[jdk18] Integrated: 8278790: Inner loop of long loop nest runs for too few iterations

[Roland Westrelin]

On Thu, 16 Dec 2021 08:44:59 GMT, Roland Westrelin <roland at openjdk.org> wrote:

> Given a counted loop that iterates in [A, Z), when long range checks
> are transformed into int range checks, a loop nest is created and
> the inner loop iterates in [0, Z2).
> 
> The limits of the inner loop are adjusted to guarantee no overflow for
> the range of values of the inner loop. That is for a range check:
> 
> i * scale + offset <u length
> 
> 1) the bounds of the inner loop are adjusted to roughly [0,
> max_jint/scale).
> 
> Also, we don't want to loose what we know about the bounds of the loop
> being transformed.
> 
> 2) So the bound of the inner loop are also adjusted to [0, min(Z2, Z - A))
> 
> The bug here is that 2) is performed before 1). This was spotted with
> a micro benchmarks where the initial loop had only ~2000
> iterations. The transformed loop is expected to run for the same 2000
> iterations but instead ran for 2000/scale iterations.

This pull request has now been integrated.

Changeset: bb7efb35
Author:    Roland Westrelin <roland at openjdk.org>
URL:       https://git.openjdk.java.net/jdk18/commit/bb7efb3517b0ac66a55607c14aae3aef1f11c892
Stats:     9 lines in 1 file changed: 5 ins; 4 del; 0 mod

8278790: Inner loop of long loop nest runs for too few iterations

Reviewed-by: chagedorn, neliasso

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PR: https://git.openjdk.java.net/jdk18/pull/35