Std: Clamp generates less efficient assembly than std:min(max,std:max(min,v))

Re:Std: Clamp generates less efficient assembly than std:min... by pavlov on Tuesday January 16, 2024 @10:25AM (#39014273)

This would only affect code that uses the old-school x87 floating point instructions, though? The x87 FPU unit indeed has scary global state that can make your doubles behave like floats in secret and silence.

I would think practically all modern FPU code on x86-64 would be using the SIMD registers which have explicit widths.

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Re:Std: Clamp generates less efficient assembly than std:min... by borodi on Tuesday January 16, 2024 @10:41AM (#39014527)

So it was a bit more pervasive than this, the issue was that flushing subnormals (values very close to 0) to 0 is a register that gets set, so if a library is built with the fastmath flags and it gets loaded, it sets the register, causing the whole process to flush it's subnormals. i.e https://github.com/llvm/llvm-project/issues/57589

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Re:Std: Clamp generates less efficient assembly than std:min... by jcranmer on Tuesday January 16, 2024 @11:14AM (#39015025)

> This would only affect code that uses the old-school x87 floating point instructions, though?

Actually, no, the x87 FPU instructions are the only ones that won't be affected.

It sets the FTZ/DAZ bits, which exist for SSE instructions but not x87 instructions.

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Re:Std: Clamp generates less efficient assembly than std:min... by mhh__ on Tuesday January 16, 2024 @11:23AM (#39015126)

You're mistaking something else for the rounding mode and subnormal handling flags.

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