dotnet/runtime

Implicit tail calls block folding of intrinsics in value numbering

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#52,280 opened on May 5, 2021

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 (1 comment) (0 reactions) (1 assignee)C# (5,445 forks)batch import
Priority:3area-CodeGen-coreclrhelp wanted

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Description

Reproduction:

private static double NotFolded(double a)
{
    var b = 1.0d;
    if (a is 0)
        return 0;

    return Math.Cos(b);
}

Codegen, Windows x64:

G_M24041_IG01:
       vzeroupper 
G_M24041_IG02:
       vxorps   xmm1, xmm1
       vucomisd xmm0, xmm1
       jp       SHORT G_M24041_IG05
       jne      SHORT G_M24041_IG05
G_M24041_IG03:
       vxorps   xmm0, xmm0
G_M24041_IG04:
       ret      
G_M24041_IG05:
       vmovsd   xmm0, qword ptr [reloc @RWD00]
G_M24041_IG06:
       jmp      System.Math:Cos(double):double

Value numbering today supports folding some intrinsics implemented by user calls. However, that support is only for actual GT_INTRINSIC nodes. Tail calls to intrinsics implemented via user calls are imported as plain GT_CALLs, blocking the optimization.

Issue is for tracking purposes, I do not expect this to be a big problem in real-world scenarios.

category:cq theme:tail-call skill-level:beginner cost:small impact:small

Contributor guide

Research direction
Examine the importer code in src/coreclr/jit/importer.cpp around line 4620-4626 where tail calls to intrinsics are imported as GT CALL instead of GT INTRINSIC. Trace why this blocks folding in value numbering and implement a fix to import them as GT INTRINSIC so that value numbering can fold them.
Tech stack
csharp
Domain
backend
Issue type
Bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
2
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
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Active
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Clear
Prerequisites
C#Git
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
80