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Make abs
, wrapping_abs
, overflowing_abs
const functions
#63786
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I am not a big fan of making code less readable just to please our const qualification. :/
Cc @oli-obk @eddyb
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We've had variables for a while, please use them.
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Has it been verified that this optimizes to identical LLVM IR?
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@eddyb Since this has already passed final comment period and been merged, shall I open a new PR to use variables?
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@nikic I can't read LLVM IR; what I did do before submitting the PR was to use llvm-mca, and although the assembly is not identical, the performance measure from llvm-mca was the same, or if I remember well, in some tests the reverse throughput improved by something tiny like 0.1, but not regressed. I only used llvm-mca with i686 or x86-64 instructions, and I don't think I tested all types comprehensively.
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@nikic Hmm, using variables as @eddyb suggested seems to hit two birds with one stone. If I insert a variable for the sign, I get one implementation and an alias: https://godbolt.org/z/m-seT0
Does that mean that the performance will not regress?
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@tspiteri Oops, I simply made a typo in my test and that's why the result is different, duh :) It actually does produce the same IR, both with variable and without. So forget everything I said, this change is fine from the optimization perspective.
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@nikic While
abs
andwrapping_abs
seem to be fine performance-wise,overflowing_abs
from this PR actually does suffer from the problem discussed, so I will not forget everything you said! :) It's easily fixed by making it return(self.wrapping_abs(), self == Self::min_value())
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And also, the code with the sign factored out is more readable. ;)
That said, I still think we should rather wait for CTFE to support conditionals than compromise code readability, and even the
let sign
version is still arcane magic. But that call is up to T-libs.There was a problem hiding this comment.
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No, we should definitely make as many of these small ops into
const
as soon as we can.