dotnet/runtime

Consider reordering the instrsxarch.h encodings to save space

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#10,908 opened on Aug 15, 2018

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area-CodeGen-coreclrenhancementhelp wanted

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Description

The instrsxarch.h encodings are currently ordered as follows:

  • R/M[reg]
  • R/M,icon
  • reg,R/M
  • eax,i32
  • register

This ordering appears to have been done in order to save space, as not all instructions require all encodings However, with the addition of HardwareIntrinsics, a large number of INST3 encodings were added and often, only one or two of the encodings are used.

It may be beneficial to consider reordering these encodings in order to save additional space. Looking briefly at the encodings, I see the following:

encoding # entries # bad_code
R/M[reg] 527 366
R/M,icon 444 390
reg,R/M 421 56
eax,i32 21 10
register 9 1

category:implementation theme:emitter skill-level:beginner cost:medium impact:small

Contributor guide

Research direction
Open the instrsxarch.h file and analyze the encoding entries. For each instruction group (R/M[reg], R/M,icon, etc.), count how many entries have only one or two sub encodings. Consider reordering the groups so that frequently used sub encodings are placed together, reducing gaps and saving space. Verify by rebuilding and checking the binary size.
Tech stack
cpp
Domain
backend
Issue type
Refactor
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
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/C++
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
80