[LLVMdev] Question about NoWrap flag for SCEVAddRecExpr

[Adam Nemet]

> On Jun 10, 2015, at 6:17 PM, Sanjoy Das <sanjoy at playingwithpointers.com> wrote:
> 
> I'm not sure if inbounds can be used to prove <nuw>.  If an object
> %OBJ is allocated at address -1 then "gep inbounds %OBJ 1" is not
> poison, but the underlying computation unsigned-overflows.

I think that this should yield poison per langref because the signed arithmetic implied by the indices should be performed with infinite precision. Base is treated as unsigned so 0xff…ff + 1 would be 0x100…00 which would be out of bounds (sort of).  See also this which I think is even more explicit: http://llvm.org/docs/GetElementPtr.html#what-happens-if-a-gep-computation-overflows <http://llvm.org/docs/GetElementPtr.html#what-happens-if-a-gep-computation-overflows>

Adam

> It may be possible to use inbounds to derive <nsw> but that too is
> tricky since inbounds does not directly imply anything about no-wrap
> as far as I can tell.
> 
> On Wed, Jun 10, 2015 at 5:29 PM, Adam Nemet <anemet at apple.com> wrote:
>> [+Arnold]
>> 
>>> On Jun 10, 2015, at 1:29 PM, Sanjoy Das <sanjoy at playingwithpointers.com> wrote:
>>> 
>>> [+CC Andy]
>>> 
>>>> Can anyone familiar with ScalarRevolution tell me whether this is an
>>>> expected behavior or a bug?
>>> 
>>> Assuming you're talking about 2*k, this is a bug.  ScalarEvolution
>>> should be able to prove that {0,+,4} is <nsw> and <nuw>.
>> 
>> I also find it surprising that the inbounds gep does not allow us to prove nuw of the pointer here.  LAA has logic for this.
>> 
>> Not to mention that we’re trying to figure out the distance of x[2*k] against *itself* which should be zero regardless of wrapping.
>> 
>> A probably more interesting case is x[2*k] = x[2*k+2] + … which would require non-wrapping.
>> 
>> Looks like we only consider an inbounds gep non-wrapping if it uses a unit stride.  I am not sure why…
>> 
>> Adam
>> 
>>> 
>>> Part of the problem is that in this case ScalarEvolution does not try
>>> to prove that {0,+,4} is <nsw> when the expression is constructed
>>> (since proving that has non-trivial cost) [1].  To get ScalarEvolution
>>> to try to prove that {0,+,4} has no-wrap properties, the client needs
>>> to construct a sign-extend expression of {0,+,4}.  SCEV will try to
>>> change a sext of an add-rec to an add-rec of sexts and try to prove
>>> no-wrap in the process [2].
>>> 
>>> To easily do this from IR, you can just add a dummy sext instruction
>>> (like in the IR fragment below) and run
>>> 'opt -analyze -scalar-evolution -scalar-evolution' (just running SCEV
>>> won't dce the unused instruction):
>>> 
>>> ; ModuleID = '<stdin>'
>>> target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
>>> target triple = "x86_64-apple-macosx10.10.0"
>>> 
>>> @x = common global [1024 x float] zeroinitializer, align 16
>>> @y = common global [1024 x float] zeroinitializer, align 16
>>> 
>>> ; Function Attrs: nounwind ssp uwtable
>>> define void @myloop1() {
>>> bb:
>>>   br label %bb2
>>> 
>>> bb1:                                              ; preds = %bb2
>>>   ret void
>>> 
>>> bb2:                                              ; preds = %bb2, %bb
>>>   %k.01 = phi i64 [ 0, %bb ], [ %tmp15, %bb2 ]
>>>   %tmp = shl nsw i64 %k.01, 1
>>>   %dummy_sext = sext i64 %tmp to i128
>>>   %tmp3 = getelementptr inbounds [1024 x float], [1024 x float]* @x,
>>> i64 0, i64 %tmp
>>>   %tmp4 = load float, float* %tmp3, align 16, !tbaa !2
>>>   %tmp5 = getelementptr inbounds [1024 x float], [1024 x float]* @y,
>>> i64 0, i64 %k.01
>>>   %tmp6 = load float, float* %tmp5, align 8, !tbaa !2
>>>   %tmp7 = fadd float %tmp4, %tmp6
>>>   store float %tmp7, float* %tmp3, align 16, !tbaa !2
>>>   %tmp8 = or i64 %k.01, 1
>>>   %tmp9 = shl nsw i64 %tmp8, 1
>>>   %tmp10 = getelementptr inbounds [1024 x float], [1024 x float]*
>>> @x, i64 0, i64 %tmp9
>>>   %tmp11 = load float, float* %tmp10, align 8, !tbaa !2
>>>   %tmp12 = getelementptr inbounds [1024 x float], [1024 x float]*
>>> @y, i64 0, i64 %tmp8
>>>   %tmp13 = load float, float* %tmp12, align 4, !tbaa !2
>>>   %tmp14 = fadd float %tmp11, %tmp13
>>>   store float %tmp14, float* %tmp10, align 8, !tbaa !2
>>>   %tmp15 = add nsw i64 %k.01, 2
>>>   %exitcond.1 = icmp eq i64 %tmp15, 512
>>>   br i1 %exitcond.1, label %bb1, label %bb2
>>> }
>>> 
>>> !0 = !{i32 1, !"PIC Level", i32 2}
>>> !1 = !{!"clang version 3.7.0 (clang-stage2-configure-Rlto_build 239114)"}
>>> !2 = !{!3, !3, i64 0}
>>> !3 = !{!"float", !4, i64 0}
>>> !4 = !{!"omnipotent char", !5, i64 0}
>>> !5 = !{!"Simple C/C++ TBAA"}
>>> 
>>> 
>>> However, adding a dummy sext only proves <nuw> for {0,+,4} and not
>>> <nsw>.  The problem is that when constructing sext({0,+,4}) SCEV
>>> realizes that since {0,+,4} is always positive, sext({0,+,4}) ==
>>> zext({0,+,4}); and to change a zext of an add-rec to an add-rec of
>>> zexts, SCEV only needs to prove <nuw> and not <nsw>.
>>> 
>>> 
>>> I wonder if it makes sense to add a hook to SCEV that gets it to try
>>> as hard as it can to prove <nuw> / <nsw> for specific add recurrences.
>>> 
>>> 
>>> 
>>> [1]: It will try to prove nuw and nsw in cases where it is "easy", but
>>>    not in this specific case.
>>> 
>>> [2]: So a worthwhile project is to have the vectorizer construct sign
>>>    extend expressions of add recurrences that it really cares about
>>>    proving no-wrap of and then check the flags on the
>>>    SCEVAddRecExpr.  It may consume too much compile time, so there's
>>>    a tricky trade-off here.
>>> 
>>> 
>>> -- Sanjoy
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>> 

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20150610/fa42e095/attachment.html>