Ch. 5 Small Issues - Githubissues

IUCompilerCourse / Essentials-of-Compilation

A book about compiling Racket and Python to x86-64 assembly

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Ch. 5 Small Issues #139

Closed AndrewTolmach closed 1 year ago

AndrewTolmach commented 1 year ago

p. 73: The discussion about implementing logical negation presupposes the choice of boolean encoding that is only made explicit at the start of 5.8.
Discussion at start of 5.5 is a bit misleading: the translation of and and or is essential to get the desired short-circuit semantics.
Relatedly, footnote 1 on p. 76 could allude to the translation of and and or as a source of nested conditionals, rather than talking about inlining.
p. 83: re the return statement; the "conclusion of the main function" is mysterious here; perhaps add a forward pointer to 5.11 ?
p. 84 first para: "However, there is no harm..." This seems an odd way to put it. We have no choice but to union the live-before sets.
5.10: It might be worth noting that a better solution to comparing two immediates at runtime is to partially evaluate the comparison away!

jsiek commented 1 year ago

Thanks Andrew! I've put in all but the last. Regarding that one, do you know how to set the eflags register directly? I think that would be needed for what you're proposing regarding the partial evaluation of the comparison. Or did you have something else in mind?

AndrewTolmach commented 1 year ago

Re the last point: I actually had in mind partial evaluation earlier on, e.g. at the source level, which could, e.g. convert 2<3 to True. (Of course, one would really also want to do if-simplification to get rid of the dead arms of conditionals that test constants.)
One could do something similar in patch-instructions, but only if it was allowed to match on multiple instructions at one time (e.g. convert cmpq $3,$2; jl to jmp). Obviously this is a bit fragile and may go beyond what you'd want to consider as a "patch." I don't see how to get an improvement over the current scheme if we can only match on the cmpq in isolation; I believe there is an instruction (sahf) that let you write the flags directly, but it requires loading %ah first, so we'd still need two instructions.

AndrewTolmach commented 1 year ago

I guess the larger point was just that students may (should?!) find it a bit odd that we up trying to generate code to perform a comparison at runtime whose outcome is known statically. No odder than adding 1+2 at runtime, of course!

jsiek commented 1 year ago

Ahh, the partial evaluation earlier on makes perfect sense, thanks!