dotnet/runtime

[JIT] X64 - Possible comparison optimization using `and` with a small type

Open

#81,520 opened on Feb 2, 2023

View on GitHub
 (1 comment) (0 reactions) (0 assignees)C# (5,445 forks)batch import
arch-x64area-CodeGen-coreclrhelp wantedtenet-performance

Repository metrics

Stars
 (17,886 stars)
PR merge metrics
 (Avg merge 12d 11h) (661 merged PRs in 30d)

Description

using System.Runtime.CompilerServices;

static class Program
{
    [MethodImpl(MethodImplOptions.NoInlining)]
    public static void SideEffect()
    {
    }

    public static bool Test(byte p, int i)
    {
        if ((p & 0xffffff00) != 0)
        {
            SideEffect();
            return true;
        }
        return false;
    }
}

The (p & 0xffffff00) != 0 condition in this case can never be true. So, we could optimize this by folding it into false thus will eliminate the true path.

This will probably not have a lot of impact.

Contributor guide

Research direction
Inspect the condition `(p & 0xffffff00) != 0` for a byte variable p, which is always false. Implement a constant folding optimization in the JIT that eliminates this check.
Tech stack
csharpcpp
Domain
compilers
Issue type
Performance
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
1
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
Under 1 hour
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#basic compiler knowledge
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
80