[llvm-commits] [llvm] r137570 - in /llvm/trunk: lib/Transforms/InstCombine/InstructionCombining.cpp test/Transforms/InstCombine/nsw.ll

[Nick Lewycky]

Eli Friedman wrote:
> On Sat, Aug 13, 2011 at 6:45 PM, Nick Lewycky<nicholas at mxc.ca>  wrote:
>> Author: nicholas
>> Date: Sat Aug 13 20:45:19 2011
>> New Revision: 137570
>>
>> URL: http://llvm.org/viewvc/llvm-project?rev=137570&view=rev
>> Log:
>> Teach instcombine to preserve the nsw bit by doing an after-the-fact analysis
>> when combining add and sub instructions. Patch by Pranav Bhandarkar!
>
> This appears to perform an incorrect transform for the following testcase:
>
> define i32 @f(i32 %x) {
>    %y = add i32 %x, 1
>    %z = add nsw i32 %y, 2
>    ret i32 %z
> }

Thanks. Fixed in r137572.

On a related note, I'm concerned about the reassociate pass. Consider:

$ cat nsw.ll
define i8 @test1(i8 %x) nounwind {
entry:
   %add = sub nsw i8 100, %x
   %add3 = add nsw i8 %add, 80
   ret i8 %add3
}
$ opt -reassociate nsw.ll -S
; ModuleID = 'nsw.ll'

define i8 @test1(i8 %x) nounwind {
entry:
   %x.neg = sub i8 0, %x
   %add3 = add nsw i8 %x.neg, -76
   ret i8 %add3
}

Nick