SPIR-V Error: OpCopyLogical Issue In Cbuffers With Arrays

Introduction

Hey guys! Today, we're diving deep into a tricky issue encountered when working with SPIR-V, specifically the OpCopyLogical instruction and its interaction with arrays within constant buffers, or cbuffers. This problem arises when the result type of the OpCopyLogical instruction inadvertently matches the operand type, leading to an invalid SPIR-V code generation. We'll break down the scenario, the root cause, and potential solutions, making it super clear for everyone, from beginners to experienced graphics developers.

The core of the issue revolves around how the DirectX Shader Compiler (dxc) handles arrays within cbuffers when compiling to SPIR-V, particularly for Vulkan. When you have arrays defined inside a cbuffer and try to copy them, the compiler might generate an OpCopyLogical instruction where the source and destination types are the same. This is a no-no in SPIR-V, resulting in a validation error. Let's get into the nitty-gritty details with a real-world example and see how we can tackle this problem head-on.

The Problem: Identical Result and Operand Types in OpCopyLogical

The heart of the matter is the OpCopyLogical instruction within the SPIR-V intermediate representation. This instruction is designed to perform a logical copy operation. However, the SPIR-V specification mandates that the result type and the operand type for OpCopyLogical must be distinct. When the compiler mistakenly generates an instruction where these types are identical, it violates this rule, and the SPIR-V validator flags it as an error. This typically occurs in scenarios involving arrays within constant buffers (cbuffers) in HLSL code compiled for Vulkan using DirectX Shader Compiler (dxc).

Imagine you're building a house, and you're trying to move bricks from one pile to another, but somehow, the instruction tells you to move the pile itself – that doesn't make sense, right? Similarly, OpCopyLogical needs a clear distinction between what's being copied and where it's being copied to. When these become the same, the operation falls apart. Now, let's explore a specific example to illustrate how this issue manifests in practice. By understanding the concrete scenario, we can better grasp the underlying problem and devise effective solutions. It's all about getting those bricks in the right place, so your house – or in this case, your shader – stands strong and performs flawlessly!

Code Example and Compilation

Let’s examine a practical example using HLSL code to illustrate this issue. Consider the following HLSL shader code snippet:

cbuffer CBArrays : register(b0) {
  float c1[2];
  int4 c2[2][2];
  bool c3[2];
}

struct Arrays {
  float c1[2];
  int4 c2[2][2];
  bool c3[2];
};

RWStructuredBuffer<Arrays> Out : register(u1);

[numthreads(1,1,1)]
void main() {
  Out[0].c1 = c1;
  Out[0].c2 = c2;
  Out[0].c3 = c3;
}

This HLSL code defines a cbuffer named CBArrays containing several arrays: c1 (an array of two floats), c2 (a 2x2 array of int4), and c3 (an array of two booleans). It also defines a structure Arrays that mirrors the layout of the cbuffer and a read-write structured buffer Out. The main function then copies the contents of the arrays from the cbuffer to the structured buffer.

Now, let’s look at the compilation command used to process this code:

dxc -spirv -fspv-target-env=vulkan1.3 -fvk-use-dx-layout -T cs_6_5 source.hlsl

This command instructs the DirectX Shader Compiler (dxc) to compile the HLSL source code (source.hlsl) into SPIR-V. The -spirv flag indicates that the output should be in SPIR-V format. The -fspv-target-env=vulkan1.3 flag specifies that the SPIR-V code should be compatible with Vulkan 1.3. The -fvk-use-dx-layout flag tells the compiler to use a layout compatible with DirectX, and -T cs_6_5 specifies that the target shader model is Compute Shader 6.5. However, when you compile this code, you might encounter the following error:

fatal error: generated SPIR-V is invalid: Result Type must not equal the Operand type
  %41 = OpCopyLogical %_arr__arr_v4int_uint_2_uint_2 %39

This error message clearly indicates that the generated SPIR-V code contains an invalid OpCopyLogical instruction. Specifically, the result type (%_arr__arr_v4int_uint_2_uint_2) is the same as the operand type (%39), which violates the SPIR-V specification. This is where the crux of the problem lies. The compiler, in handling the array copies within the cbuffer, has produced an instruction that doesn't conform to the rules of SPIR-V.

Root Cause Analysis

To really squash this bug, we need to understand what's causing it. In this scenario, the DirectX Shader Compiler (dxc) is generating an OpCopyLogical instruction where the result type is identical to the operand type. This goes against the SPIR-V specification, which mandates these types be distinct. This often occurs when dealing with arrays inside cbuffers, particularly when targeting Vulkan. It’s like telling someone to copy a book onto itself – makes no sense, right?

The reason this happens is rooted in how the compiler interprets and translates HLSL array operations into SPIR-V. When you copy an array from a cbuffer to another memory location, the compiler needs to generate instructions to move the data. In some cases, the compiler incorrectly infers that the source and destination are of the same type, leading to the invalid OpCopyLogical instruction. It's a subtle hiccup in the translation process, but it can cause major headaches if left unchecked.

Think of it like a translation error between two languages. The HLSL code is the original language, and SPIR-V is the translated version. Sometimes, the translator (dxc) misinterprets a phrase and produces a grammatically incorrect sentence in the target language. In our case, the