https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109637
Bug ID: 109637
Summary: unnecessary range check in complete switch on bitfield
Product: gcc
Version: 13.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: middle-end
Assignee: unassigned at gcc dot gnu.org
Reporter: mattiase at acm dot org
Target Milestone: ---
This fully populated switch still produces some kind of useless range check:
struct S { unsigned x : 2; };
int f (struct S *s) {
switch(s->x) {
case 0: return 0;
case 1: return 1;
case 2: return 2;
case 3: return 3;
}
}
->
movzbl (%rdi), %eax
andl $3, %eax
leal -1(%rax), %edx
movzbl %al, %eax
cmpb $3, %dl
movl $0, %edx
cmovnb %edx, %eax
ret
GCC apparently understands that the switch is complete at some level since
anything after the switch is recognised as dead code, so the range check is a
bit puzzling.
The code is fine if we explicitly mask the switch value as in `switch(s->x &
3)`:
movzbl (%rdi), %eax
andl $3, %eax
ret
