https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114661
Bug ID: 114661
Summary: Bit operations not optimized to multiplication
Product: gcc
Version: 14.0
Status: UNCONFIRMED
Keywords: missed-optimization
Severity: normal
Priority: P3
Component: middle-end
Assignee: unassigned at gcc dot gnu.org
Reporter: antoshkka at gmail dot com
Target Milestone: ---
Consider the example:
unsigned mul(unsigned char c) {
if (c > 3) __builtin_unreachable();
return c << 18 | c << 15 |
c << 12 | c << 9 |
c << 6 | c << 3 | c;
}
GCC with -O2 generates the following assembly:
mul(unsigned char):
movzx edi, dil
lea edx, [rdi+rdi*8]
lea eax, [0+rdx*8]
mov ecx, edx
sal edx, 15
or eax, edi
sal ecx, 9
or eax, ecx
or eax, edx
ret
However it could be optimized to just:
mul(unsigned char):
imul eax, edi, 299593
ret
Compiling with -Os does not help.
Godbolt playground: https://godbolt.org/z/YszzMbovK
P.S.: without `c << 18 | c << 15 |` the bit operations are transformed to
multiplication.
