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by pizlonator 2253 days ago
Llvm makes some questionable choices about how to do SSA, alias analysis, register allocation, and instruction selection. Also it goes all in on UB optimizations even when experience from other compilers shows that it’s not really needed. Maybe those choices are really fundamental and there is no escaping them to get peak perf - but you’re not going to know for sure until folks try alternatives. Those alternatives likely require building something totally new from scratch because we are talking about things that are fundamental to llvm even if they aren’t fundamental to compilers in general.
2 comments
temac 2253 days ago
I dislike UB, but I do at language level. When LLVM is reached, UB can only have and only be continued to be removed, never added (from a global point of view, applying general as-if rules a compiler can always generate its own boilerplate in which it knows something can not happen, then maybe latter leverage "UB" to e.g. trim impossible paths, that are really impossible in this case -- at least barring other language level "real" UB). So are there really any drawback to internal exploitation of "UB" (maybe we should call it otherwise then) if for example the source language had none?
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pizlonator 2253 days ago
It is true that compilers sometimes have to have operations that have a semantics that are defined only if some conditions hold. But LLVM's and C's interpretation of what happens when the conditions don't hold is extraordinarily liberal and I'm not sure that is either beneficial or sane.

Like, LLVM tries not to add UB, but design choices it made to support optimization with UB do sometimes result in new UB being introduced, like the horror show that happens with `undef` and code versioning.

So, I think that optimizing with UB internally is fine but only if it's some kind of bounded UB where you promise something stronger than nasal demons.

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aw1621107 2253 days ago
> Llvm makes some questionable choices about how to do SSA, alias analysis, register allocation, and instruction selection.

Do you mind expanding more on these points or directing me to some places where I can learn more about them? Compilers are a fairly new field for me, so anything I can learn about their design decisions and tradeoffs are worth their weight in gold.

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