How to avoid some built-in expansions in gcc?

[Michael Matz]

Hello,

On Sat, 1 Jun 2024, Richard Biener via Gcc wrote:

> >>> You have a pointer how to define a target optab? I looked into optabs code but found no appropriate hook.  For isinf<mode> is seems is is enough to provide a failing expander, but other functions like isnan don't have an optab entry, so there is a hook mechanism to extend optabs?
> >> Just add corresponding optabs for the missing cases (some additions are pending, like isnornal).  There’s no hook to prevent folding to FP compares nor is that guarded by other means (like availability of native FP ops).  Changing the guards would be another reasonable option.
> >> Richard
> > 
> > There are many other such folds, e.g. for isdigit().  The AVR libraries have all this in hand-optimized assembly, and all these built-in expansions are bypassing that.  Open-coded C will never beat that assemlbly code, at least not with the current GCC capabilities.
> 
> The idea is that isdigit() || isalpha() or similar optimize without 
> special casing the builtins.
> 
> > How would I go about disabling / bypassing non-const folds from ctype.h and the many others?
> 
> I think this mostly shows we lack late recognition of open-coded isdigit 
> and friends, at least for the targets where inlining them is not 
> profitable.
> 
> A pragmatic solution might be a new target hook, indicating a specified 
> builtin is not to be folded into an open-coded form.

Well, that's what the mechanism behind -fno-builtin-foobar is supposed to 
be IMHO.  Hopefully the newly added additional mechanism using optabs and 
ifns (instead of builtins) heeds it.

> A good solution would base this on (size) costs, the perfect solution 
> would re-discover the builtins late and undo inlining that didn’t turn 
> out to enable further simplification.
> 
> How is inlined isdigit bad on AVR?  Is a call really that cheap 
> considering possible register spilling around it?

On AVR with needing to use 8bit registers to do everything?  I'm pretty 
sure the call is cheaper, yeah :)


Ciao,
Michael.