Re: How to return markup conditionally?

[David Kastrup]

Jean Abou Samra <jean@abou-samra.fr> writes:

> Lol, now I understand.
>
> `module/language/cps/lower-primcalls.scm`:
>
> ```
> ;; precondition: pair is mutable pair
> (define-primcall-lowerer (set-car! cps k src #f (pair val))
>   (with-cps cps
>     (build-term
>       ($continue k src
>         ($primcall 'scm-set!/immediate '(pair . 0) (pair val))))))
> ```
>
>
> One of these lovely “optimizations” that the Guile developers are so
> fond of.

Frankly, one of the most annoying "optimisation" fights I had was with
GCC's standard library because the then-head-honcho insisted on keeping
"abort ()" marked as "no-return".  Because it does not return.

What it does, however, is create a core dump with a stack trace, and
that stack trace is comparatively useless if the compiler gets to assume
that the stack can be treated as outdated trash anyway.

I debugged for half an eternity on a failed assertion because the failed
assertion pinpointed a call site that was not actually being used other
than some unrelated code jumping to its abort () call in order to save a
byte of instruction.

So why would one tell the compiler that it does not need to retain a
useful stack when generating a stack trace?  Because the option is not
called "invalidate stack" but "no return".

In contrast, I find the optimisation that assumes that constants will
not be modified comparatively understandable.  Without that assumption,
you'll never be able to fold multiple constants.

-- 
David Kastrup