https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101200
Bug ID: 101200
Summary: Unneeded AND after shift
Product: gcc
Version: 11.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: steinar+gcc at gunderson dot no
Target Milestone: ---
The code after reduction is:
struct {
int b[6];
} c;
unsigned char d;
void e() {
unsigned char a = d >> 4, f = d & 15;
c.b[a] = c.b[f];
}
with g++-11 -O2, this produces
movzbl d(%rip), %eax
movq %rax, %rdx
shrq $4, %rax
andl $15, %edx
andl $15, %eax
movl c(,%rdx,4), %edx
movl %edx, c(,%rax,4)
ret
The second AND with 15 is unneeded and should have been optimized away by VRP
as I understand it. I can't reproduce it with ARM, though, so maybe there's
something x86-specific?
Compiler is
gcc version 11.1.0 (Debian 11.1.0-3)
The same code is generated back to at least 4.9. Also present in
gcc version 12.0.0 20210527 (experimental) [master revision
262e75d22c3:7bb6b9b2f47:9d3a953ec4d2695e9a6bfa5f22655e2aea47a973] (Debian
20210527-1)
