dotnet/runtime

Consider a new flag COMPlus_EnableVEX

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#10,808 opened on Jul 31, 2018

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area-CodeGen-coreclrhelp wantedtest-enhancement

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Description

Proposal

Introduce a new flag (COMPlus_EnableVEX) and make some minor changes to the existing flag (COMPlus_EnableAVX).

Rationale

Prior to the HWIntrinsics feature, the COMPlus_EnableAVX flag only controlled support for AVX/AVX2 (which it did by enabling/disabling the VEX-encoding path in the emitter).

However, now that HWIntrinsics exist, there are a number of additional ISAs that are impacted by this flag (BMI1, BMI2, FMA). Since this flag now controls more than its original scope, and that may cause additional confusion later on, we should instead split the functionality into separate logical flags before the HWIntrinsics feature ships.

Changes

Introduce COMPlus_EnableVEX, this would control whether or not the VEX-encoding is support by the emitter. It would impact all ISAs that use the VEX encoding.

Change COMPlus_EnableAVX to only control the AVX/AVX2 ISAs.

category:testing theme:emitter skill-level:beginner cost:small impact:small

Contributor guide

Research direction
Understand the current implementation of COMPlus EnableAVX flag and the emitter's VEX encoding logic, then create a new flag COMPlus EnableVEX that controls VEX encoding and adjust COMPlus EnableAVX to only cover AVX/AVX2. Locate relevant code in the runtime repository, especially in the JIT/emitter area.
Tech stack
csharp
Domain
backendcompilers
Issue type
Feature
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.
1-3 hours
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#
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
80