LLVM should better understand subtraction of zext i1s #42543
Description
| Bugzilla Link | 43198 |
| Version | trunk |
| OS | Windows NT |
| Reporter | LLVM Bugzilla Contributor |
| CC | @RKSimon,@rotateright |
Extended Description
I found this trying to make a better implementation of Rust's Ord::cmp for integers.
C++ repro https://godbolt.org/z/tL0-oW
int spaceship(int a, int b) {
return (a > b) - (a < b);
}
bool my_lt(int a, int b) {
return spaceship(a, b) == -1;
}
my_lt there should be foldable down to just a < b, but that doesn't happen
define dso_local zeroext i1 @​_Z5my_ltii(i32 %0, i32 %1) local_unnamed_addr #​0 {
%3 = icmp sgt i32 %0, %1
%4 = zext i1 %3 to i32
%5 = icmp slt i32 %0, %1
%6 = zext i1 %5 to i32
%7 = sub nsw i32 %4, %6
%8 = icmp eq i32 %7, -1
ret i1 %8
}
(And running it through opt again doesn't help either, https://godbolt.org/z/-hl5H0)