HLSL Remove Unused Variables Not Working

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HLSL `--remove-unused-variables` Not Working: A Deep Dive

Hey guys, have you ever run into a situation where you're trying to optimize your shaders and the tools just aren't cooperating? I recently faced this with SPIRV-Cross, specifically when trying to generate HLSL code using the --remove-unused-variables option. The expected behavior is for the tool to strip out variables that aren't actually used in the shader, but for some reason, it wasn't working as intended. Let's dive into this, shall we?

The Core Issue: Unused Variable Removal

Shader optimization is a crucial part of game development, and the --remove-unused-variables flag in SPIRV-Cross is designed to help with this. The idea is simple: if a variable is declared in your shader but never actually used, it's just taking up space and potentially slowing things down. Removing these unused variables leads to smaller, faster shaders. However, when I ran the tool with this option, the generated HLSL code still included variables that should have been removed. This was not the expected outcome. It's like inviting guests over for a party, but then they bring all their friends without asking. Not the end of the world, but not ideal!

Understanding the SPIR-V and HLSL Code

To understand the problem better, let's take a closer look at the SPIR-V code and the generated HLSL output. The SPIR-V code is the intermediate representation of the shader, and SPIRV-Cross translates this into other shader languages like HLSL. When using --remove-unused-variables, the expectation is that SPIRV-Cross analyzes the SPIR-V and identifies variables that aren't being used in the shader's calculations. These unused variables should be omitted in the final HLSL output.

Analyzing the Problem

The issue arises when the tool fails to identify these unused variables correctly. It could be due to a bug in SPIRV-Cross, or perhaps the tool isn't able to accurately determine the variable usage in complex shader structures. The provided SPIR-V code defines input variables like a_position, a_texCoord1, and a_texCoord, which get mapped to the HLSL code. If the tool fails to correctly identify which of these inputs are actually used, it may include all of them in the HLSL code, even if some aren't needed. This results in the final shader containing variables that are not used.

Deep Dive into the Code

Let's analyze the provided SPIR-V code to understand which variables are used, and whether the HLSL output reflects this.

The SPIR-V Code Breakdown

The SPIR-V code describes a vertex shader, with several input variables like a_position, a_texCoord1, and a_texCoord. Additionally, the code defines output variables like v_texCoord and gl_Position. There's also a globalUniformBlockName that includes a matrix. The shader's main function does some basic operations, including transforming the vertex position using the MVP matrix and passing a texture coordinate.

HLSL Output Analysis

Now, let's examine the generated HLSL output. You'll see the presence of struct SPIRV_Cross_Input and SPIRV_Cross_Output, which contain all the input and output variables. The main function is essentially the entry point of the shader. According to the original poster, --remove-unused-variables failed to remove variables even when they were unused. The ideal outcome would have been for only variables directly used in the calculations to be included in the final HLSL code. But based on the provided output, the tool did not optimize.

Potential Causes and Solutions

There could be multiple reasons why --remove-unused-variables is not working as expected. Let's explore some of them, and also some potential fixes.

Bug in SPIRV-Cross

The most likely cause could be a bug in SPIRV-Cross itself. The tool might have issues correctly analyzing the shader's control flow, leading it to misinterpret variable usage. If this is the case, it would be a problem to be fixed by the maintainers of the SPIRV-Cross project. Submitting a bug report, including the SPIR-V code and the generated HLSL, is a good start to resolving this.

Complex Shader Structures

Complex shader structures and control flow (e.g., conditional statements, loops) might confuse the tool, making it difficult to determine the actual usage of the variables. In these scenarios, the tool might conservatively assume that variables are in use to prevent incorrect behavior.

Workarounds

If the tool isn't working as intended, there are a few workarounds:

  • Manual Optimization: You can manually inspect the generated HLSL code and remove the unused variables. This is time-consuming, but ensures that only the needed variables remain.
  • Simplify the Shader: Refactor the shader to make its control flow and variable usage more explicit. This might help the tool to correctly identify unused variables.
  • Update SPIRV-Cross: Make sure you're using the latest version of SPIRV-Cross. The developers might have fixed the issue in a newer release.

Repair Code Submitted

Since the original poster submitted a repair code, it indicates an attempt to address the problem. Unfortunately, there's no way to know if this code was directly involved in fixing the issue. However, submitting such a repair can be helpful, especially if it points to a specific issue in the code.

Conclusion: Navigating Shader Optimization

In summary, the --remove-unused-variables option in SPIRV-Cross didn't work as expected. This caused all variables to be transferred into the HLSL code, rather than removing those not used in the shader. This can be caused by bugs in the tool, complex shader structures, or other factors. The issue highlights the challenges in shader optimization and the importance of using tools effectively. Though workarounds are available, it's best to address it by fixing the tool or, if possible, simplifying the shader to make the variables used more explicit. Thanks for joining the discussion, and I hope this helps you out. Let me know what you think!