Fix bug constants and sub instructions
When finding constants in a chain starting with the RHS operator of
sub instructions, we were negating the constant before zero extending
it, which is incorrect.
Unfortunately, I was unable to find a simple way to implement this
transformation correctly, so for now I just disabled this optimization
for constants that feed into the RHS of a sub.
Resolves#62379
Transformation from alive2.llvm.org:
define i16 @src(i8 %a, i8 %b, i8 %c) {
entry:
%0 = sub nuw nsw i8 %c, %a
%1 = sub nuw nsw i8 %b, %0
%2 = zext i8 %1 to i16
ret i16 %2
}
Before/Bad:
define i16 @tgt(i8 %a, i8 %b, i8 %c) {
entry:
%0 = zext i8 %a to i16
%1 = zext i8 %b to i16
%c_neg = sub i8 0, %c
%c_zext = zext i8 %c_neg to i16
%2 = sub i16 0, %0
%3 = sub i16 %1, %2
%4 = add i16 %3, %c_zext
ret i16 %4
}
Correct:
define i16 @tgt(i8 %a, i8 %b, i8 %c) {
entry:
%0 = zext i8 %a to i16
%1 = zext i8 %b to i16
%c_zext = zext i8 %c to i16
%c_neg = sub i16 0, %c_zext
%2 = sub i16 0, %0
%3 = sub i16 %1, %2
%4 = add i16 %3, %c_neg
ret i16 %4
}
Reviewed By: nikic
Differential Revision: https://reviews.llvm.org/D149507
2e3cabe172