dotnet/runtime

Optimize Math.Pow(X, C) in JIT

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#13,336 opened on Aug 29, 2019

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 (6 comments) (8 reactions) (0 assignees)C# (5,445 forks)batch import
area-CodeGen-coreclrhelp wantedoptimization

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Description

I noticed that it's quite popular to use constants with Math.Pow, e.g.

double z = Math.Pow(x, 2);
// can be optimized to just
double z = x * x;

Codegen:

C.Test1(Double)
    L0000: vzeroupper
    L0003: vmovsd xmm1, qword [rip+0x15]
    L000b: mov rax, 0x7ffec7f7cca0
    L0015: jmp rax

C.Test2(Double)
    L0000: vzeroupper
    L0003: vmulsd xmm0, xmm0, xmm0
    L0007: ret

E.g. Burgers benchmark uses it a lot https://github.com/dotnet/performance/blob/master/src/benchmarks/micro/coreclr/Burgers/Burgers.cs#L22

I am currently converting Math methods in Mono to LLVM intrinsic and noticed LLVM's InstCombine actually does it, see https://github.com/mono/mono/pull/16561

category:cq theme:floating-point skill-level:intermediate cost:medium

Contributor guide

Research direction
Study how LLVM's InstCombine optimizes Math.Pow with constant integer exponents and implement similar pattern in .NET JIT. Focus on identifying common exponent values (e.g., 2, 3, 0.5) and replacing with multiplication or sqrt.
Tech stack
csharp
Domain
performancebackend
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.
1-2 days
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.
Mostly clear
Prerequisites
C#JIT compilation concepts
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
40