Using mix() with step() to simulate conditional assignments in shaders is often less efficient than directly using branch instructions. While seemingly branchless, this mix()/step() approach can introduce extra computations and potentially disrupt hardware optimizations related to predication. Modern GPUs are adept at handling branches efficiently, especially when they are predictable, so relying on them is often faster and simpler than employing arithmetic workarounds. Therefore, default to standard branching unless profiling reveals a specific performance bottleneck that can be demonstrably addressed by a mix()/step() alternative.
Inigo Quilez's blog post, "Don't 'optimize' conditional moves in shaders with mix()+step()," argues against a common but misguided optimization technique used in shader programming. This technique attempts to replace explicit conditional statements (like if and else) with a combination of the mix() and step() functions, believing it will improve performance. Quilez contends that this perceived optimization is often counterproductive on modern GPUs and can actually lead to worse performance and even introduce subtle visual artifacts.
The core issue stems from how GPUs handle branching. While older GPUs suffered performance penalties from branching due to their sequential architecture, modern GPUs utilize a Single Instruction Multiple Data (SIMD) architecture. This means they execute the same instruction across multiple data points simultaneously. When encountering a branch (an if statement), both branches are evaluated for all data points, and the relevant result is then selected based on the condition. While this might seem wasteful, it avoids the complexities of thread divergence and maintains the efficiency of the SIMD architecture.
The proposed "optimization" using mix(a, b, step(x, y)) emulates a conditional move. It works by utilizing the step() function, which returns 0 if x<y and 1 otherwise. This result is then fed into the mix() function, which linearly interpolates between a and b based on the third parameter. Effectively, if x<y, mix() returns a (because the third parameter is 0); otherwise, it returns b (because the third parameter is 1). While logically equivalent to a conditional, this approach forces the GPU to evaluate both a and b regardless of the condition, even if only one result is ultimately used. This is precisely the same behavior the "optimization" was intended to avoid.
Moreover, the step() function introduces potential issues with precision and edge cases. Due to floating-point limitations, values very near the threshold of the step function can lead to unexpected blending between a and b, creating subtle visual artifacts, especially when dealing with sharp transitions or discontinuities in the data.
Quilez further emphasizes that compilers are often smart enough to recognize simple conditional statements and optimize them appropriately for the target hardware. Manually trying to outsmart the compiler with tricks like the mix()+step() combination often hinders the compiler’s ability to perform more effective optimizations.
In conclusion, Quilez advises against using mix()+step() as a replacement for conditional statements in shaders. He advocates for writing clear, readable code using explicit conditionals and trusting the compiler to generate optimized code for modern GPUs. The perceived performance gains from this "optimization" are generally illusory and can lead to performance degradation and visual artifacts. Clear and explicit code is generally preferred for maintainability and allows the compiler to perform more robust optimizations.
Summary of Comments ( 7 )
https://news.ycombinator.com/item?id=42990324
HN users generally agreed that the article's advice is sound, particularly for modern GPUs. Several pointed out that
mix()andstep()can be more efficient than branching, especially when dealing with SIMD architectures where branching can lead to thread divergence. Some emphasized that profiling is crucial, as the optimal approach can vary depending on the specific GPU and shader complexity. One commenter noted that while branching might be faster in simple cases,mix()offers more predictable performance as shader complexity increases. Another cautioned against premature optimization and recommended focusing on algorithmic improvements first. A few users shared alternative techniques like using lookup textures or bitwise operations for certain conditional scenarios. Finally, there was discussion about the evolution of GPU architecture and how older advice regarding branching might no longer apply.The Hacker News post "Don't "optimize" conditional moves in shaders with mix()+step()" sparked a discussion with several insightful comments. The central theme revolves around the performance implications of using
mix()andstep()to simulate conditional moves in shaders, as opposed to using actual conditional statements (e.g.,ifstatements).Several commenters pointed out that the performance characteristics of
mix()/step()vs. conditional branching can vary significantly depending on the specific GPU architecture and the surrounding shader code. While the original article suggestsmix()/step()can be less efficient, commenters noted that modern GPUs often handle branching efficiently, sometimes even converting branches into predicated instructions similar to whatmix()/step()achieves. Therefore, a blanket statement about one approach always being superior is inaccurate.One commenter highlighted the importance of profiling and benchmarking to determine the best approach for a given situation. They emphasized that theoretical considerations and general advice can be misleading, and empirical testing is crucial. Another user concurred, suggesting tools like Shader Playground for easy experimentation and performance comparison.
The impact of compiler optimizations was also discussed. Commenters noted that compilers can sometimes transform code in surprising ways, potentially negating the perceived benefits of one technique over another. Therefore, relying on assumptions about how the code will be executed at the hardware level can be problematic.
Some commenters delved into the nuances of GPU architectures, explaining how branching can affect occupancy and warp divergence. They explained how a branch might cause threads within a warp to take different paths, leading to serialization and reduced performance. However, it was also pointed out that modern GPUs have mechanisms to mitigate this, and the actual performance impact can be complex.
A few users discussed the readability and maintainability trade-offs. While
mix()/step()might seem more concise, it can sometimes obscure the intent of the code compared to a more explicitifstatement. This can make debugging and future modifications more challenging.Finally, some commenters offered alternative approaches for handling conditional logic in shaders, such as using lookup tables or specialized instructions available on certain GPUs. These suggestions highlighted the importance of exploring different techniques and considering the specific hardware target when optimizing shader code.