https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96625
Bug ID: 96625
Summary: Unnecessarily large assembly generated when a
bit-offsetted higher-end end of a uint64_t-backed
bitfield is shifted toward the high end (left) by its
bit-offset
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: pskocik at gmail dot com
Target Milestone: ---
(Bitfields backed by 32-bit unsigneds are handled well.)
My example (https://gcc.godbolt.org/z/Yac38T):
#include <stdint.h>
#define FRONTSZ 3
#define UTYPE uint64_t
struct s{ union {
UTYPE whole;
struct {
UTYPE front:FRONTSZ,
tail:8*sizeof(UTYPE)-FRONTSZ; };
};};
UTYPE hiShifted_tail(struct s X) { return X.tail<<FRONTSZ; }
//better codegen:
UTYPE hiShifted_tail2(struct s X) { return X.whole>>FRONTSZ<<FRONTSZ; }
UTYPE hiShifted_tail3(struct s X) { return X.whole &
(0xffffffffffffffff<<FRONTSZ); }
x86-64 assembly generated for hiShifted_tail{,2,3}:
0000000000000000 <hiShifted_tail> (14 bytes):
0: 48 b8 f8 ff ff ff ff ff ff 1f movabs rax,0x1ffffffffffffff8
a: 48 21 f8 and rax,rdi
d: c3 ret
0000000000000000 <hiShifted_tail{2,3}> (8 bytes):
0: 48 89 f8 mov rax,rdi
3: 48 83 e0 f8 and rax,0xfffffffffffffff8
7: c3 ret
The codegen follows the same pattern for other front-sizes.
hiShifted_tail() on clang (regardless of whether uint64_t or uint32_t is used
as the backing type) and on gcc with uint32_t rather than uin64_t used as the
bitfield-backing-type follows the smaller codegen patter of
hiShifted_tail{2,3}.
