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.

configure.ac

@@ -3227,8 +3227,13 @@ to configure.])
 fi
 
 # Configure gnulib.
-# Although this does not affect CFLAGS or LIBS permanently.
-# it temporarily reverts them to their pre-pkg-config values,
+#
+# Do this after any CC or CFLAGS changes that affect Gnulib,
+# and before any tests that depend on gnulib tests,
+# e.g., the system_malloc=no test below.
+#
+# Do not affect CFLAGS or LIBS permanently.
+# Instead, temporarily revert them to their pre-pkg-config values,
 # because gnulib needs to work with both src (which uses the
 # pkg-config stuff) and lib-src (which does not).  For example, gnulib
 # may need to determine whether CLOCK_TIME_LIB should contain -lrt,
@@ -3266,9 +3271,19 @@ AC_CACHE_CHECK(
    fi])
 doug_lea_malloc=$emacs_cv_var_doug_lea_malloc
 
-AC_CHECK_ALIGNOF([int])
-AC_CHECK_ALIGNOF([long])
-AC_CHECK_ALIGNOF([long long])
+AC_CHECK_ALIGNOF([EMACS_INT],
+  [[#include <limits.h>
+    #include <stddef.h>
+    #include <stdint.h>
+    #if INTPTR_MAX <= INT_MAX && !defined WIDE_EMACS_INT
+    typedef int EMACS_INT;
+    #elif INTPTR_MAX <= LONG_MAX && !defined WIDE_EMACS_INT
+    typedef long int EMACS_INT;
+    #elif INTPTR_MAX <= LLONG_MAX
+    typedef long long int EMACS_INT;
+    #endif
+  ]])
+
 AC_CHECK_SIZEOF([long])
 
 AC_CACHE_CHECK([for struct alignment],
@@ -3293,40 +3308,27 @@ system_malloc=yes
 # adequately positioned memory, enable the GNU malloc, which more
 # consistently provides allocations at low addresses, as is required for
 # the pdumper to load dump files at a representable location.
-AS_IF([test "$with_pdumper" = "yes" && test "$with_wide_int" != "yes"],
-  AC_CHECK_HEADERS([stdalign.h])
-  [AC_CACHE_CHECK([whether alignas is required yet unavailable],
-    [emacs_cv_alignas_unavailable],
-    [AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[
-#ifdef HAVE_STDALIGN_H
-#include <stdalign.h>
-#endif
-      #include <limits.h>
-     ]], [[
-#define IDEAL_GCALIGNMENT 8
-#if INTPTR_MAX <= INT_MAX && !defined WIDE_EMACS_INT
-#  define ALIGNOF_EMACS_INT ALIGNOF_INT
-# elif INTPTR_MAX <= LONG_MAX && !defined WIDE_EMACS_INT
-#  define ALIGNOF_EMACS_INT ALIGNOF_LONG
-# elif INTPTR_MAX <= LLONG_MAX
-#  define ALIGNOF_EMACS_INT ALIGNOF_LONG_LONG
-# else
-# error "INTPTR_MAX too large"
-#endif
+AS_CASE([$emacs_cv_struct_alignment%$gl_cv_header_working_stdalign_h%$with_wide_int%$ac_cv_alignof_EMACS_INT%$with_pdumper],
+  [*%*%*%*%*%*],
+    [AC_MSG_ERROR([configuration values cannot contain '%'])],
 
-#if (ALIGNOF_EMACS_INT < IDEAL_GCALIGNMENT && !defined alignas	\
-     && !defined HAVE_STRUCT_ATTRIBUTE_ALIGNED	       		\
-     && !defined __alignas_is_defined				\
-     && __STDC_VERSION__ < 202311 && __cplusplus < 201103)
-#error "!USE_LSB_TAG required"
-#endif
-   ]])], [emacs_cv_alignas_unavailable=no],
-        [emacs_cv_alignas_unavailable=yes])])
-   AS_IF([test "$emacs_cv_alignas_unavailable" = "yes"],
-     [system_malloc=no
-       AC_MSG_WARN([The GNU memory manager will be enabled as your system
-does not guarantee that the portable dumper can allocate memory at a suitably
-low address.])])])
+  dnl There is no problem in most situations, i.e.:
+  [dnl if emacs_cv_struct_alignment
+   yes%* \
+   dnl or if gl_cv_header_working_stdalign_h
+   | *%yes*%*%*%* \
+   dnl or if with_wide_int (as in practice tag bits are high-order)
+   | *%*%yes%*%* \
+   dnl or if EMACS_INT's alignment is at least 8.
+   | *%*%*%8%* | *%*%*%[1-9]?*%*],
+    [],
+
+  dnl Otherwise, if using the portable dumper, enable GNU malloc.
+  [*%yes],
+    [system_malloc=no
+     AC_MSG_WARN([The GNU memory manager will be enabled as your system
+       does not guarantee that the portable dumper can allocate memory
+       at a suitably low address.])])
 
 dnl This must be before the test of $ac_cv_func_sbrk below.
 AC_CHECK_FUNCS_ONCE([sbrk])

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