instrcombine causes miscompilation with fcmp and itofp

[llvmbot]

Bugzilla Link	3021
Resolution	FIXED
Resolved on	Nov 08, 2008 22:27
Version	unspecified
OS	Linux
Reporter	LLVM Bugzilla Contributor
CC	@isanbard

Extended Description

Running opt -instcombine turns this:

define i1 @​bug(i32 %val) {
%1 = uitofp i32 %val to double
%2 = fcmp ole double %1, 0.000000e+00
ret i1 %2
}

into this:

define i1 @​bug(i32 %val) {
%1 = icmp slt i32 %val, 1 ; [#uses=1]
ret i1 %1
}

This transformation is invalid (consider the case %val = 0xffffffff).

[isanbard]

It would seem that if we're dealing with an FCMP against a UIToFP instruction, the resulting instcombine should be unsigned rather than signed. If I change the code to do this, it gives me this:

define i1 @​bug1(i32 %val) {
%1 = icmp eq i32 %val, 0 ; [#uses=1]
ret i1 %1
}

which seems more reasonable.

[isanbard]

Fixed here:
http://llvm.org/viewvc/llvm-project?rev=58929&view=rev