instcombine doesn't fold x*y+x*z to x*(y+z)

[nlewycky]

Bugzilla Link	1976
Resolution	FIXED
Resolved on	Feb 03, 2008 01:43
Version	trunk
OS	All
Attachments	proposed patch (a little ugly)

Extended Description

Instcombine should be able to fold:

define i8 @​test1(i8 %x, i8 %y, i8 %z) {
%A = mul i8 %x, %y
%B = mul i8 %x, %z
%C = add i8 %A, %B
ret i8 %C
}

into this:

define i8 @​test2(i8 %x, i8 %y, i8 %z) {
%A = add i8 %y, %z
%B = mul i8 %x, %A
ret i8 %B
}

It currently implements this only when %y and %z are ConstantInt. That's wrong, this transformation works in general.

Patch attached. (Note that we shouldn't remove the existing "XC1 + XC2 --> X * (C1+C2)" trafo because it also does shift-left combining.)

[nlewycky]

assigned to @nlewycky

[lattner]

The patch looks good, but the variable names don't make any sense. I'd suggest keeping the names generic like A/B/C/D or W/X/Y/Z and then changing the comment to be something like this:

// WX + YZ --> W * (X+Z) iff W == Y

[lattner]

Oh, this should not trigger for FP btw.

[nlewycky]

Fixed. Patch here: http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20080128/057877.html