https://bugs.llvm.org/show_bug.cgi?id=42456
Bug ID: 42456
Summary: [InstCombine] Dropping pointless masking before shift
Product: libraries
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: enhancement
Priority: P
Component: Scalar Optimizations
Assignee: unassignedb...@nondot.org
Reporter: lebedev...@gmail.com
CC: llvm-bugs@lists.llvm.org
https://godbolt.org/z/cuuf0P
https://rise4fun.com/Alive/PcZk
int bad(unsigned input, unsigned len) {
unsigned shamt = (32 - len);
unsigned diff = input >> shamt;
return (int(diff) << shamt) >> shamt;
}
int good(unsigned input, unsigned len) {
return int(input) >> (32 - len);
}
Name:
%sub = sub i32 32, %len
%zz = shl i32 -1, %sub
%shl = and i32 %zz, %input
%r = ashr i32 %shl, %sub
=>
%r = ashr i32 %input, %sub
--
You are receiving this mail because:
You are on the CC list for the bug._______________________________________________
llvm-bugs mailing list
llvm-bugs@lists.llvm.org
https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-bugs
