[PATCH] libcpp: Use constexpr for _cpp_trigraph_map initialization for C++14

[Jakub Jelinek]

Hi!

The _cpp_trigraph_map initialization used to be done for C99+ using
designated initializers, but can't be done that way for C++ because
the designated initializer support in C++ as array designators are just
an extension there and don't allow skipping anything nor going backwards.

But, we can get the same effect using C++14 constexpr constructor.
With the following patch we get rid of the runtime initialization
and the array can be in .rodata.

Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?

2024-09-18  Jakub Jelinek  <ja...@redhat.com>

        * internal.h (_cpp_trigraph_map_s): New type for C++14 or later.
        (_cpp_trigraph_map_d): New variable for C++14 or later.
        (_cpp_trigraph_map): Define to _cpp_trigraph_map_d.map for C++14 or
        later.
        * init.cc (init_trigraph_map): Define to nothing for C++14 or later.
        (TRIGRAPH_MAP, END, s): Define differently for C++14 or later.

--- libcpp/internal.h.jj        2024-09-12 18:16:49.993409101 +0200
+++ libcpp/internal.h   2024-09-18 09:45:36.832570227 +0200
@@ -666,6 +666,12 @@ struct cpp_embed_params
    compiler that supports C99.  */
 #if HAVE_DESIGNATED_INITIALIZERS
 extern const unsigned char _cpp_trigraph_map[UCHAR_MAX + 1];
+#elif __cpp_constexpr >= 201304L
+extern const struct _cpp_trigraph_map_s {
+  unsigned char map[UCHAR_MAX + 1];
+  constexpr _cpp_trigraph_map_s ();
+} _cpp_trigraph_map_d;
+#define _cpp_trigraph_map _cpp_trigraph_map_d.map
 #else
 extern unsigned char _cpp_trigraph_map[UCHAR_MAX + 1];
 #endif
--- libcpp/init.cc.jj   2024-09-13 16:09:32.701455021 +0200
+++ libcpp/init.cc      2024-09-18 09:49:43.671189585 +0200
@@ -41,8 +41,8 @@ static void read_original_directory (cpp
 static void post_options (cpp_reader *);
 
 /* If we have designated initializers (GCC >2.7) these tables can be
-   initialized, constant data.  Otherwise, they have to be filled in at
-   runtime.  */
+   initialized, constant data.  Similarly for C++14 and later.
+   Otherwise, they have to be filled in at runtime.  */
 #if HAVE_DESIGNATED_INITIALIZERS
 
 #define init_trigraph_map()  /* Nothing.  */
@@ -52,6 +52,15 @@ __extension__ const uchar _cpp_trigraph_
 #define END };
 #define s(p, v) [p] = v,
 
+#elif __cpp_constexpr >= 201304L
+
+#define init_trigraph_map()  /* Nothing.  */
+#define TRIGRAPH_MAP \
+constexpr _cpp_trigraph_map_s::_cpp_trigraph_map_s () : map {} {
+#define END } \
+constexpr _cpp_trigraph_map_s _cpp_trigraph_map_d;
+#define s(p, v) map[p] = v;
+
 #else
 
 #define TRIGRAPH_MAP uchar _cpp_trigraph_map[UCHAR_MAX + 1] = { 0 }; \

        Jakub