[PATCH] SCEV should use NSW to get trip count for loop with nonunit stride.

[Hal Finkel]

----- Original Message -----
> I'm not able to defend SCEV's current inability to handle loops with
> non-unit stride. In other cases, SCEV is rightly conservative in
> ignoring NSW, and I've been defending that approach, but this
> particular case keeps coming up and I don't have a good answer.
> 
> For example:
> void foo(int n, int *x) {
>  for (int i = 0; i < n; i += 3) {
>    x[i] = i;
>    x[i+1] = i+1;
>    x[i+2] = i+2;
>  }
> }
> 
> In ScalarEvolution::HowManyLessThans we ignore the NSW flag when
> considering whether the loop counter increment may skip the loop
> test. However, if signed-wrap occurs, the result of the loop test is
> undefined. Providing a loop trip count to downstream optimization
> seems harmless. The only assumption we've made is that if executing
> the loop causes the counter to wrap, then the loop test will fail.
> In practice the test may not fail, but the assumption is still
> valid.

Andy,

Thanks very much for doing this!

 -Hal

> 
> There is one cryptic test case (below) attempting to enforce the
> conservative behavior. But it doesn't reveal why this case is
> problematic. I haven't been able to pick up clues from old bugs
> either. I know several related bugs in this area have been fixed
> since this test case was added. My guess is that either this was
> test case was preemtive or the original problem was fixed elsewhere.
> 
> ---
> ; Be careful with this one. If %n is INT4_MAX, %i.next will wrap. The
> nsw bit
> ; says that the result is undefined, but ScalarEvolution must respect
> that
> ; subsequent passes may result the undefined behavior in predictable
> ways.
> ; CHECK: Determining loop execution counts for: @nsw_step2
> ; CHECK: Loop %loop: Unpredictable backedge-taken count.
> ; CHECK: Loop %loop: Unpredictable max backedge-taken count.
> define void @nsw_step2(i4 %n) {
> entry:
>   %s = icmp sgt i4 %n, 0
>   br i1 %s, label %loop, label %exit
> loop:
>   %i = phi i4 [ 0, %entry ], [ %i.next, %loop ]
>   %i.next = add nsw i4 %i, 2
>   %t = icmp slt i4 %i.next, %n
>   br i1 %t, label %loop, label %exit
> exit:
>   ret void
> }
> 

-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory