https://bugs.llvm.org/show_bug.cgi?id=43198
Bug ID: 43198
Summary: LLVM should better understand subtraction of `zext
i1`s
Product: new-bugs
Version: trunk
Hardware: PC
OS: Windows NT
Status: NEW
Severity: enhancement
Priority: P
Component: new bugs
Assignee: unassignedb...@nondot.org
Reporter: me22...@gmail.com
CC: htmldevelo...@gmail.com, llvm-bugs@lists.llvm.org
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)
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