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Description
(This example comes from looking at Rust's discriminant code from https://rust.godbolt.org/z/1138aGqdb)
Take this input IR:
define noundef zeroext i1 @is_foo(i8 noundef range(i8 -1, 5) %x) unnamed_addr {
start:
%0 = sub i8 %x, 2
%1 = zext i8 %0 to i64
%2 = icmp ule i8 %0, 2
%3 = add i64 %1, 1
%_2 = select i1 %2, i64 %3, i64 0
%_0 = icmp eq i64 %_2, 0
ret i1 %_0
}Today, LLVM does simplify it a bunch, getting it down to https://llvm.godbolt.org/z/G7as7Y6o5
define noundef zeroext i1 @is_foo(i8 noundef range(i8 -1, 5) %x) unnamed_addr #0 {
%0 = add nsw i8 %x, -5
%1 = icmp ult i8 %0, -3
ret i1 %1
}It could do better, though. The range information was used to determine the nsw, but that doesn't really help the unsigned icmp.
Specifically, the range restriction is enough that it'd be allowed to be just https://alive2.llvm.org/ce/z/fkVEwL
define noundef zeroext i1 @is_foo(i8 noundef range(i8 -1, 5) %x) unnamed_addr #0 {
start:
%1 = icmp slt i8 %x, 2
ret i1 %1
}Eliminating the need for the add altogether.