ada: Rework the Android sigtramp implementation

The initial signal handling code introduced for aarch64-android
overlooked details of the tasking runtime, not in the initial testing
perimeter.

Specifically, a reference to __gnat_sigtramp from __gnat_error_handler,
initially introduced for the arm port, was prevented if !arm on the
grounds that other ports would rely on kernel CFI. aarch64-android
does provide kernel CFI and __gnat_sigtramp was not provided for this
configuration.

But there is a similar reference from s-intman__android, which kicks in
as soon as the tasking runtime gets activated, triggering link failures.

Testing for more precise target specific parameters from Ada
code is inconvenient and replicating the logic is not attractive in
any case, so this change addresses the problem in the following
fashion:

- Always provide a __gnat_sigtramp entry point, common to the
  tasking and non-tasking signal handling code for all the Android
  configurations,

- There (C code), from target definition macros, select a path
  that either routes directly to the actual signal handler or goes
  through the intermediate layer providing hand crafted CFI
  information which allows unwinding up to the interrupted code.

- Similarily to what was done for VxWorks, move the arm specific
  definitions to a separate header file to make the general structure
  of the common C code easier to grasp,

- Adjust the comments in the common sigtramp.h header to
  account for such an organisation possibility.

gcc/ada/ChangeLog:
	* sigtramp-armdroid.c: Refactor into ...
	* sigtramp-android.c, sigtramp-android-asm.h: New files.
	* Makefile.rtl (arm/aarch64-android section): Add
	sigtramp-android.o to EXTRA_LIBGNAT_OBJS unconditionally. Add
	sigtramp.h and sigtramp-android-asm.h to EXTRA_LIBGNAT_SRCS.
	* init.c (android section, __gnat_error_handler): Defer to
	__gnat_sigramp unconditionally again.
	* sigtramp.h: Adjust comments to allow neutral signal handling
	relays, merely forwarding to the underlying handler without any
	intermediate CFI magic.

1 files changed, 112 insertions, 0 deletions

diff --git a/gcc/ada/sigtramp-android-asm.h b/gcc/ada/sigtramp-android-asm.h
new file mode 100644
index 0000000..72cebae
--- /dev/null
+++ b/gcc/ada/sigtramp-android-asm.h
@@ -0,0 +1,112 @@
+/****************************************************************************
+ *                                                                          *
+ *                         GNAT COMPILER COMPONENTS                         *
+ *                                                                          *
+ *                      S I G T R A M P - T A R G E T                       *
+ *                                                                          *
+ *                     Asm Implementation Include File                      *
+ *                                                                          *
+ *           Copyright (C) 2024, Free Software Foundation, Inc.             *
+ *                                                                          *
+ * GNAT is free software;  you can  redistribute it  and/or modify it under *
+ * terms of the  GNU General Public License as published  by the Free Soft- *
+ * ware  Foundation;  either version 3,  or (at your option) any later ver- *
+ * sion.  GNAT is distributed in the hope that it will be useful, but WITH- *
+ * OUT ANY WARRANTY;  without even the  implied warranty of MERCHANTABILITY *
+ * or FITNESS FOR A PARTICULAR PURPOSE.                                     *
+ *                                                                          *
+ * As a special exception under Section 7 of GPL version 3, you are granted *
+ * additional permissions described in the GCC Runtime Library Exception,   *
+ * version 3.1, as published by the Free Software Foundation.               *
+ *                                                                          *
+ * In particular,  you can freely  distribute your programs  built with the *
+ * GNAT Pro compiler, including any required library run-time units,  using *
+ * any licensing terms  of your choosing.  See the AdaCore Software License *
+ * for full details.                                                        *
+ *                                                                          *
+ * GNAT was originally developed  by the GNAT team at  New York University. *
+ * Extensive contributions were provided by Ada Core Technologies Inc.      *
+ *                                                                          *
+ ****************************************************************************/
+
+/*****************************************************************
+ * CPU specific parts of the __gnat_sigtramp service for Android *
+ *****************************************************************/
+
+/* The intended use mode of this header is to provide macros
+   and a prologue to the generation of an asm function, as in
+
+     #include <this-header>
+     asm (SIGTRAMP_START(<symbol-name>));
+     asm (SIGTRAMP_BODY);
+     asm (SIGTRAMP_END(<symbol-name>));
+
+   and nothing else after.  */
+
+/* asm string construction helpers.  */
+
+#define STR(TEXT) #TEXT
+/* stringify expanded TEXT, surrounding it with double quotes.  */
+
+#define S(E) STR(E)
+/* stringify E, which will resolve as text but may contain macros
+   still to be expanded.  */
+
+/* asm (TEXT) outputs <tab>TEXT. These facilitate the output of
+   multiline contents:  */
+#define TAB(S) "\t" S
+#define CR(S)  S "\n"
+
+#undef TCR
+#define TCR(S) TAB(CR(S))
+
+#if defined(__arm__)
+
+/* Trampoline body block
+   ---------------------  */
+
+#define SIGTRAMP_BODY \
+CR("") \
+TCR("# Allocate frame and also save r2 which is the argument register") \
+TCR("# containing the sigcontext, so that we can restore it during") \
+TCR("# unwinding and thereby load the rest of the desired context.") \
+TCR("stmfd	sp!, {r2, r3, lr}") \
+TCR("# The unwinder undo's these operations in reverse order so starting") \
+TCR("# from bottom, restore r2 from the current vsp location, move r2 into") \
+TCR("# the vsp, add 12 bytes to get the start of the register save area") \
+TCR("# then restore the 15 general purpose registers of the frame which") \
+TCR("# raised the signal.") \
+TCR(".save {r0-r15}") \
+TCR(".pad #12") \
+TCR(".movsp r2") \
+TCR(".save {r2}") \
+TCR("# Call the real handler. The signo, siginfo and sigcontext") \
+TCR("# arguments are the same as those we received in r0, r1 and r2.") \
+TCR("blx	r3") \
+TCR("# Restore our callee-saved items, release our frame and return") \
+TCR("# (should never get here!).") \
+TCR("ldmfd	sp, {r2, r3, pc}")
+
+/* Symbol definition block
+   -----------------------  */
+
+#define SIGTRAMP_START(SYM) \
+CR("# " S(SYM) " unwind trampoline") \
+TCR(".type " S(SYM) ", %function") \
+CR("") \
+CR(S(SYM) ":") \
+TCR(".fnstart")
+
+/* Symbol termination block
+   ------------------------  */
+
+#define SIGTRAMP_END(SYM) \
+CR(".fnend") \
+TCR(".size " S(SYM) ", .-" S(SYM))
+
+#endif
+
+/* Text section start.  The compiler isn't aware of that switch.  */
+
+asm (".text\n"
+     TCR(".align 2"));