dotnet/runtime

Chained comparison of two integers against a constant is not coalesced

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#101,347 opened on Apr 21, 2024

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area-CodeGen-coreclrhelp wantedin-prtenet-performance

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Description

Description

Given the snippet

static bool Test(int a, int b) => a is 0x80 && b is 0x80;

.NET emits

       cmp      ecx, 128
       jne      SHORT G_M000_IG05
       xor      eax, eax
       cmp      edx, 128
       sete     al
       ret      
G_M000_IG05:                ;; offset=0x0014
       xor      eax, eax
       ret

While Clang emits

        xor     edi, 128
        xor     esi, 128
        or      esi, edi
        sete    al
        ret

It would be nice if JIT/ILC could emit branchless comparison for such patterns.

Configuration

.NET 8.0.3

Regression?

No

Contributor guide

Research direction
Examine the JIT's code generation for pattern matching with 'is' operator. Look for existing optimizations for chained comparisons and see if you can apply a branchless technique like using XOR and OR. Focus on the lowering or codegen phase for x64. Test your changes with the given snippet.
Tech stack
csharp
Domain
backendperformance
Issue type
Feature
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
Half day
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
Active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
C#Git
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
30