dotnet/runtime

F# typeof comparison suboptimal codegen

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#82,394 opened on Feb 20, 2023

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area-CodeGen-coreclrhelp wanted

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Description

The following F# snippet:

let test() = typeof<float> = typeof<bool>

Emits this IL:

ldtoken !!a
call class [netstandard]System.Type [netstandard]System.Type::GetTypeFromHandle(valuetype [netstandard]System.RuntimeTypeHandle)
ldtoken [System.Runtime]System.Boolean
call class [netstandard]System.Type [netstandard]System.Type::GetTypeFromHandle(valuetype [netstandard]System.RuntimeTypeHandle)
tail.
call bool [FSharp.Core]Microsoft.FSharp.Core.LanguagePrimitives/HashCompare::GenericEqualityIntrinsic<class [System.Runtime]System.Type>(!!0, !!0)
ret

and this codegen:

_.test()
    L0000: push rsi
    L0001: sub rsp, 0x20
    L0005: mov rcx, 0x7ff9f93c77a8
    L000f: call 0x00007ffa58e77670
    L0014: mov rsi, rax
    L0017: mov rcx, 0x7ff9f934b3c0
    L0021: call 0x00007ffa58e77670
    L0026: mov r8, rax
    L0029: mov rdx, rsi
    L002c: mov rcx, 0x7ff9febc1e88
    L0036: mov rax, 0x7ff9feb43be8
    L0040: add rsp, 0x20
    L0044: pop rsi
    L0045: jmp qword ptr [rax]

expected codegen:

_.test()
    L0000: xor eax, eax
    L0002: ret

Contributor guide

Research direction
Investigate how the F# compiler emits code for typeof comparisons and why it doesn't constant fold them. Check if the JIT can optimize the type handle comparison at compile time.
Tech stack
csharp
Domain
performance
Issue type
Performance
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.
Over 1 week
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
Basic knowledge of .NET runtime and F#
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
40