[gcc r15-4125] libcpp: Use constexpr for _cpp_trigraph_map initialization for C++14

[Jakub Jelinek]

https://gcc.gnu.org/g:e4c0595ec4ed3cd2f6fb471081a9d2d3960e1672

commit r15-4125-ge4c0595ec4ed3cd2f6fb471081a9d2d3960e1672
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Mon Oct 7 21:25:22 2024 +0200

    libcpp: Use constexpr for _cpp_trigraph_map initialization for C++14
    
    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.
    
    2024-10-07  Jakub Jelinek  <jakub@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.

Diff:
---
 libcpp/init.cc    | 13 +++++++++++--
 libcpp/internal.h |  6 ++++++
 2 files changed, 17 insertions(+), 2 deletions(-)

diff --git a/libcpp/init.cc b/libcpp/init.cc
index 2c80d63a491d..3e4a2bc0ae79 100644
--- a/libcpp/init.cc
+++ b/libcpp/init.cc
@@ -41,8 +41,8 @@ static void read_original_directory (cpp_reader *);
 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_map[UCHAR_MAX + 1] = {
 #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 }; \
diff --git a/libcpp/internal.h b/libcpp/internal.h
index b69a0377f024..2379fbba8996 100644
--- a/libcpp/internal.h
+++ b/libcpp/internal.h
@@ -668,6 +668,12 @@ 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