looks good but copy16 is weird: the bytes are copied to an aligned
location on stack and then loaded back.
On Godbolt it happens with GCC 13.2.0 on RV32, RV64, ARM64 (but only if using
-mstrict-align), MIPS64EL, and SPARC & SPARC64 (comparison needs big endian
bytes16). For ARM64 and MIPS64EL the oldest GCC on Godbolt is GCC 5.4 and the
same thing happens with that too.
32-bit reads with -O2 behave similarly. With -Os a call to memcpy is emitted
for copy32 but not for bytes32.
#include <stdint.h>
#include <string.h>
uint32_t bytes32(const uint8_t *b)
{
return (uint32_t)b[0]
| ((uint32_t)b[1] << 8)
| ((uint32_t)b[2] << 16)
| ((uint32_t)b[3] << 24);
}
uint32_t copy32(const uint8_t *b)
{
uint32_t v;
memcpy(&v, b, sizeof(v));
return v;
}
riscv64-linux-gnu-gcc -march=rv64gc -O2
bytes32:
lbu a4,1(a0)
lbu a3,0(a0)
lbu a5,2(a0)
lbu a0,3(a0)
slli a4,a4,8
or a4,a4,a3
slli a5,a5,16
or a5,a5,a4
slli a0,a0,24
or a0,a0,a5
sext.w a0,a0
ret
copy32:
lbu a2,0(a0)
lbu a3,1(a0)
lbu a4,2(a0)
lbu a5,3(a0)
addi sp,sp,-16
sb a2,12(sp)
sb a3,13(sp)
sb a4,14(sp)
sb a5,15(sp)
lw a0,12(sp)
addi sp,sp,16
jr ra
riscv64-linux-gnu-gcc -march=rv64gc -Os
bytes32:
lbu a4,1(a0)
lbu a5,0(a0)
slli a4,a4,8
or a4,a4,a5
lbu a5,2(a0)
lbu a0,3(a0)
slli a5,a5,16
or a5,a5,a4
slli a0,a0,24
or a0,a0,a5
sext.w a0,a0
ret
copy32:
addi sp,sp,-32
mv a1,a0
li a2,4
addi a0,sp,12
sd ra,24(sp)
call memcpy@plt
ld ra,24(sp)
lw a0,12(sp)
addi sp,sp,32
jr ra
I probably cannot test any proposed fixes but I hope this report is still
useful. Thanks!
