[m-rev.] diff: use global registers on aarch64

[Julien Fischer]

The diff below is sufficient to allow Mercury to bootcheck in grade
reg.gc on my aarch64 Linux system.  I'll update the documentation and
NEWS after I've bootchecked in further reg.* grades.

------------------------------

Use global registers on aarch64.

runtime/machdeps/aarch64_regs.h:
runtime/mercury_regs.h:
      Define global registers for aarch64.

configure.ac:
      Compile with -fomit-frame-pointer on aarch64.  (Doing so, frees
      up register x29.)

      Compile with -fPIC instead of -fpic on aarch64-*-linux* systems
      in order to avoid size limits on the global offset table.

Julien.

diff --git a/configure.ac b/configure.ac
index c7afb1255..ea491cf41 100644
--- a/configure.ac
+++ b/configure.ac
@@ -2,7 +2,7 @@
  # vim: ts=4 sw=4 expandtab
  #-----------------------------------------------------------------------------#
  # Copyright (C) 1995-2012 The University of Melbourne.
-# Copyright (C) 2013-2020 The Mercury team.
+# Copyright (C) 2013-2021 The Mercury team.
  # This file may only be copied under the terms of the GNU General
  # Public Licence - see the file COPYING in the Mercury distribution.
  #-----------------------------------------------------------------------------#
@@ -2420,10 +2420,11 @@ case "$ac_cv_prog_gcc" in yes)
              mercury_cv_gcc_labels=no
              ;;
          aarch64*)
-        # The test for GCC labels goes into a loop on AArch64.
+            # The test for GCC labels goes into a loop on AArch64.
              MERCURY_MSG("gcc labels do not work on AArch64")
              mercury_cv_asm_labels=no
              mercury_cv_gcc_labels=no
+            CFLAGS_FOR_REGS="-fno-omit-frame-pointer"
              ;;
      esac
      ;;
@@ -4186,6 +4187,9 @@ if test "$mercury_cv_enable_shared_libs" = "yes"; then
              ;;
          aarch64*-linux|aarch64*-linux-gnu)
              AC_MSG_RESULT(yes)
+            # On aarch64 machines we must use -fPIC rather than -fpic in order
+            # to avoid limits on the size of the global offset table.
+            CFLAGS_FOR_PIC="-fPIC -DMR_PIC"
              EXT_FOR_SHARED_LIB=so
              ;;
          *-freebsd*)
diff --git a/runtime/machdeps/aarch64_regs.h b/runtime/machdeps/aarch64_regs.h
index e69de29bb..9b612310a 100644
--- a/runtime/machdeps/aarch64_regs.h
+++ b/runtime/machdeps/aarch64_regs.h
@@ -0,0 +1,73 @@
+/*
+** Copyright (C) 2021 The Mercury team.
+** This file is distributed under the terms specified in COPYING.LIB.
+*/
+#ifndef MR_MACHDEPS_AARCH64_REGS_H
+#define MR_MACHDEPS_AARCH64_REGS_H
+
+#define MR_NUM_REAL_REGS 4
+
+/*
+** Machine registers MR_mr0 - MR_mr36 for the ARM architecture.
+*/
+
+register        MR_Word MR_mr0 __asm__("x19");  /* sp */
+register        MR_Word MR_mr1 __asm__("x20");  /* succip */
+register        MR_Word MR_mr2 __asm__("x21");  /* r1 */
+register        MR_Word MR_mr3 __asm__("x22");  /* r2 */
+
+#define MR_save_regs_to_mem(save_area) (        \
+        save_area[0] = MR_mr0,                  \
+        save_area[1] = MR_mr1,                  \
+        save_area[2] = MR_mr2,                  \
+        save_area[3] = MR_mr3,                  \
+        (void)0                                 \
+ )
+
+#define MR_restore_regs_from_mem(save_area) (   \
+        MR_mr0 = save_area[0],                  \
+        MR_mr1 = save_area[1],                  \
+        MR_mr2 = save_area[2],                  \
+        MR_mr3 = save_area[3],                  \
+        (void)0                                 \
+)
+
+#define MR_save_transient_regs_to_mem(save_area)        ((void)0)
+#define MR_restore_transient_regs_from_mem(save_area)   ((void)0)
+ 
+#define MR_mr4  MR_fake_reg[4]
+#define MR_mr5  MR_fake_reg[5]
+#define MR_mr6  MR_fake_reg[6]
+#define MR_mr7  MR_fake_reg[7]
+#define MR_mr8  MR_fake_reg[8]
+#define MR_mr9  MR_fake_reg[9]
+#define MR_mr10 MR_fake_reg[10]
+#define MR_mr11 MR_fake_reg[11]
+#define MR_mr12 MR_fake_reg[12]
+#define MR_mr13 MR_fake_reg[13]
+#define MR_mr14 MR_fake_reg[14]
+#define MR_mr15 MR_fake_reg[15]
+#define MR_mr16 MR_fake_reg[16]
+#define MR_mr17 MR_fake_reg[17]
+#define MR_mr18 MR_fake_reg[18]
+#define MR_mr19 MR_fake_reg[19]
+#define MR_mr20 MR_fake_reg[20]
+#define MR_mr21 MR_fake_reg[21]
+#define MR_mr22 MR_fake_reg[22]
+#define MR_mr23 MR_fake_reg[23]
+#define MR_mr24 MR_fake_reg[24]
+#define MR_mr25 MR_fake_reg[25]
+#define MR_mr26 MR_fake_reg[26]
+#define MR_mr27 MR_fake_reg[27]
+#define MR_mr28 MR_fake_reg[28]
+#define MR_mr29 MR_fake_reg[29]
+#define MR_mr30 MR_fake_reg[30]
+#define MR_mr31 MR_fake_reg[31]
+#define MR_mr32 MR_fake_reg[32]
+#define MR_mr33 MR_fake_reg[33]
+#define MR_mr34 MR_fake_reg[34]
+#define MR_mr35 MR_fake_reg[35]
+#define MR_mr36 MR_fake_reg[36]
+#define MR_mr37 MR_fake_reg[37]
+
+#endif /* not MR_MACHDEPS_ARM_REGS_H */
diff --git a/runtime/mercury_regs.h b/runtime/mercury_regs.h
index daec4f9de..f7f611a82 100644
--- a/runtime/mercury_regs.h
+++ b/runtime/mercury_regs.h
@@ -95,6 +95,8 @@
      #include "machdeps/ia64_regs.h"
    #elif defined(__arm__)
      #include "machdeps/arm_regs.h"
+  #elif defined(__aarch64__)
+    #include "machdeps/aarch64_regs.h"
    #else
      #error "MR_USE_GCC_GLOBAL_REGISTERS not yet supported on this machine."
    #endif