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Re: Time to merge scratch/correct-warning-pos into master, perhaps?

From: Stefan Monnier
Subject: Re: Time to merge scratch/correct-warning-pos into master, perhaps?
Date: Tue, 25 Jan 2022 08:27:55 -0500
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/29.0.50 (gnu/linux)

> The C level EQ operation takes twice as long when the operands do not
> match, and the same time as before when they do match.

I highly doubt reality is that simple because CPU performance is a lot
more complex than just counting instructions.

> When one of the operands is a literal nil, there is no increase in
> execution time.

That's an oversimplification as well: it's only when we use the `NILP`
macro that the extra cost is avoided.  Any use of `EQ` is still slowed
down when it happens to receive a nil as one of the arguments.

I do find the slowdowns discussed here rather worrisome.
I thought the original agreement was that it was OK to install this
change if the slowdown could be brought down to about 1% or below (for
the non-compilation case).

More importantly, I wonder how slowing down EQ by a factor of 2 can end
up costing 10% of runtime when running the test suite.  I think this
deserves investigation.


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