https://bugs.llvm.org/show_bug.cgi?id=48683
Bug ID: 48683
Summary: Missing known bits info for x*x
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]
bool foo(unsigned x) {
return (x * x) & 2;
}
Square is always congruent to 0 or 1 mod 4 so we can fold it to return false
(For knownbits, second LSB is not set).
define i1 @src(i32 %0) {
%1:
%2 = mul i32 %0, %0
%3 = and i32 %2, 2
%4 = icmp ne i32 %3, 0
ret i1 %4
}
=>
define i1 @tgt(i32 %0) {
%1:
ret i1 0
}
Transformation seems to be correct!
https://en.wikipedia.org/wiki/Quadratic_reciprocity
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