Re:Six times faster than C
by bruce343434 on Saturday July 08, 2023 @03:15PM
(#36647746)
I looked at it closer. In C, it is a side effect to assign to a variable. For an extern function not annotated __attribute__((pure)) the compiler has to assume the function call generates side effects. This prevents it from reordering the assignment and function call. Since x86-64 ABI has caller saved registers, in the case where it calls eff() first, it has to save x and y, and after the call, restore them.
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The work on e-graphs I refer to is <https://egraphs-good.github.io/>—amortised rebuilds.
My example has nothing whatsoever to do with abi, and everything to do with ir. f and g are exactly semantically equivalent, and this equivalence is trivial to show; that the compilers generate different code for each demonstrates redundancies in their ir.
> it is a side effect to assign to a variable
But that variable is not aliased here.