JIT: Use faster mod for uint16 values by MihaZupan · Pull Request #128509 · dotnet/runtime

MihaZupan · 2026-05-22T23:41:50Z

Another attempt at #111535
Closes #111492

Change the transformation for

int Mod(char c) => c % 42;

-int Mod(char c) => (int)(c - (uint)(((ulong)((uint)c >> 1) * 818089009u) >> 34) * 42);
+int Mod(char c) => (int)(((ulong)(102261127u * c) * 42) >> 32);

    movzx    rax, di
-   mov      ecx, eax
-   shr      ecx, 1
-   imul     rcx, rcx, 0x30C30C31
-   shr      rcx, 34
-   imul     ecx, ecx, 42
-   sub      eax, ecx
+   imul     eax, eax, 0x6186187
+   imul     rax, rax, 42
+   shr      rax, 32
    ret

Let's see what CI thinks.

Diffs: https://gist.github.com/MihuBot/38b0b6eeabd80de528a6e1967a7df7cb

dotnet-policy-service · 2026-05-22T23:42:59Z

Tagging subscribers to this area: @JulieLeeMSFT, @jakobbotsch
See info in area-owners.md if you want to be subscribed.

MihaZupan · 2026-05-22T23:43:22Z

@MihuBot

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Pull request overview

This PR updates CoreCLR JIT morphing/assertion propagation/lowering to enable a cheaper remainder sequence when both operands are proven to fit in uint16 and the divisor is a non-zero constant, and adds a JIT regression test that exercises relevant % const patterns (including char-based modulo).

Changes:

Teach morphing to avoid rewriting % const into a - (a / b) * b when lowering can apply a cheaper uint16 FastMod-style sequence (and convert MOD→UMOD when safe).
Add a new GTF_UMOD_UINT16_OPERANDS hint set by assertion propagation and consumed by lowering to trigger the specialized expansion.
Add a new JIT test under src/tests/JIT/opt/Divide/Regressions/ to cover representative modulo patterns.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/coreclr/jit/morph.cpp	Skips early MOD-to-SUB/MUL/DIV morphing (and may flip MOD→UMOD) so lowering can apply the cheaper uint16 modulo path.
src/coreclr/jit/lower.cpp	Implements the uint16-specialized FastMod lowering for `GT_UMOD` by constant divisors.
src/coreclr/jit/assertionprop.cpp	Improves `IntegralRange` reasoning and sets `GTF_UMOD_UINT16_OPERANDS` when uint16-range operands are proven.
src/coreclr/jit/gentree.h	Introduces the new `GTF_UMOD_UINT16_OPERANDS` flag.
src/coreclr/jit/gentree.cpp	Ensures tree comparison accounts for the new mod/div-related flag.
src/tests/JIT/opt/Divide/Regressions/Regression4_Divide.csproj	Adds the new regression test project.
src/tests/JIT/opt/Divide/Regressions/Regression4_Divide.cs	Adds the new regression test cases exercising modulo scenarios.

MihaZupan · 2026-05-23T17:11:25Z

/azp run Fuzzlyn

azure-pipelines · 2026-05-23T17:11:38Z

Azure Pipelines successfully started running 1 pipeline(s).

Copilot

Pull request overview

Copilot reviewed 7 out of 7 changed files in this pull request and generated 1 comment.

+            BlockRange().InsertBefore(divMod, multiplier, mul1, castUp);
+
+            // Reuse the existing constant divisor as a TYP_LONG operand for the second multiply.
+            divisor->BashToConst(static_cast<int64_t>(divisorValue));


The VN-based small-type refinement in IntegralRange::ForNode for GT_LCL_VAR claimed a tight range whenever the local's conservative VN was a CAST to a small type. That is unsound for normalize-on-load locals: their storage only contains the small-type bits in the low byte (upper bytes are stale), but the tight range let fgOptimizeCast drop a required sign/zero extending load downstream, causing it to read those stale bits. Restrict the refinement to locals whose storage is fully normalized: either non-small locals, or small locals with lvNormalizeOnStore set. Repros fuzzlyn seed 12902382323863156506 (and others) where '(short)(arg0 ^ arg2)' with sbyte arg0 began returning the unsigned byte interpretation in release. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

MihaZupan · 2026-05-24T00:41:00Z

/azp run Fuzzlyn

azure-pipelines · 2026-05-24T00:41:14Z

Azure Pipelines successfully started running 1 pipeline(s).

jakobbotsch · 2026-05-24T10:31:50Z

+            // extending load and read those stale bits.
+            if ((compiler->vnStore != nullptr) && (!varTypeIsSmall(varDsc->TypeGet()) || varDsc->lvNormalizeOnStore()))
+            {
+                ValueNum  vn = compiler->vnStore->VNConservativeNormalValue(node->gtVNPair);


I do not think IntegralRange::ForNode should depend on VNs. VNs are a flow sensitive concept that is only valid during specific phases. I think this is too general a utility to start using VNs.

I would be ok with introducing specific helpers that can refine integral ranges based on VNs though, and then using it explicitly from phases where VNs are known to be valid. But in the end you would probably want to use range check instead.

JIT: Use faster mod for uint16 values

cea3809

MihaZupan added this to the 11.0.0 milestone May 22, 2026

MihaZupan self-assigned this May 22, 2026

Copilot AI review requested due to automatic review settings May 22, 2026 23:41

MihaZupan added the area-CodeGen-coreclr CLR JIT compiler in src/coreclr/src/jit and related components such as SuperPMI label May 22, 2026

Copilot started reviewing on behalf of MihaZupan May 22, 2026 23:42 View session

MihuBot mentioned this pull request May 22, 2026

[JitDiff X64] [MihaZupan] JIT: Use faster mod for uint16 values MihuBot/runtime-utils#1924

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Copilot AI reviewed May 22, 2026

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Comment thread src/coreclr/jit/lower.cpp

Comment thread src/coreclr/jit/morph.cpp

MihuBot mentioned this pull request May 23, 2026

[JitDiff ARM64] [MihaZupan] JIT: Use faster mod for uint16 values MihuBot/runtime-utils#1925

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build-analysis Bot mentioned this pull request May 23, 2026

Multiple Helix work items fail on maccatalyst/tvos CoreCLR Release #126460

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Add a MinOpts check

56f1398

Copilot AI review requested due to automatic review settings May 23, 2026 22:04

Copilot started reviewing on behalf of MihaZupan May 23, 2026 22:05 View session

Copilot AI reviewed May 23, 2026

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Comment thread src/coreclr/jit/lower.cpp

BlockRange().InsertBefore(divMod, multiplier, mul1, castUp);

// Reuse the existing constant divisor as a TYP_LONG operand for the second multiply.

divisor->BashToConst(static_cast<int64_t>(divisorValue));

MihaZupan force-pushed the uint16-mod branch from 65babc5 to 897296a Compare May 23, 2026 22:25

MihuBot mentioned this pull request May 23, 2026

[JitDiff X64] [MihaZupan] JIT: Use faster mod for uint16 values MihuBot/runtime-utils#1933

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jakobbotsch reviewed May 24, 2026

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JIT: Use faster mod for uint16 values#128509

JIT: Use faster mod for uint16 values#128509
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