Re: [PATCH v2] _Generic.3: New page documenting _Generic()

[Alejandro Colomar <alx.manpages@gmail.com>]

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Hi Florian,

On 8/23/22 09:58, Florian Weimer wrote:
> Note that this approach does not really work that well in practice
> because macros using _Generic expand all the alternatives (in current
> implementations; doing this differently requires deviating from the
> layered implementation strategy suggested in the C standard).  This
> means that _Generic-using macros can only be nested maybe three or four
> levels deep, depending on the number of _Generic alternatives on each
> level.  For <tgmath.h>, this is really not enough, so a high-quality
> implementation of <tgmath.h> using _Generic is not feasible.  GCC
> provides __builtin_tgmath, which is designed in such a way that when
> used in a macro, the macro argument is only expanded once.
> 
> Maybe mention this under BUGS?
> 
> C++ templates do not suffer from this particular problem.


Heh, I don't know how this didn't oocur to me before.  Well, maybe it's 
because it's non-standard (but the standard might very well benefit from 
adding this, IMO).

An always_inline function with no extern definition behaves as if it 
were a macro (trying to take a pointer to it, or something that needs a 
linker symbol, will result in linker errors, which is not the most 
readable error, but good enough), in the sense that it doesn't have ABI 
issues, but has the benefit of not creating code exponentially.


Cheers,

Alex

---

diff --git a/man3/_Generic.3 b/man3/_Generic.3
index f3daf98c1..3bd5f306c 100644
--- a/man3/_Generic.3
+++ b/man3/_Generic.3
@@ -30,7 +30,9 @@ C11 and later.
  The following program demonstrates how to write
  a replacement for the standard
  .BR imaxabs (3)
-function, which being a function can't really provide what it promises:
+function, which being an
+.I extern
+function can't really provide what it promises:
  seamlessly upgrading to the widest available type.
  .PP
  .\" SRC BEGIN (_Generic.c)
@@ -39,11 +41,16 @@ seamlessly upgrading to the widest available type.
  #include <stdio.h>
  #include <stdlib.h>

-#define my_imaxabs(j)  _Generic(INTMAX_C(0),  \e
-    long:           labs(j),                  \e
-    long long:      llabs(j)                  \e
- /* long long long: lllabs(j) */              \e
-)
+[[gnu::always_inline]]
+inline intmax_t
+my_imaxabs(intmax_t j)
+{
+    return _Generic(j,
+        long:           labs(j),
+        long long:      llabs(j)
+     /* long long long: lllabs(j) */
+    )
+}

  int
  main(void)




-- 
Alejandro Colomar
<http://www.alejandro-colomar.es/>

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