8330158: C2: Loop strip mining uses ABS with min int #18813

rwestrel · 2024-04-17T10:45:04Z

This fixes 3 calls to ABS with a min int argument. I think all of them
are harmless:

in PhaseIdealLoop::exact_limit(), I removed the call to ABS. The
check is for a stride of 1 or -1.
in OuterStripMinedLoopNode::adjust_strip_mined_loop(), for the
computation of scaled_iters_long, the stride is passed to ABS()
and then implicitly casted to long. I now cast the stride to long
before ABS(). For a min int stride, LoopStripMiningIter * stride
overflows the int range for all values of LoopStripMiningIter
except 0 or 1. Those values are handled early on in that method. So
for a min in stride:
```
(jlong)scaled_iters != scaled_iters_long
```
is always true and the method returns early.
in OuterStripMinedLoopNode::adjust_strip_mined_loop(), the
computation of short_scaled_iters also calls ABS() with the
stride as argument. But the result of that computation is only used
if the test for:
```
(jlong)scaled_iters != scaled_iters_long
```
doesn't cause an early return of the method. I reordered statements
so the ABS() calls happens after that test which will cause an early
return if the stride is min int.

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Issue

JDK-8330158: C2: Loop strip mining uses ABS with min int (Bug - P4)

Reviewers

Aleksey Shipilev (@shipilev - Reviewer) ⚠️ Review applies to ecd940f3
Vladimir Kozlov (@vnkozlov - Reviewer)
Dean Long (@dean-long - Reviewer) ⚠️ Review applies to ecd940f3
Martin Balao (@martinuy - Committer)

Reviewing

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Checkout this PR locally:
$ git fetch https://git.openjdk.org/jdk.git pull/18813/head:pull/18813
$ git checkout pull/18813

Update a local copy of the PR:
$ git checkout pull/18813
$ git pull https://git.openjdk.org/jdk.git pull/18813/head

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Checkout this PR locally:
$ git pr checkout 18813

View PR using the GUI difftool:
$ git pr show -t 18813

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Download this PR as a diff file:
https://git.openjdk.org/jdk/pull/18813.diff

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bridgekeeper · 2024-04-17T10:45:42Z

👋 Welcome back roland! A progress list of the required criteria for merging this PR into master will be added to the body of your pull request. There are additional pull request commands available for use with this pull request.

openjdk · 2024-04-17T10:46:03Z

@rwestrel This change now passes all automated pre-integration checks.

ℹ️ This project also has non-automated pre-integration requirements. Please see the file CONTRIBUTING.md for details.

After integration, the commit message for the final commit will be:

8330158: C2: Loop strip mining uses ABS with min int

Reviewed-by: shade, kvn, dlong, mbalao

You can use pull request commands such as /summary, /contributor and /issue to adjust it as needed.

At the time when this comment was updated there had been 126 new commits pushed to the master branch:

f1d0e71: 8324577: [REDO] - [IMPROVE] OPEN_MAX is no longer the max limit on macOS >= 10.6 for RLIMIT_NOFILE
74b11cc: 8331004: Serial: Remove unused GenClosure
45ed97f: 8330464: hserr generic events - add entry for the before_exit calls
e311ba3: 8330626: ZGC: Windows address space placeholders not managed correctly
e923dfe: 8314592: Add shortcut to SymbolLookup::find
1519081: 8330303: Crash: assert(_target_jt == nullptr || _target_jt->vthread() == target_h()) failed
2bb5cf5: 8330155: Serial: Remove TenuredSpace
7bb59dc: 8330670: Obsolete ScavengeBeforeFullGC
174d626: 8328741: serviceability/jvmti/ObjectMonitorUsage/ObjectMonitorUsage.java failed with unexpected owner
e681e9b: 8330819: C2 SuperWord: bad dominance after pre-loop limit adjustment with base that has CastLL after pre-loop
... and 116 more: https://git.openjdk.org/jdk/compare/bfff02eef68c80f623419a3f6ceb9fe3121b88f4...master

As there are no conflicts, your changes will automatically be rebased on top of these commits when integrating. If you prefer to avoid this automatic rebasing, please check the documentation for the /integrate command for further details.

➡️ To integrate this PR with the above commit message to the master branch, type /integrate in a new comment.

openjdk · 2024-04-17T10:46:36Z

@rwestrel The following label will be automatically applied to this pull request:

hotspot-compiler

When this pull request is ready to be reviewed, an "RFR" email will be sent to the corresponding mailing list. If you would like to change these labels, use the /label pull request command.

mlbridge · 2024-04-17T10:50:23Z

Webrevs

01: Full - Incremental (d876cbc9)
00: Full (ecd940f3)

shipilev

All right, I agree with this reasoning.

Have you tried running tests with #18751 applied?

rwestrel · 2024-04-17T15:49:59Z

Thanks for reviewing this.

Have you tried running tests with #18751 applied?

I only ran the particular test that you mentioned in the bug.

shipilev · 2024-04-17T15:59:38Z

I only ran the particular test that you mentioned in the bug.

All right, let me run tests with #18751 applied and see if we have any surprises.

vnkozlov

two comments

vnkozlov · 2024-04-17T16:14:28Z

src/hotspot/share/opto/loopnode.cpp

  int scaled_iters = (int)scaled_iters_long;
-  int short_scaled_iters = LoopStripMiningIterShortLoop* ABS(stride);
+  if ((jlong)scaled_iters != scaled_iters_long) {
+    // Remove outer loop and safepoint (too few iterations)


Please put more extended comment here. What you have in PR description would be nice.

I added a comment. Does it look good to you?

vnkozlov · 2024-04-17T16:21:54Z

src/hotspot/share/opto/loopnode.cpp

+    remove_outer_loop_and_safepoint(igvn);
+    return;
+  }
+  int short_scaled_iters = LoopStripMiningIterShortLoop * ABS(stride);


So stride is not MIN_INT here but the expression still can overflow. Should we use jlong for expression and short_scaled_iters? iter_estimate is jlong.

Good catch. I updated the patch as suggested.

dean-long · 2024-04-17T18:53:13Z

src/hotspot/share/opto/loopnode.cpp

+    remove_outer_loop_and_safepoint(igvn);
+    return;
+  }
+  int short_scaled_iters = LoopStripMiningIterShortLoop * ABS(stride);


Isn't it true that stride can be MIN_INT here, if LoopStripMiningIter == 1?

There's a test for LoopStripMiningIter == 1 earlier in the method that causes the method to return.

dean-long · 2024-04-17T23:30:41Z

There's also a less obvious use of an abs() idiom in LoopLimitNode::Ideal, when it does

2579      stride_p = -stride_con;

if stride_con is negative. Does it make sense to fix that as part of this PR?

vnkozlov

Looks good.

shipilev · 2024-04-19T18:40:34Z

All right, let me run tests with #18751 applied and see if we have any surprises.

I ran Maven CTW, Fuzzer tests, and the rest of OpenJDK jtregs with my ABS-checking patch applied, and there are no surprises. Looks good!

shipilev · 2024-04-22T08:55:48Z

@rwestrel, if you could integrate this, we can then go forward with #18751. Thanks!

martinuy

Looks good to me.

rwestrel · 2024-04-29T07:35:01Z

@shipilev @dean-long @vnkozlov @martinuy thanks for the reviews

rwestrel · 2024-04-29T07:35:07Z

/integrate

openjdk · 2024-04-29T07:36:17Z

Going to push as commit c615c18.
Since your change was applied there have been 174 commits pushed to the master branch:

fb63cba: 8330684: ClassFile API runs into StackOverflowError while parsing certain class' bytes
4e5c25e: 8331108: Unused Math.abs call in java.lang.FdLibm.Expm1#compute
16c7dcd: 8330523: Reduce runtime and improve efficiency of KeepAliveTest
c3372c4: 8331134: Port SimpleStringBuilderStrategy to use ClassFile API
a078b5e: 8331114: Further improve performance of MethodTypeDesc::descriptorString
e3eb652: 8296543: Update exception documentation for ExecutorService.invokeAll overriders as required
aa2edd4: 8330686: Fix typos in the DatabaseMetaData javadoc
aee91fb: 8296175: Output warning if generated docs contain diagnostic markers
a920af2: 8303689: javac -Xlint could/should report on "dangling" doc comments
f3bb3e2: 8331153: JFR: Improve logging of jdk/jfr/api/consumer/filestream/TestOrdered.java
... and 164 more: https://git.openjdk.org/jdk/compare/bfff02eef68c80f623419a3f6ceb9fe3121b88f4...master

Your commit was automatically rebased without conflicts.

openjdk · 2024-04-29T07:36:22Z

@rwestrel Pushed as commit c615c18.

💡 You may see a message that your pull request was closed with unmerged commits. This can be safely ignored.

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8330158: C2: Loop strip mining uses ABS with min int #18813

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8330158: C2: Loop strip mining uses ABS with min int #18813

8330158: C2: Loop strip mining uses ABS with min int #18813

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