[LLVMdev] Question about NoWrap flag for SCEVAddRecExpr

[Gerolf Hoflehner]

Hi Sanjoy,

Interesting. I took a closer look at SCEV when I saw Tong’s example. I’m curious about "[1]: It will try to prove nuw and nsw in cases where it is "easy", but not in this specific case.” Could you describe “easy” and “difficult” algorithmically?

Thanks
Gerolf

> 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>.
> 
> 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
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