https://bugs.llvm.org/show_bug.cgi?id=42399
Bug ID: 42399
Summary: InstCombine: shift amount reassociation when doing bit
test
Product: libraries
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: enhancement
Priority: P
Component: Scalar Optimizations
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected]
https://godbolt.org/z/Kq82aC
https://rise4fun.com/Alive/kBb
bool bad(unsigned x, unsigned y, unsigned skip0, unsigned skip1) {
x >>= skip0;
return x & (y << skip1);
}
bool good(unsigned x, unsigned y, unsigned skip0, unsigned skip1) {
return x & (y << (skip1+skip0));
}
Name:
Pre: C1+C2 u< 32
%o4 = lshr i32 %x, C1
%o5 = shl i32 %y, C2
%o6 = and i32 %o5, %o4
%r = icmp ne i32 %o6, 0
=>
%n4 = add i32 C1, C2
%n5 = shl i32 %y, %n4
%n6 = and i32 %n5, %x
%r = icmp ne i32 %n6, 0
Still from real code..
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