bug#63556: 29.0.90; Use of _Generic breaks Emacs build on GCC <4.9

[Paul Eggert <eggert@cs.ucla.edu>]

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On 2023-05-17 10:38, Spencer Baugh wrote:
> I think it's because I'm on gcc (GCC) 4.8.5 20150623 (Red Hat 4.8.5-44),
> and _Generic was added in 4.9?

Thanks for reporting the problem. Since _Generic is used here merely as 
a micro-optimization on 32-bit platforms, it's not worth whatever 
porting hassle you've run into, so I installed the attached patch so 
that modiff no longer uses _Generic.

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From 0bba1b8c3df8b7b53e08fd69dcc832d253e7a2d1 Mon Sep 17 00:00:00 2001
From: Paul Eggert <eggert@cs.ucla.edu>
Date: Wed, 17 May 2023 10:53:26 -0700
Subject: [PATCH] Port modiff changes to GCC 4.8.5
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Problem reported by Spencer Baugh.
* src/lisp.h (elogb): Now simply a function on unsigned long long int.
This avoids problems porting to GCC 4.8.5 20150623 (Red Hat 4.8.5-44).
There may be a minor runtime cost on 32-bit platforms but it’s
not worth worrying about.
---
 src/lisp.h | 17 +++++------------
 1 file changed, 5 insertions(+), 12 deletions(-)

diff --git a/src/lisp.h b/src/lisp.h
index c9a64f07427..57c1c7395c1 100644
--- a/src/lisp.h
+++ b/src/lisp.h
@@ -3906,18 +3906,11 @@ integer_to_uintmax (Lisp_Object num, uintmax_t *n)
 }
 
 /* Return floor (log2 (N)) as an int, where 0 < N <= ULLONG_MAX.  */
-#if (201112 <= __STDC_VERSION__ && INT_MAX <= UINT_MAX \
-     && LONG_MAX <= ULONG_MAX && LLONG_MAX <= ULLONG_MAX)
-# define elogb(n) \
-    _Generic (+(n), \
-	      int:           UINT_WIDTH   - 1 - count_leading_zeros    (n), \
-	      unsigned int:  UINT_WIDTH   - 1 - count_leading_zeros    (n), \
-	      long:          ULONG_WIDTH  - 1 - count_leading_zeros_l  (n), \
-	      unsigned long: ULONG_WIDTH  - 1 - count_leading_zeros_l  (n), \
-	      default:       ULLONG_WIDTH - 1 - count_leading_zeros_ll (n))
-#else
-# define elogb(n) (ULLONG_WIDTH - 1 - count_leading_zeros_ll (n))
-#endif
+INLINE int
+elogb (unsigned long long int n)
+{
+  return ULLONG_WIDTH - 1 - count_leading_zeros_ll (n);
+}
 
 /* A modification count.  These are wide enough, and incremented
    rarely enough, so that they should never overflow a 60-bit counter
-- 
2.39.2