https://bugs.llvm.org/show_bug.cgi?id=42206
Bug ID: 42206
Summary: Missed optimization for (unsigned) log2((unsigned) 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]
It seems we could do following transformations:
(T) log2((T) x) -> x == 0 ? 0 : sizeof(T) * CHAR_BIT - 1 -__builtin_clz(x);
(T) floor(log2((T) x)) -> x == 0 ? 0 : sizeof(T) * CHAR_BIT - 1
-__builtin_clz(x);
where T is unsigned integer type.
Test program:
#include <stdlib.h>
#include <math.h>
#include <limits.h>
#include <stdio.h>
#define T unsigned
#define TT unsigned
TT f(T x) {
return log2(x);
}
TT f2(T x) {
return x == 0 ? 0 : sizeof(T) * CHAR_BIT - 1 - __builtin_clz(x);
}
int main(int argc, char **argv) {
for (T i = 0; i < INT_MAX; ++i) {
if (f(i) != f2(i)) {
abort();
}
}
return 0;
}
I see no crash so I think this is valid transformation. Am I wrong?
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