Add configure tests for C11-ish _Alignof and _Alignas.

[dxcommon.git] / m4 / align.m4

diff --git a/m4/align.m4 b/m4/align.m4
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+# Copyright © 2024 Nick Bowler
+#
+# License WTFPL2: Do What The Fuck You Want To Public License, version 2.
+# This is free software: you are free to do what the fuck you want to.
+# There is NO WARRANTY, to the extent permitted by law.
+
+# DX_C_ALIGNOF
+#
+# Probe whether the C compiler understands _Alignof(T).
+#
+# If not supported, _Alignof(T) is defined as a function-like macro which
+# uses offsetof to guess the alignment of T, which must be a type such that
+# T x; is a valid declaration of x as an object of type T.  It is therefore
+# necessary that the caller includes <stddef.h> before using _Alignof.
+#
+# We skip the test if Autoconf has previously determined that the C compiler
+# supports C11 or a newer standard, since the C11 test program checks this.
+#
+# Annoyingly, autoconf-2.71 removed the assignment of ac_cv_prog_cc_c11,
+# even though this was actual documented behaviour...
+AC_DEFUN([DX_C_ALIGNOF],
+[AS_CASE([${ac_cv_prog_cc_c11-no}/${ac_prog_cc_stdc-no}],
+  [no/no|*/c89|*/c99],
+    [AC_CACHE_CHECK([if $CC supports _Alignof], [dx_cv_have_alignof],
+      [AC_COMPUTE_INT([_dx_tmp], [_Alignof(char)], [@&t@], [_dx_tmp=0])
+AS_CASE([$_dx_tmp], [1], [dx_cv_have_alignof=yes], [dx_cv_have_alignof=no])])],
+  [dx_cv_have_alignof=yes])
+AS_CASE([$dx_cv_have_alignof], [no], [AC_DEFINE([_Alignof(T)],
+  [offsetof(struct { char a; T b; }, b)],
+  [Define _Alignof(T) to a suitable fallback if _Alignof is unsupported.])])])
+
+# DX_C_ALIGNAS
+#
+# Probe whether the C compiler understands _Alignas(X).
+#
+# If not supported, but the compiler supports the GNU __attribute__ or
+# Microsoft __declspec methods to set alignment, _Alignas(X) is defined
+# as a function-like macro which expands to such syntax.  These only
+# work if X is an integer constant (as opposed to a type name).
+#
+# If there is no known method of declaring a variable with increased
+# alignment, then _Alignas(X) is defined as a function-like macro which
+# expands to nothing, which will compile but has no runtime effect.
+#
+# We skip the test if Autoconf has previously determined that the C
+# compiler supports C11 or a newer standard, since the C11 test program
+# checks this.
+AC_DEFUN([DX_C_ALIGNAS],
+[AS_CASE([${ac_cv_prog_cc_c11-no}/${ac_prog_cc_stdc-no}],
+  [no/no|*/c89|*/c99],
+    [AC_CACHE_CHECK([if $CC supports _Alignas], [dx_cv_have_alignas],
+[dx_cv_have_alignas=no
+for _dx_alignas
+in '_Alignas(X)' '__attribute__((__aligned__(X)))' '__declspec(align(X))'
+do
+AC_COMPUTE_INT([_dx_tmp],
+  [sizeof (struct { char a; char ALIGNAS_TEST(4) b; }) >= 8],
+  [#define ALIGNAS_TEST(X) $_dx_alignas
+],[_dx_tmp=0])
+AS_CASE([$_dx_tmp/$_dx_alignas],
+  [1/_Alignas*], [dx_cv_have_alignas=yes; break],
+  [1/*], [dx_cv_have_alignas=$_dx_alignas; break])
+done])],
+  [dx_cv_have_alignas=yes])
+AS_CASE([$dx_cv_have_alignas],
+  [*'(X)'*], [AC_DEFINE_UNQUOTED([_Alignas(X)], [$dx_cv_have_alignas],
+    [Define _Alignas(X) to a suitable fallback if _Alignas is unsupported.])],
+  [no], [AC_DEFINE([_Alignas(X)], [/**/])])])