https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120842
Bug ID: 120842
Summary: optimization for check many equal by bits or
Product: gcc
Version: 16.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: rockeet at gmail dot com
Target Milestone: ---
bool many_eq_max_13(unsigned int val) {
[[assume(val < 32)]];
return val == 1 || val == 4 || val == 7 || val == 9 ||
val == 10 || val == 13;
}
bool many_eq_max_31(unsigned int val) {
[[assume(val < 32)]];
return val == 1 || val == 4 || val == 7 || val == 9 ||
val == 10 || val == 13 || val == 31;
}
bool many_eq_max_63(unsigned int val) {
[[assume(val < 64)]];
return val == 1 || val == 4 || val == 7 || val == 9 ||
val == 10 || val == 13 || val == 31 || val == 63;
}
gcc just produce optimal code for many_eq_max_31:
(https://godbolt.org/z/jrf8rexj7)
"many_eq_max_13(unsigned int)":
mov eax, 658
mov edx, 9216
bt rax, rdi
setc al
bt rdx, rdi
setc dl
test al, al
cmove eax, edx
ret
"many_eq_max_31(unsigned int)":
mov eax, 2147493522
bt rax, rdi
setc al
ret
"many_eq_max_63(unsigned int)":
mov eax, 9874
bt rax, rdi
setc al
test al, al
je .L7
ret
.L7:
and edi, -33
cmp edi, 31
sete al
ret
both gcc and clang is not perfect for this case, clang is a bit better.
