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JIT: clone "i != arr.Length" loops to eliminate bounds checks - #129268

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AndyAyersMS:fix-84697-ne-cloning
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JIT: clone "i != arr.Length" loops to eliminate bounds checks#129268
AndyAyersMS merged 1 commit into
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AndyAyersMS:fix-84697-ne-cloning

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@AndyAyersMSAndyAyersMS commented Jun 11, 2026

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Loops with i != end previously kept their per-iteration bounds checks while the equivalent < / > forms did not, because loop cloning bailed on GT_NE.

Recognize GT_NE with stride exactly +/-1 as an increasing or decreasing loop in NaturalLoopIterInfo, extend optDeriveLoopCloningConditions to emit the right per-access and zero-trip conditions for GT_NE, and add a new "init RELOP end" cloning condition so that a misordered init (which for GT_NE would wrap the IV through the type rather than exit at the first test) falls back to the slow path. Builds on #129176 which fixes the latent decreasing-loop soundness gap.

A follow-on change to assertion prop / range check will catch the remaining NE-loop BCE cases that fall outside loop cloning (e.g. loops the cloning heuristic rejects).

Contributes to #84697.

@AndyAyersMS
JIT: clone "i != arr.Length" loops to eliminate bounds checks
f46c3dd
Loops with `i != end` previously kept their per-iteration bounds
checks while the equivalent `<` / `>` forms did not, because loop
cloning bailed on GT_NE.
Recognize GT_NE with stride exactly +/-1 as an increasing or decreasing
loop in NaturalLoopIterInfo, extend optDeriveLoopCloningConditions to
emit the right per-access and zero-trip conditions for GT_NE, and add a
new "init RELOP end" cloning condition so that a misordered init (which
for GT_NE would wrap the IV through the type rather than exit at the
first test) falls back to the slow path. Builds on dotnet#129176 which fixes
the latent decreasing-loop soundness gap.
A follow-on change to assertion prop / range check will catch the
remaining NE-loop BCE cases that fall outside loop cloning (e.g. loops
the cloning heuristic rejects).
Contributes to dotnet#84697.
CopilotAI review requested due to automatic review settings June 11, 2026 01:33
@github-actionsgithub-actionsBot added the area-CodeGen-coreclr CLR JIT compiler in src/coreclr/src/jit and related components such as SuperPMI label Jun 11, 2026
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AndyAyersMS commented Jun 11, 2026

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Splitting #129199 into two PRs. This is the first.

@EgorBo PTAL
fyi @dotnet/jit-contrib

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Pull request overview

This PR extends CoreCLR JIT loop cloning analysis to treat certain i != limit loops (specifically stride ±1) as monotonic increasing/decreasing loops, enabling the same cloning-based bounds-check elimination previously available for ordered relational loop conditions.

Changes:

  • Recognize GT_NE loop tests as clonable when the induction variable step is exactly +1 or -1 (NaturalLoopIterInfo::IsIncreasingLoop/IsDecreasingLoop).
  • Update loop-cloning condition derivation to accept GT_NE, emit appropriate zero-trip/per-access guards, and add an extra init-vs-limit ordering guard for GT_NE.
  • Add a new JIT regression test covering != loops across arrays/strings/spans, including soundness scenarios.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

FileDescription
src/coreclr/jit/loopcloning.cppExtends cloning-condition derivation to handle GT_NE loops (stride ±1) with additional guards for safety.
src/coreclr/jit/flowgraph.cppClassifies GT_NE + stride ±1 loops as increasing/decreasing in NaturalLoopIterInfo.
src/tests/JIT/Regression/Regression_o_3.csprojIncludes the new regression test source file in the JIT regression project.
src/tests/JIT/Regression/JitBlue/Runtime_84697/Runtime_84697.csAdds a regression test for i != Length loop BCE and key soundness cases.

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AndyAyersMS merged commit b426c12 into dotnet:mainJun 11, 2026
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@dotnet-milestone-botdotnet-milestone-botBot added this to the 11.0-preview6 milestone Jun 17, 2026
@AndyAyersMSAndyAyersMS mentioned this pull request Jun 18, 2026
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dotnet/perf-autofiling-issues#74735

eiriktsarpalis pushed a commit that referenced this pull request Jul 15, 2026
@AndyAyersMS@eiriktsarpalis
JIT: clone "i != arr.Length" loops to eliminate bounds checks (#129268)
ac041cc
Loops with `i != end` previously kept their per-iteration bounds checks
while the equivalent `<` / `>` forms did not, because loop cloning
bailed on GT_NE.
Recognize GT_NE with stride exactly +/-1 as an increasing or decreasing
loop in NaturalLoopIterInfo, extend optDeriveLoopCloningConditions to
emit the right per-access and zero-trip conditions for GT_NE, and add a
new "init RELOP end" cloning condition so that a misordered init (which
for GT_NE would wrap the IV through the type rather than exit at the
first test) falls back to the slow path. Builds on #129176 which fixes
the latent decreasing-loop soundness gap.
A follow-on change to assertion prop / range check will catch the
remaining NE-loop BCE cases that fall outside loop cloning (e.g. loops
the cloning heuristic rejects).
Contributes to #84697.
@github-actionsgithub-actionsBot locked and limited conversation to collaborators Jul 19, 2026
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