Don't "optimize" conditional moves in shaders with mix()+step()

Re:Don't "optimize" conditional moves in shaders with mix()+... by ajross on Sunday February 09, 2025 @10:51AM (#42991387)

> it's also concerning that we have syntax where an if is some times a branch and some times not.

That's true on scalar CPUs too though. The CMOV instruction arrived with the P6 core in 1995, for example. Branches are expensive everywhere, even in scalar architectures, and compilers do their best to figure out when they should use an alternative strategy. And sometimes get it wrong, but not very often.

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Re:Don't "optimize" conditional moves in shaders with mix()+... by masklinn on Sunday February 09, 2025 @12:58PM (#42992224)

For scalar CPUs, historically CMOV used to be relatively slow on x86, and notably for reliable branching patterns (>75% reliable) branches could be a lot faster.

cmov also has dependencies on all three inputs, so if there's a high level of bias towards the unlikely input having a much higher latency than the likely one a cmov can cost a fair amount of waiting.

Finally cmov were absolutely terrible on P4 (10-ish cycles), and it's likely that a lot of their lore dates back to that.

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