JIT: Elide transitive array bounds checks by EgorBo · Pull Request #130588 · dotnet/runtime

EgorBo · 2026-07-12T21:47:30Z

static int Test(int[] items, int size, int index)
{
    if ((uint)size <= (uint)items.Length && (uint)index < (uint)size)
    {
        return items[index];
    }
    return 0;
}

 G_M24430_IG01:
-       sub      rsp, 40
 
 G_M24430_IG06:
-       cmp      r8d, r10d
-       jae      SHORT G_M24430_IG08
        mov      eax, r8d
        mov      eax, dword ptr [rcx+4*rax+0x10]
 
-G_M24430_IG08:
-       call     CORINFO_HELP_RNGCHKFAIL
-       int3
-
-; Total bytes of code: 48
+; Total bytes of code: 23

Diffs

dotnet-policy-service · 2026-07-12T21:48:38Z

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Copilot

Pull request overview

This PR extends JIT assertion propagation + range analysis so it can chain certain unsigned relations involving checked-bound VNs (e.g., size u<= items.Length) to prove a subsequent index relation (e.g., index u< size) and thereby elide the transitive array bounds check for items[index].

Changes:

Teach optCreateJTrueBoundsAssertion to create/selectively admit unsigned VN-to-VN and checked-bound assertions that can participate in chaining.
Update RangeCheck::MergeEdgeAssertionsWorker to (a) accept LE_UN checked-bound assertions and (b) derive bounds through transitive unsigned VN relations while preserving checked-bound-shaped limits.
Add a JIT opt test covering the “index < size <= length” pattern and asserting no CORINFO_HELP_RNGCHKFAIL appears in the generated code.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
src/tests/JIT/opt/RangeChecks/ElidedBoundsChecks.cs	Adds a new regression/opt-pattern method and invocation to validate transitive unsigned bounds-check elimination.
src/coreclr/jit/rangecheck.cpp	Enables transitive reasoning across unsigned relations (with guardrails) and preserves checked-bound limits when following VN chains.
src/coreclr/jit/assertionprop.cpp	Broadens assertion creation to support the unsigned checked-bound chaining needed by the rangecheck optimization, with heuristics to limit table pressure.

        // Bounds-check coalescing: 4 constant indices, same length VN.
        int[] arr4 = new int[] { 10, 20, 30, 40 };
+        if (IndexLessThanSizeLessThanOrEqualToLength(arr4, 3, 2) != 30)
+            return 0;


+    // Restrict optional unsigned relation assertions to small methods, where they cannot crowd out
+    // later bounds-check assertions in an already pressured assertion table.
+    const bool canCreateUnsignedRelation = optMaxAssertionCount == 64;


Copilot

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

            budget--;
-            Range otherRange = GetRangeFromAssertionsWorker(comp, otherVN, assertions, budget, visited);
-            if (!otherRange.IsConstantRange())
+            int   nextBudget = budget;
+            Range otherRange = GetRangeFromAssertionsWorker(comp, otherVN, assertions, nextBudget, visited);
+
+            // Preserve checked-bound-shaped limits while following a chain such as
+            // "index u< size" and "size u<= array.Length".
+            if (canUseCheckedBounds)
+            {
+                MergeEdgeAssertionsWorker(comp, otherVN, preferredBoundVN, assertions, &otherRange, canUseCheckedBounds,
+                                          nextBudget, visited);
+            }


Track bounds-check index VNs and let range analysis propagate symbolic bounds through unsigned VN comparisons. Preserve the distinction between index operands and non-negative length bounds. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com> Copilot-Session: b997c5bf-c48e-4c43-af7f-3062fc691e31

Restrict symbolic unsigned bound propagation to unresolved ranges and non-strict intermediate array-length bounds. Keep legacy length-only assertion consumers from treating marked index VNs as lengths. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com> Copilot-Session: b997c5bf-c48e-4c43-af7f-3062fc691e31

Keep length and index VN markers separate, and recognize the exact short-circuit predecessor pattern at bounds-check assertion propagation. Do not infer non-negativity from checked-bound membership. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com> Copilot-Session: b997c5bf-c48e-4c43-af7f-3062fc691e31

Copilot

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

When an index has an unsigned upper-bound assertion against an intermediate VN, resolve that VN through assertions using the current preferred array-length bound. Keep existing unsigned-bound recognition precedence and require explicit non-negativity facts. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com> Copilot-Session: b997c5bf-c48e-4c43-af7f-3062fc691e31

Copilot

Pull request overview

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

+            if (boundRange.LowerLimit().IsConstant() && (boundRange.LowerLimit().GetConstant() >= 0) &&
+                boundRange.UpperLimit().IsBinOpArray() && (boundRange.UpperLimit().vn == preferredBoundVN))


+        // Current assertion is "normalLclVN u< boundVN". Get boundVN's range using our preferred bound.
+        else if (canUseCheckedBounds && pRange->LowerLimit().IsUnknown() && pRange->UpperLimit().IsUnknown() &&
+                 curAssertion.KindIs(Compiler::OAK_LT_UN) &&
+                 (curAssertion.GetOp1().GetVN() == normalLclVN) &&
+                 curAssertion.GetOp2().KindIs(Compiler::O2K_VN_ADD_CNS) &&
+                 (curAssertion.GetOp2().GetVN() != preferredBoundVN) && (curAssertion.GetOp2().GetCns() == 0) &&
+                 (budget > 0))


Copilot AI review requested due to automatic review settings July 12, 2026 21:47

github-actions Bot added the area-CodeGen-coreclr CLR JIT compiler in src/coreclr/src/jit and related components such as SuperPMI label Jul 12, 2026

Copilot started reviewing on behalf of EgorBo July 12, 2026 21:47 View session

dotnet-policy-service Bot assigned EgorBo Jul 12, 2026

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Copilot AI review requested due to automatic review settings July 12, 2026 22:20

EgorBo force-pushed the egorbo/jit-elide-transitive-bounds-check branch from 8e84a81 to d67fb6b Compare July 12, 2026 22:20

Copilot started reviewing on behalf of EgorBo July 12, 2026 22:20 View session

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build-analysis Bot mentioned this pull request Jul 13, 2026

System.IO.FileSystem.Tests.WorkItemExecution CI timeout on net9.0-osx-Debug-x64-Mono_Minijit_Debug-OSX.1200.Amd64.Open #101423

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EgorBo and others added 3 commits July 14, 2026 02:58

Copilot AI review requested due to automatic review settings July 14, 2026 02:13

EgorBo force-pushed the egorbo/jit-elide-transitive-bounds-check branch from d67fb6b to 19159f8 Compare July 14, 2026 02:13

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Comment thread src/coreclr/jit/assertionprop.cpp Outdated

Comment thread src/coreclr/jit/valuenum.cpp

Comment thread src/coreclr/jit/assertionprop.cpp Outdated

Copilot AI review requested due to automatic review settings July 14, 2026 02:26

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This was referenced Jul 14, 2026

slow macOS - "##[error]The job running on agent Azure Pipelines 9 ran longer than the maximum time of 60 minutes." dotnet/dnceng#1883

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The Operation will be canceled. The next steps may not contain expected logs. dotnet/dnceng#3008

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JIT: Elide transitive array bounds checks#130588

JIT: Elide transitive array bounds checks#130588
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EgorBo commented Jul 12, 2026 •

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		if (boundRange.LowerLimit().IsConstant() && (boundRange.LowerLimit().GetConstant() >= 0) &&
		boundRange.UpperLimit().IsBinOpArray() && (boundRange.UpperLimit().vn == preferredBoundVN))

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EgorBo commented Jul 12, 2026 •

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