instcombine doesn't merge mixed sign-unsign comparisons

[nlewycky]

Bugzilla Link	1941
Resolution	LATER
Resolved on	Oct 15, 2008 11:33
Version	trunk
OS	All

Extended Description

Instcombine will merge comparisons like (x >= 10) && (x < 20) by producing (x - 10) u< 10, but only when the comparisons have matching sign.

This program:

define i1 @​test(i8 %x) {
%A = icmp uge i8 %x, 5
%B = icmp slt i8 %x, 20
%C = and i1 %A, %B
ret i1 %C
}

could be converted using the same technique. There's a lot of corner cases here that would need to be thought through thoroughly.

(This was found by inspection, I doubt if any llvm-gcc generated input would trigger this.)

[lattner]

Yep, this could be done.

[lattner]

Moved this to target/readme.txt