[PATCH] SimplifyIndVar: Forget loops that are made infinite

[Arnold Schwaighofer]

After talking to Andy, I don’t think this is the right approach.

We cannot assume if there is back-edge taken count there will be a certain loop structure.


On May 8, 2013, at 4:31 PM, Arnold Schwaighofer <aschwaighofer at apple.com> wrote:

> We can transform loops into infinite ones. When this happens we should "forget"
> the SCEV for such loops.
> 
> Later transformations might depend on the SCEVs to be correct. For example, the
> backedge taken count. The loop vectorizer - rightly in my opinion - assumes that
> if there is a backedge take count - there should be an induction variable.
> 
> Fixes PR15926.
> 
> 
> long long a;
> volatile int x;
> void fn1 () {
>  for (a = 0; a >= 0; ++a)
>    x = 0;
> }
> 
> We (indvars) used to transform this from:
> 
> 
> define void @fn1() {
> entry:
>  store i64 0, i64* @a, align 8
>  br label %for.body
> 
> for.body:
>  %inc1 = phi i64 [ 0, %entry ], [ %inc, %for.body ]
>  store volatile i32 0, i32* @x, align 4
>  %inc = add nsw i64 %inc1, 1
>  %cmp = icmp sgt i64 %inc1, -2
>  br i1 %cmp, label %for.body, label %for.end
> 
> for.end:
>  %inc.lcssa = phi i64 [ %inc, %for.body ]
>  store i64 %inc.lcssa, i64* @a, align 8
>  ret void
> }
> 
> to:
> 
> define void @fn1() {
> entry:
>  store i64 0, i64* @a, align 8, !tbaa !0
>  br label %for.body
> 
> for.body:                                         ; preds = %entry, %for.body
>  store volatile i32 0, i32* @x, align 4, !tbaa !3
>  br i1 true, label %for.body, label %for.end
> 
> for.end:                                          ; preds = %for.body
>  store i64 -9223372036854775807, i64* @a, align 8, !tbaa !0
>  ret void
> }
> 
> 
> But kept the original SCEVs alive.
> 
> 
> 
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