[LLVMdev] Question about NoWrap flag for SCEVAddRecExpr

[Sanjoy Das]

[+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