compiler-explorer/compiler-explorer

[REQUEST] Consider using gdc -fno-moduleinfo -fno-weak by default [D language]

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#1,961 opened on May 11, 2020

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enhancementhelp wantedlang-drequest

Description

So, both gdc and ldc do emit quite a bit of extra stuff into object code by default, like module info, type info, static module constructors, etc. It makes it a bit messy.

gdc does support -fno-moduleinfo.

I think it would be nice if you select gdc from the list of D compilers, and the options are empty, to maybe set options to -fno-moduleinfo ?

-fno-weak might also be worth too, as it will get a rid of dangling instantiations of various templates that are inlined into the code anyway by other stages, and not used by anything else.

Take a look here at a massive difference in readability of the assembler output:

https://godbolt.org/z/E3oN9q (14 vs 54 vs 211 lines of assembler)

For LDC compiler you could try --fvisibility=hidden, but in my experience it doesn't do much. The various templates instantentiations are marked weak AFAIK in ELF object (this can be changed to be once odr), but to get rid of them one would need to ask linker to remove them probably. Maybe there is some magic ldc or LLVM flag to do it directly tho.

Contributor guide

Tech stack
typescript
Domain
tooling
Issue type
feature
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.
stale
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
Familiarity with D compiler flagsUnderstanding of compiler explorer's compiler configuration
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
60
Research direction
Look at how default compiler options are configured for other compilers in the compiler explorer codebase (e.g., in the `lib/compilers` directory). Find the D compiler definitions for gdc and ldc. The flags ` fno moduleinfo` and ` fno weak` should be added as default options when no user specified options are provided. Review the existing compiler configuration patterns and apply similar logic for D compilers.