https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110613
Bug ID: 110613
Summary: optimization about combined store of adjacent
bitfields
Product: gcc
Version: 14.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: lh_mouse at 126 dot com
Target Milestone: ---
This is a piece of code taken from a WebSocket frame parser:
```
#include <stdint.h>
struct Header
{
// 1st byte
uint8_t opcode : 4;
uint8_t rsv3 : 1;
uint8_t rsv2 : 1;
uint8_t rsv1 : 1;
uint8_t fin : 1;
// 2nd byte
uint8_t reserved_1 : 7;
uint8_t mask : 1;
// 3rd and 4th bytes
uint8_t reserved_2;
uint8_t reserved_3;
// 5th to 7th bytes
union {
char mask_key[4];
uint32_t mask_key_u32;
};
// 8th to 15th bytes
uint64_t payload_len;
};
void
set_header(Header* ph, const uint8_t* bptr)
{
uint8_t f = bptr[0];
uint8_t t = bptr[1];
ph->fin = f >> 7;
ph->rsv1 = f >> 6;
ph->rsv2 = f >> 5;
ph->rsv3 = f >> 4;
ph->opcode = f;
ph->mask = t >> 7;
ph->reserved_1 = t;
}
```
The structure is designed to match x86_64 ABI (little endian), so
```
ph->fin = f >> 7;
ph->rsv1 = f >> 6;
ph->rsv2 = f >> 5;
ph->rsv3 = f >> 4;
ph->opcode = f;
```
should be a simple move (https://gcc.godbolt.org/z/9vTqs7axj), and
```
ph->mask = t >> 7;
ph->reserved_1 = t;
```
should also be a simple move (https://gcc.godbolt.org/z/KdchWvEn1), but!
When put these two pieces of code together, guess what?:
(godbolt: https://gcc.godbolt.org/z/hbEaeb3MT)
```
set_header(Header*, unsigned char const*):
movzx edx, BYTE PTR [rsi]
movzx ecx, BYTE PTR [rsi+1]
mov eax, edx
mov esi, edx
shr al, 4
and esi, 15
and eax, 1
sal eax, 4
or eax, esi
mov esi, edx
shr sil, 5
and esi, 1
sal esi, 5
or eax, esi
mov esi, edx
shr dl, 7
shr sil, 6
movzx edx, dl
and esi, 1
sal edx, 7
sal esi, 6
or eax, esi
or eax, edx
mov edx, ecx
shr cl, 7
and edx, 127
sal ecx, 15
sal edx, 8
or eax, edx
or eax, ecx
mov WORD PTR [rdi], ax
ret
```
