gfx-rs/wgpu

Backends should not need two passes to handle wrapped functions

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#7,106 opened on Feb 11, 2025

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area: naga back-endgood first issuekind: refactorlang: HLSLlang: Metallang: SPIR-Vnaga

Description

Once #7012 lands, Naga's Metal Shading Language, HLSL, and SPIR-V backends will make a prepass over each function's expressions to see which wrapped functions it will need, and then generate their definitions before the function's own translation. This means that both the prepass and the actual code generation need to repeat the logic for deciding which expressions need wrapped functions.

Instead, we should generate output for the Module's functions, and note which wrapped functions we needed by adding them to the backend's wrapped_functions table. Then, after all the Module's functions have been translated, we should simply iterate over the wrapped_functions table and generate the definitions we need.

Since MSL and HLSL require the wrapped function definitions to precede their uses, in those backends the two stages should write to separate Strings, which the backend should concatenate as a final step. Those copies should be pretty cheap. SPIR-V allows OpFunctionCall to refer to functions defined later, so in that case no reordering is necessary.

Contributor guide

Tech stack
rust
Domain
backend
Issue type
refactor
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
4
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.
fresh
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
Rust proficiencyNaga backend familiarityUnderstanding of wrapped functions
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
25
Research direction
Begin by reviewing the referenced PR #7012 to understand the current prepass logic. Then examine the backend files in naga/src/back/ (msl.rs, hlsl.rs, spirv.rs) to see how wrapped functions are used. The goal is to modify the code generation to collect needed wrapped functions during function translation and generate them afterwards, avoiding the prepass. For MSL and HLSL, implement separate string buffers that are concatenated at the end. The SPIR V backend can simply reorder the output since it allows forward references.