https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102495
Bug ID: 102495
Summary: optimize some consecutive byte load pattern to word
load
Product: gcc
Version: 11.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: other
Assignee: unassigned at gcc dot gnu.org
Reporter: mytbk920423 at gmail dot com
Target Milestone: ---
I use the following code get a 32-bit word from a byte array by loading each
byte and shifting them, but GCC doesn't optimize the code to a single word load
when I put the byte load in a loop.
Clang trunk can optimize all of the follows:
https://gcc.godbolt.org/z/KfWE67K5c
```
#define SHL(a,b) ((uint32_t)(a) << (b))
// both GCC and Clang optimize to *(uint32_t*)(vv)
uint32_t getword_b(const uint8_t *vv)
{
return SHL(vv[3], 24) | SHL(vv[2], 16) | SHL(vv[1], 8) | SHL(vv[0], 0);
}
// GCC cannot optimize this, Clang can
uint32_t getword_forloop(const uint8_t *vv)
{
uint32_t res = 0;
for (size_t i = 0; i < 4; i++) {
res |= SHL(vv[i], (i * 8));
}
return res;
}
// both GCC and Clang optimize to ((uint32_t*)(vec))[word_idx]
uint32_t getword_from_vec(const uint8_t *vec, size_t word_idx)
{
size_t byte_idx = word_idx * 4;
const uint8_t *vv = vec + byte_idx;
return SHL(vv[3], 24) | SHL(vv[2], 16) | SHL(vv[1], 8) | SHL(vv[0], 0);
}
// neither GCC nor Clang 12.0.1 can optimize this, Clang trunk can
uint32_t getword_from_vec_forloop(const uint8_t *vec, size_t word_idx)
{
size_t byte_idx = word_idx * 4;
uint32_t res = 0;
for (size_t i = 0; i < 4; i++) {
res |= SHL(vec[byte_idx + i], (i * 8));
}
return res;
}
```
