HLSL Bug: Negation Error In Vector Multiplication
Hey guys! Today, we're diving deep into a rather intriguing issue in HLSL (High-Level Shading Language) that can cause some unexpected behavior in your shader code. Specifically, we're talking about a bug where the result of an integer-vector and integer-vector multiplication is incorrectly negated when one of the integer vectors is logically NOT'd (!). This can lead to some head-scratching moments, especially when your shader output isn't quite what you anticipated. Let's break down the problem, explore how it manifests, and see how we can work around it.
Understanding the Issue
The core of the problem lies in how HLSL handles the combination of logical negation and integer-vector multiplication. In theory, negating a boolean vector should simply invert its truth values. When you multiply this negated vector with another integer vector, you'd expect element-wise multiplication based on the inverted boolean values. However, due to this bug, the result can sometimes be unexpectedly negated, leading to incorrect calculations and visual artifacts.
The Technical Details
To get a clearer picture, let's look at a specific example. Consider the following HLSL code snippet:
export uint32_t2 foo(uint32_t2 a, uint32_t2 b) {
return a * !b;
}
export uint32_t2 bar(uint32_t2 a, uint32_t2 b) {
return a * !bool2(b);
}
In this example, we have two functions, foo and bar, both designed to achieve the same outcome: multiplying an integer vector a by the logical negation of another integer vector b. The foo function directly uses the ! operator on b, while the bar function explicitly converts b to a bool2 vector before applying the negation. Logically, these two functions should produce identical results. However, this is where the bug kicks in.
The Unexpected Behavior
When compiled using Clang, the above HLSL code generates the following LLVM Intermediate Representation (IR):
; RUN: clang-dxc -T lib_6_7 -Xclang -emit-llvm %s
defined noundef <2 x i32> @foo(unsigned int vector[2], unsigned int vector[2])(<2 x i32> noundef %0, <2 x i32> noundef %1) local_unnamed_addr #0 {
%3 = icmp eq <2 x i32> %1, zeroinitializer
%4 = sub <2 x i32> zeroinitializer, %0
%5 = select <2 x i1> %3, <2 x i32> %4, <2 x i32> zeroinitializer
ret <2 x i32> %5
}
defined noundef <2 x i32> @bar(unsigned int vector[2], unsigned int vector[2])(<2 x i32> noundef %0, <2 x i32> noundef %1) local_unnamed_addr #0 {
%3 = icmp eq <2 x i32> %1, zeroinitializer
%4 = select <2 x i1> %3, <2 x i32> %0, <2 x i32> zeroinitializer
ret <2 x i32> %5
}
attributes #0 = { alwaysinline mustprogress nofree norecurse nosync nounwind willreturn memory(none) "frame-pointer"="all" "no-infs-fp-math"="true" "no-nans-fp-math"="true" "no-signed-zeros-fp-math"="true" "no-trapping-math"="true" "stack-protector-buffer-size"="8" }
Notice the crucial difference between the IR generated for foo and bar. In the foo function, the result is calculated as sub <2 x i32> zeroinitializer, %0, which effectively negates the vector %0 (which corresponds to a). This negation is then conditionally selected based on the comparison %3 = icmp eq <2 x i32> %1, zeroinitializer. If %1 (corresponding to b) is zero, the negated value is used; otherwise, zero is used. This is incorrect.
In contrast, the bar function correctly selects between %0 ( a) and zero based on the same condition, without the unexpected negation. This discrepancy highlights the bug: directly negating an integer vector with ! within a multiplication operation leads to an erroneous negation in the generated IR.
Why This Matters
This bug can have significant implications for your shader code. Imagine you're using this type of multiplication as part of a complex lighting calculation or a procedural effect. The unexpected negation can introduce subtle but noticeable errors in your visuals, leading to incorrect colors, shading artifacts, or distorted patterns. Debugging these issues can be particularly challenging because the code looks correct, and the problem only manifests under specific conditions related to the logical negation of integer vectors.
How to Reproduce the Issue
The easiest way to reproduce this issue is to use the Godbolt link provided in the original report: https://godbolt.org/z/n8zYee3W1. This link provides a live compiler environment where you can directly input the HLSL code and observe the generated LLVM IR. By comparing the IR for the foo and bar functions, you can clearly see the incorrect negation occurring in foo.
Alternatively, you can set up a local HLSL compilation environment using Clang and the DirectX Shader Compiler (DXC). Compile the code with the -Xclang -emit-llvm flags to generate the LLVM IR, and then examine the output for the discrepancies described above.
Workarounds and Solutions
Fortunately, there are several ways to work around this bug and ensure your shader code behaves as expected.
1. Explicitly Convert to Boolean Vectors
The most straightforward workaround is to explicitly convert the integer vector to a boolean vector before applying the logical NOT operator. This is exactly what the bar function in our example code does:
export uint32_t2 bar(uint32_t2 a, uint32_t2 b) {
return a * !bool2(b);
}
By using !bool2(b) instead of !b, you force HLSL to treat the negation as a boolean operation, preventing the erroneous negation in the generated IR. This is the recommended approach for avoiding the bug.
2. Use Conditional Statements
Another approach is to use conditional statements to explicitly handle the cases where the vector elements are zero or non-zero. This can be more verbose but provides fine-grained control over the calculation:
export uint32_t2 baz(uint32_t2 a, uint32_t2 b) {
uint32_t2 result = 0;
for (int i = 0; i < 2; ++i) {
if (b[i] == 0) {
result[i] = a[i];
}
}
return result;
}
In this baz function, we iterate through the elements of the vectors and conditionally assign the corresponding element from a to the result if the element in b is zero. This avoids the direct use of the ! operator and multiplication, thus bypassing the bug.
3. Compiler Updates
It's also crucial to stay updated with the latest compiler versions. Compiler developers are constantly working to fix bugs and improve code generation. It's possible that this particular issue may be resolved in a future release of Clang or DXC. Regularly updating your compiler toolchain can help you benefit from these fixes and avoid potential issues.
Conclusion
The HLSL bug involving the incorrect negation of integer-vector multiplication with logical NOT is a subtle but potentially impactful issue. By understanding the problem, recognizing its symptoms, and applying the workarounds discussed above, you can ensure the accuracy and reliability of your shader code. Remember to always test your shaders thoroughly and inspect the generated IR when encountering unexpected behavior. Happy shading, guys! And keep an eye out for those sneaky bugs!