[AArch64] Return the regnum for PC (32) on aarch64

This will allow the unwind info to explicitly specify a different value
for the return address from the link register.
Such usage, although uncommon, is valid and useful for signal frames.
It is also supported by aadwarf64 from ARM (Note 9 in [1]).

Ref https://sourceware.org/pipermail/gdb/2022-May/050091.html

[1] https://github.com/ARM-software/abi-aa/blob/2022Q1/aadwarf64/aadwarf64.rst#dwarf-register-names

Signed-off-by: Luis Machado <luis.machado@arm.com>

gdb/aarch64-tdep.c

@@ -2219,6 +2219,9 @@ aarch64_dwarf_reg_to_regnum (struct gdbarch *gdbarch, int reg)
   if (reg == AARCH64_DWARF_SP)
     return AARCH64_SP_REGNUM;
 
+  if (reg == AARCH64_DWARF_PC)
+    return AARCH64_PC_REGNUM;
+
   if (reg >= AARCH64_DWARF_V0 && reg <= AARCH64_DWARF_V0 + 31)
     return AARCH64_V0_REGNUM + reg - AARCH64_DWARF_V0;
 

gdb/aarch64-tdep.h

@@ -34,6 +34,7 @@ struct regset;
 /* AArch64 Dwarf register numbering.  */
 #define AARCH64_DWARF_X0   0
 #define AARCH64_DWARF_SP  31
+#define AARCH64_DWARF_PC  32
 #define AARCH64_DWARF_RA_SIGN_STATE  34
 #define AARCH64_DWARF_V0  64
 #define AARCH64_DWARF_SVE_VG   46

gdb/testsuite/gdb.arch/aarch64-unwind-pc.S

@@ -0,0 +1,48 @@
+/* Copyright 2022 Free Software Foundation, Inc.
+
+   This file is part of GDB.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+        .text
+        .globl main
+        .type main,#function
+main:
+        .cfi_startproc
+        stp     x29, x30, [sp, -16]!
+        .cfi_def_cfa sp, 16
+        .cfi_offset x29, 0
+        .cfi_offset x30, 8
+        bl test_func
+        ldp     x29, x30, [sp], 16
+        .cfi_restore x29
+        .cfi_restore x30
+        .cfi_def_cfa sp, 0
+        mov     x0, 0
+        ret
+        .cfi_endproc
+
+        .globl test_func
+test_func:
+        .cfi_startproc
+        // Unwind x30 to a different value
+        // CFA_val_expression x30 const2u 0x1234
+        .cfi_escape 0x16, 30, 0x03, 0x0a, 0x34, 0x12
+        // CFA_val_expression pc breg30 0
+        .cfi_escape 0x16, 32, 0x02, 0x8e, 0x00
+        mov     x0, x30
+        .cfi_register 32, x0
+        mov     x30, 0x1234
+        ret     x0
+        .cfi_endproc

gdb/testsuite/gdb.arch/aarch64-unwind-pc.exp

@@ -0,0 +1,70 @@
+# Copyright 2022 Free Software Foundation, Inc.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+# This file is part of the gdb testsuite.
+
+# Test explicitly unwinding the PC DWARF register on aarch64
+
+if {![is_aarch64_target]} then {
+    verbose "Skipping ${gdb_test_file_name}."
+    return
+}
+
+standard_testfile .S
+
+if { [prepare_for_testing "failed to prepare" $testfile $srcfile] } {
+    return -1
+}
+
+if ![runto_main] {
+    return -1
+}
+
+proc test_reg_vals {} {
+    gdb_test "p \$pc - &main" "= 8" "p \$pc"
+    gdb_test "p/x \$x30" "= 0x1234" "p \$x30"
+}
+
+proc test_unwind_pc { inst } {
+    gdb_test "si" "$inst" "single step"
+    gdb_test "backtrace" \
+	".*#1.*in main ().*" \
+	"backtrace"
+    gdb_test "up" "in main ().*" "parent frame"
+    test_reg_vals
+}
+
+# Ready to enter the function
+gdb_test "si" "bl test_func" "call site"
+# Step through the 3 instructions in the function to make sure that
+# we have the same unwind info throughout.
+with_test_prefix "1st stepi" {
+    test_unwind_pc "mov     x0, x30"
+}
+with_test_prefix "2nd stepi" {
+    test_unwind_pc "mov     x30, 0x1234"
+}
+with_test_prefix "3rd stepi" {
+    test_unwind_pc "ret     x0"
+}
+# Check again after we returned
+with_test_prefix "final" {
+    # Check that we've stepped out (si prints out the new function name)
+    gdb_test "si" ".*main *().*" "single step out"
+    gdb_test "backtrace" \
+	"#0\[\t \]+main ().*" \
+	"backtrace"
+    test_reg_vals
+}