[llvm] r209545 - Fix and improve SCEV ComputeBackedgeTankCount.

[Eric Christopher]

Hey nice! Couple of trivial things I noticed while reading through.

> +; Here we have a must-exit loop latch that is not computabe and a

"Computable"

> +; may-exit early exit that can only have one non-exiting iteration
> +; before the check is forever skipped.
> +;
> +; CHECK-LABEL: @cannot_compute_mustexit
> +; CHECK: Loop %for.body.i: <multiple exits> Unpredictable backedge-taken
count.
> +; CHECK: Loop %for.body.i: Unpredictable max backedge-taken count.
> + at b = common global i32 0, align 4
> +
> +define i32 @cannot_compute_mustexit() {
> +entry:
> +  store i32 -1, i32* @a, align 4
> +  br label %for.body.i
> +
> +for.body.i:                                       ; preds = %for.cond.i,
%entry
> +  %storemerge1.i = phi i32 [ -1, %entry ], [ %add.i.i, %for.cond.i ]
> +  %tobool.i = icmp eq i32 %storemerge1.i, 0
> +  %add.i.i = add nsw i32 %storemerge1.i, 2
> +  br i1 %tobool.i, label %bar.exit, label %for.cond.i
> +
> +for.cond.i:                                       ; preds = %for.body.i
> +  store i32 %add.i.i, i32* @a, align 4
> +  %ld = load volatile i32* @b
> +  %cmp.i = icmp ne i32 %ld, 0
> +  br i1 %cmp.i, label %for.body.i, label %bar.exit
> +
> +bar.exit:                                         ; preds = %for.cond.i,
%for.body.i
> +  ret i32 0
> +}
> +
> +; This loop has two must-exits, both of with dominate the latch. The

"Both of which"

:)

-eric

> +; MaxBECount should be the minimum of them.
> +;
> +; CHECK-LABEL: @two_mustexit
> +; CHECK: Loop %for.body.i: <multiple exits> Unpredictable backedge-taken
count.
> +; CHECK: Loop %for.body.i: max backedge-taken count is 1
> +define i32 @two_mustexit() {
> +entry:
> +  store i32 -1, i32* @a, align 4
> +  br label %for.body.i
> +
> +for.body.i:                                       ; preds = %for.cond.i,
%entry
> +  %storemerge1.i = phi i32 [ -1, %entry ], [ %add.i.i, %for.cond.i ]
> +  %tobool.i = icmp sgt i32 %storemerge1.i, 0
> +  %add.i.i = add nsw i32 %storemerge1.i, 2
> +  br i1 %tobool.i, label %bar.exit, label %for.cond.i
> +
> +for.cond.i:                                       ; preds = %for.body.i
> +  store i32 %add.i.i, i32* @a, align 4
> +  %cmp.i = icmp slt i32 %storemerge1.i, 3
> +  br i1 %cmp.i, label %for.body.i, label %bar.exit
> +
> +bar.exit:                                         ; preds = %for.cond.i,
%for.body.i
> +  ret i32 0
> +}
>
>
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