Align config.h better to src/lisp.h

This is mostly a cleanup patch to fix growing discrepancies
between src/lisp.h and configure.ac in terms of how they deduce
how EMACS_INT aligns.  The patch includes a static check that the
two methods now agree.  It also speeds up ‘configure’ a bit.
* configure.ac (ALIGNOF_INT, ALIGNOF_LONG, ALIGNOF_LONG_LONG):
Remove; no longer used.
(ALIGNOF_EMACS_INT): New macro.
(system_malloc): Set to 'no' more consistently with what’s
in src/lisp.h.
* src/lisp.h (ALIGNOF_EMACS_INT): Do not define here, as config.h
defines it now.  Check that config.h’s definition equals the
actual alignof (EMACS_INT).
(USE_USB_TAG): Set more consistently with what’s in configure.ac.
(alignas): Don’t second-guess Gnulib.

1 files changed, 11 insertions, 20 deletions

diff --git a/src/lisp.h b/src/lisp.h
index 47442a29f50..e63019cc088 100644
--- a/src/lisp.h
+++ b/src/lisp.h
@@ -90,21 +90,18 @@ DEFINE_GDB_SYMBOL_END (GCTYPEBITS)
 typedef int EMACS_INT;
 typedef unsigned int EMACS_UINT;
 enum { EMACS_INT_WIDTH = INT_WIDTH, EMACS_UINT_WIDTH = UINT_WIDTH };
-#  define ALIGNOF_EMACS_INT ALIGNOF_INT
 #  define EMACS_INT_MAX INT_MAX
 #  define pI ""
 # elif INTPTR_MAX <= LONG_MAX && !defined WIDE_EMACS_INT
 typedef long int EMACS_INT;
 typedef unsigned long EMACS_UINT;
 enum { EMACS_INT_WIDTH = LONG_WIDTH, EMACS_UINT_WIDTH = ULONG_WIDTH };
-#  define ALIGNOF_EMACS_INT ALIGNOF_LONG
 #  define EMACS_INT_MAX LONG_MAX
 #  define pI "l"
 # elif INTPTR_MAX <= LLONG_MAX
 typedef long long int EMACS_INT;
 typedef unsigned long long int EMACS_UINT;
 enum { EMACS_INT_WIDTH = LLONG_WIDTH, EMACS_UINT_WIDTH = ULLONG_WIDTH };
-#  define ALIGNOF_EMACS_INT ALIGNOF_LONG_LONG
 #  define EMACS_INT_MAX LLONG_MAX
 /* MinGW supports %lld only if __USE_MINGW_ANSI_STDIO is non-zero,
    which is arranged by config.h, and (for mingw.org) if GCC is 6.0 or
@@ -123,6 +120,7 @@ enum { EMACS_INT_WIDTH = LLONG_WIDTH, EMACS_UINT_WIDTH = ULLONG_WIDTH };
 #  error "INTPTR_MAX too large"
 # endif
 #endif
+static_assert (alignof (EMACS_INT) == ALIGNOF_EMACS_INT);
 
 /* Number of bits to put in each character in the internal representation
    of bool vectors.  This should not vary across implementations.  */
@@ -252,15 +250,13 @@ DEFINE_GDB_SYMBOL_END (INTTYPEBITS)
     b. slower, because it typically requires extra masking.
    So, USE_LSB_TAG is true only on hosts where it might be useful.  */
 DEFINE_GDB_SYMBOL_BEGIN (bool, USE_LSB_TAG)
-#if (ALIGNOF_EMACS_INT < IDEAL_GCALIGNMENT && !defined alignas  \
-     && ! (__GNUC__ || 4 <= __clang_major__)                    \
-     && __STDC_VERSION__ < 202311 && __cplusplus < 201103)      \
-     && !defined WIDE_EMACS_INT                                 \
-     && !defined HAVE_STRUCT_ATTRIBUTE_ALIGNED
-#define USE_LSB_TAG 0
-#else /* ALIGNOF_EMACS_INT >= IDEAL_GCALIGNMENT || defined alignas ... */
-#define USE_LSB_TAG (VAL_MAX / 2 < INTPTR_MAX)
-#endif /* ALIGNOF_EMACS_INT >= IDEAL_GCALIGNMENT || defined alignas ... */
+#if (ALIGNOF_EMACS_INT < IDEAL_GCALIGNMENT \
+     && !HAVE_C_ALIGNASOF && !defined alignas \
+     && !defined HAVE_STRUCT_ATTRIBUTE_ALIGNED)
+# define USE_LSB_TAG false
+#else
+# define USE_LSB_TAG (VAL_MAX / 2 < INTPTR_MAX)
+#endif
 DEFINE_GDB_SYMBOL_END (USE_LSB_TAG)
 
 /* Mask for the value (as opposed to the type bits) of a Lisp object.  */
@@ -268,14 +264,9 @@ DEFINE_GDB_SYMBOL_BEGIN (EMACS_INT, VALMASK)
 # define VALMASK (USE_LSB_TAG ? - (1 << GCTYPEBITS) : VAL_MAX)
 DEFINE_GDB_SYMBOL_END (VALMASK)
 
-/* Support 'alignas (A)' if possible, where A is an integer constant.  */
-#ifndef alignas
-# if __GNUC__ || 4 <= __clang_major__
-/* This is more reliable than the alignas operator, in GCC 14.  */
-#  define alignas(a) __attribute__ ((__aligned__ (a)))
-# elif __STDC_VERSION__ < 202311 && __cplusplus < 201103
-#  define alignas(a) /* not supported */
-# endif
+/* Ignore 'alignas' on compilers lacking it.  */
+#if !HAVE_C_ALIGNASOF && !defined alignas
+# define alignas(a) /* not supported */
 #endif
 
 /* The minimum alignment requirement for Lisp objects that is imposed by the