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

[Alan Mackenzie]

Hello, Eli.

On Sun, Jan 16, 2022 at 17:43:19 +0200, Eli Zaretskii wrote:
> > Date: Sun, 16 Jan 2022 15:26:44 +0000
> > Cc: larsi@gnus.org, emacs-devel@gnu.org
> > From: Alan Mackenzie <acm@muc.de>

> > > I thought the slowdown was in Lisp somewhere.  If it's in EQ, I'm not
> > > sure I understand how come the byte-compiler's slowdown is so much
> > > more significant than in other Lisp code.

> > The specification of the macro lisp_h_EQ in lisp.h L370 starts off:

> > #define lisp_h_EQ(x, y) ((XLI ((x)) == XLI ((y)))       \
> >   || (symbols_with_pos_enabled    \
> > .....

> > If symbols_with_pos_enabled is currently false, EQ need never execute
> > more than these first two lines.  If the variable is true, potentially
> > each of the other three cases (each variable x and y can be with or
> > without a position, giving four matching possibilities) needs to be
> > checked individually, which is slow.

> > Essentially the same code is coded up as emit_EQ inside comp.c.

> Yes, but my question was why would this affect the byte-compiler more
> than it affects any other arbitrary Lisp.

It is because symbols_with_pos_enabled is bound to true inside the byte
compiler (and the native compiler), but nowhere else.

> Are you saying that the byte-compiler is such a more intensive user of
> EQ than other Lisp programs?

Sort of - it uses all ten lines of lisp_h_EQ, rather than just the first
two lines used by all the other code.

-- 
Alan Mackenzie (Nuremberg, Germany).