dotnet/runtime

JIT doesn't fold constant multiplication to single instruction

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#91,824 opened on Sep 8, 2023

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 (5 comments) (0 reactions) (0 assignees)C# (5,445 forks)batch import
Priority:3area-CodeGen-coreclrhelp wantedtenet-performance

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Description

Description

Consider the following code:

int A(int i)
{
    return i * 4 * 8;
}

int B(int i)
{
    return i * (4 * 8);
}

These two functions should result in identical results.

But the JIT output slightly different code for each:

Program.<<Main>$>g__A|0_0(Int32)
    L0000: mov eax, ecx
    L0002: shl eax, 2
    L0005: shl eax, 3
    L0008: ret

Program.<<Main>$>g__B|0_1(Int32)
    L0000: mov eax, ecx
    L0002: shl eax, 5
    L0005: ret

This is on: ; Core CLR 7.0.823.31807 on x64

Analysis

Given that multiplication is communicative, and that this is a power of two operation, I would expect the JIT to fold them to the 2nd version, without the parenthesis.

The first version takes two instructions instead of 1.

Contributor guide

Research direction
Investigate the JIT's constant folding logic for multiplication by constants that are powers of two. The issue shows that when the multiplication is chained (i * 4 * 8), the JIT fails to combine the shifts into a single shift (i * 32). Look at the JIT's optimization pass for constant multiplication and ensure that associative folding is performed.
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.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
Half day
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Active
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Clear
Prerequisites
C#
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30