JIT: eliminate redundant span checks across loop back-edge phis by EgorBo · Pull Request #127117 · dotnet/runtime

EgorBo · 2026-04-19T11:28:59Z

Contributes to #111309

Optimize redundant range/argument checks for spans inside loops.

int Sum(ReadOnlySpan<int> data)
{
    Vector128<int> sum = default;
    while (data.Length >= Vector128<int>.Count)
    {
        sum += Vector128.Create(data);
        data = data.Slice(Vector128<int>.Count);
    }
    int result = Vector128.Sum(sum);
    foreach (var t in data)
        result += t;
    return result;
}

Codegen diff

 G_M38854_IG02:
        mov      rax, bword ptr [rcx]
        mov      ecx, dword ptr [rcx+0x08]
        vxorps   xmm0, xmm0, xmm0
        cmp      ecx, 4
        jl       SHORT G_M38854_IG04
 G_M38854_IG03:
-       cmp      ecx, 4
-       jl       SHORT G_M38854_IG09
        vpaddd   xmm0, xmm0, xmmword ptr [rax]
        add      rax, 16
        add      ecx, -4
        cmp      ecx, 4
        jge      SHORT G_M38854_IG03
 G_M38854_IG04:
        vpsrldq  xmm1, xmm0, 8
        vpaddd   xmm0, xmm1, xmm0
        vpsrldq  xmm1, xmm0, 4
        vpaddd   xmm0, xmm1, xmm0
        vmovd    edx, xmm0
        test     ecx, ecx
        je       SHORT G_M38854_IG07
 G_M38854_IG05:
        xor      r8d, r8d
 G_M38854_IG06:
        add      edx, dword ptr [rax+r8]
        add      r8, 4
        dec      ecx
        jne      SHORT G_M38854_IG06
 G_M38854_IG07:
        mov      eax, edx
 G_M38854_IG08:
-       add      rsp, 40
        ret
-G_M38854_IG09:
-       mov      ecx, 6
-       call     [System.ThrowHelper:ThrowArgumentOutOfRangeException(int)]
-       int3
-; Total bytes of code 113
+; Total bytes of code 79

Diffs

Recognize that a back-edge phi-arg's VN equals the (checkedBound + cns) form referenced by an O2K_CHECKED_BOUND_ADD_CNS assertion in RangeCheck::MergeEdgeAssertions. Read phi-arg VNs from the SSA def's m_vnPair in optVisitReachingAssertions; this is always at least as precise as phiArg->gtVNPair (which fgValueNumberPhiDef may leave as NoVN or stale when it refuses to back-patch a loop-varying VN). Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Copilot

Pull request overview

Optimize CoreCLR JIT range/argument check elimination for spans in loops by improving how edge assertions are interpreted across loop back-edge phi arguments.

Changes:

Extend RangeCheck::MergeEdgeAssertions to recognize when the tracked VN matches a checked-bound expression via ADD(boundVN, cns), enabling assertion application to loop back-edge phi-args.
Update Compiler::optVisitReachingAssertions to read the VN for a phi-arg from its SSA def’s m_vnPair (instead of the potentially stale gtVNPair), improving precision for assertion propagation.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
`src/coreclr/jit/rangecheck.cpp`	Enhances checked-bound assertion matching so loop back-edge phi args can inherit assertions and eliminate redundant checks.
`src/coreclr/jit/compiler.hpp`	Uses SSA-def VN for phi args during reaching-assertion visitation to avoid `gtVNPair` staleness/NoVN cases.

- optVisitReachingAssertions: only fall back to the SSA def's m_vnPair when phiArg->gtVNPair is NoVN, restoring the cheap fast-path for the common case. - MergeEdgeAssertions: skip the IsVNBinFuncWithConst lookup when boundCns == 0, since VN normalizes ADD(x, 0) to x, so any positive match would already be caught by the trivial check. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Copilot

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

Copilot

Pull request overview

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

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Copilot

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

EgorBo · 2026-04-20T13:55:36Z

PTAL @jakobbotsch @dotnet/jit-contrib

EgorBo · 2026-04-20T13:56:43Z

    auto  visitor  = [comp, &phiRange, &budget](ValueNum reachingVN, ASSERT_TP reachingAssertions) {
        // call GetRangeFromAssertions for each reaching VN using reachingAssertions
-        Range edgeRange = GetRangeFromAssertions(comp, reachingVN, reachingAssertions, --budget);
+        Range edgeRange = GetRangeFromAssertions(comp, reachingVN, reachingAssertions, min(3, --budget));


After I switched optVisitReachingAssertion to SSA defs instead of VNs it significantly regressed TP, this helped to reduce it 3x times

jakobbotsch · 2026-04-20T14:03:02Z

+        // fgValueNumberPhiDef may leave gtVNPair as NoVN when it refuses to back-patch
+        // a loop-varying SSA-def VN into the phi-arg slot (a hygiene constraint for general
+        // gtVNPair consumers that does not apply here, since we hand the VN to the visitor
+        // together with this edge's assertion set and never write it back into any tree).
+        // Use the SSA def's VN directly; by SSA construction it names the same value.
+        LclSsaVarDsc*  phiArgSsaDef = lvaGetDesc(phiArg)->GetPerSsaData(phiArg->GetSsaNum());
+        const ValueNum phiArgVN     = vnStore->VNConservativeNormalValue(phiArgSsaDef->m_vnPair);


Not clear to me if this is legal... See #106229 and the issue it links to. The VNs from the phi def can be from a different loop iteration and it can take on a different runtime value.

@jakobbotsch but isn't similar how rangecheck works? where we iterate phi via SSA and call MergeEdgeAssertions on them?

(we obtain VN out of the tree inside)

Copilot AI review requested due to automatic review settings April 19, 2026 11:29

github-actions bot added the area-CodeGen-coreclr CLR JIT compiler in src/coreclr/src/jit and related components such as SuperPMI label Apr 19, 2026

Copilot started reviewing on behalf of EgorBo April 19, 2026 11:29 View session

dotnet-policy-service bot assigned EgorBo Apr 19, 2026

Copilot AI reviewed Apr 19, 2026

View reviewed changes

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

EgorBo added the reduce-unsafe label Apr 19, 2026

Copilot AI review requested due to automatic review settings April 19, 2026 13:38

Copilot started reviewing on behalf of EgorBo April 19, 2026 13:39 View session

Copilot AI reviewed Apr 19, 2026

View reviewed changes

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

fix TP

ff00e93

EgorBo force-pushed the jit-rangecheck-phi-add-cns branch from e5170d0 to ff00e93 Compare April 19, 2026 14:44

cleanup

4cce476

Copilot AI review requested due to automatic review settings April 19, 2026 15:08

cleanup

c31e025

Copilot AI reviewed Apr 19, 2026

View reviewed changes

Comment thread src/coreclr/jit/rangecheck.cpp

Comment thread src/coreclr/jit/compiler.hpp Outdated

Comment thread src/coreclr/jit/rangecheck.cpp

Update src/coreclr/jit/compiler.hpp

2db2b80

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Copilot AI review requested due to automatic review settings April 19, 2026 15:21

Copilot started reviewing on behalf of EgorBo April 19, 2026 15:22 View session

Copilot AI reviewed Apr 19, 2026

View reviewed changes

Comment thread src/coreclr/jit/rangecheck.cpp

This was referenced Apr 19, 2026

[wasm] WBT SatelliteAssembliesTests.CheckThatSatelliteAssembliesAreNotAOTed failing #90458

Open

System.Net.NameResolution.Tests DNS failures: Name or service not known #126641

Open

EgorBo requested a review from jakobbotsch April 20, 2026 13:55

EgorBo commented Apr 20, 2026

View reviewed changes

jakobbotsch reviewed Apr 20, 2026

View reviewed changes

EgorBo mentioned this pull request Apr 21, 2026

Introduce GT_ASSERTION node #127193

Open

Provide feedback

Saved searches

Use saved searches to filter your results more quickly

JIT: eliminate redundant span checks across loop back-edge phis#127117

JIT: eliminate redundant span checks across loop back-edge phis#127117
EgorBo wants to merge 6 commits intomainfrom
jit-rangecheck-phi-add-cns

EgorBo commented Apr 19, 2026 •

edited

Loading

Uh oh!

Copilot AI left a comment

Uh oh!

Uh oh!

Copilot AI left a comment

Uh oh!

Uh oh!

Copilot AI left a comment

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Copilot AI left a comment

Uh oh!

Uh oh!

EgorBo commented Apr 20, 2026

Uh oh!

EgorBo Apr 20, 2026

Uh oh!

jakobbotsch Apr 20, 2026

Uh oh!

EgorBo Apr 20, 2026 •

edited

Loading

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

3 participants

Conversation

EgorBo commented Apr 19, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Codegen diff

Uh oh!

Copilot AI left a comment

Choose a reason for hiding this comment

Pull request overview

Reviewed changes

Uh oh!

Uh oh!

Copilot AI left a comment

Choose a reason for hiding this comment

Pull request overview

Uh oh!

Uh oh!

Copilot AI left a comment

Choose a reason for hiding this comment

Pull request overview

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Copilot AI left a comment

Choose a reason for hiding this comment

Pull request overview

Uh oh!

Uh oh!

EgorBo commented Apr 20, 2026

Uh oh!

EgorBo Apr 20, 2026

Choose a reason for hiding this comment

Uh oh!

jakobbotsch Apr 20, 2026

Choose a reason for hiding this comment

Uh oh!

EgorBo Apr 20, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

3 participants

EgorBo commented Apr 19, 2026 •

edited

Loading

EgorBo Apr 20, 2026 •

edited

Loading