Hello,

This patch teaches the DWARF support in gcc about RA_AUTH_CODE pseudo hard-register and also 
.save {ra_auth_code} and .cfi_offset ra_auth_code <offset> dwarf directives for the PAC feature
in Armv8.1-M architecture.

RA_AUTH_CODE register number is 107 and it's dwarf register number is 143.

When compiled with "arm-none-eabi-gcc -O2  -mthumb -march=armv8.1-m.main+pacbti -S -fasynchronous-unwind-tables -g"
command line options, the directives supported in this patch looks like below:

        ...
        push    {ip}
        .save {ra_auth_code}
        .cfi_def_cfa_offset 8
        .cfi_offset 143, -8
        ...

This patch can be committed after the patch at https://gcc.gnu.org/pipermail/gcc-patches/2021-November/583407.html
is committed.

Regression tested on arm-none-eabi target and found no regressions.

Ok for master?

Regards,
Srinath.

gcc/ChangeLog:

2021-11-12  Srinath Parvathaneni  <srinath.parvathaneni@arm.com>

        * config/arm/aout.h (ra_auth_code): Add to enum.
        * config/arm/arm.c (emit_multi_reg_push): Add RA_AUTH_CODE register to
        dwarf frame expression instead of IP_REGNUM.
        (arm_expand_prologue): Mark as frame related insn.
        (arm_regno_class): Check for pac pseudo reigster.
        (arm_dbx_register_number): Assign ra_auth_code register number in dwarf.
        (arm_unwind_emit_sequence): Print .save directive with ra_auth_code
        register.
        (arm_conditional_register_usage): Mark ra_auth_code in fixed reigsters.
        * config/arm/arm.h (FIRST_PSEUDO_REGISTER): Modify.
        (IS_PAC_Pseudo_REGNUM): Define.
        (enum reg_class): Add PAC_REG entry.
        * config/arm/arm.md (RA_AUTH_CODE): Define.

gcc/testsuite/ChangeLog:

2021-11-12  Srinath Parvathaneni  <srinath.parvathaneni@arm.com>

        * g++.target/arm/pac-1.C: New test.


###############     Attachment also inlined for ease of reply    ###############


diff --git a/gcc/config/arm/aout.h b/gcc/config/arm/aout.h
index 25a2812a663742893b928398b0d3948e97f1905b..c69e299e012f46c8d0711830125dbf2f6b2e93d7 100644
--- a/gcc/config/arm/aout.h
+++ b/gcc/config/arm/aout.h
@@ -74,7 +74,8 @@
   "wr8",   "wr9",   "wr10",  "wr11",				\
   "wr12",  "wr13",  "wr14",  "wr15",				\
   "wcgr0", "wcgr1", "wcgr2", "wcgr3",				\
-  "cc", "vfpcc", "sfp", "afp", "apsrq", "apsrge", "p0"		\
+  "cc", "vfpcc", "sfp", "afp", "apsrq", "apsrge", "p0",		\
+  "ra_auth_code"						\
 }
 #endif
 
diff --git a/gcc/config/arm/arm.h b/gcc/config/arm/arm.h
index 8e6ef41f6b065217d1af3f4f1cb85b2d8fbd0dc0..f31944e85c9ab83501f156d138e2aea1bcb5b79d 100644
--- a/gcc/config/arm/arm.h
+++ b/gcc/config/arm/arm.h
@@ -815,7 +815,8 @@ extern const int arm_arch_cde_coproc_bits[];
 	s16-s31	      S	VFP variable (aka d8-d15).
 	vfpcc		Not a real register.  Represents the VFP condition
 			code flags.
-	vpr		Used to represent MVE VPR predication.  */
+	vpr		Used to represent MVE VPR predication.
+	ra_auth_code	Pseudo register to save PAC.  */
 
 /* The stack backtrace structure is as follows:
   fp points to here:  |  save code pointer  |      [fp]
@@ -856,7 +857,7 @@ extern const int arm_arch_cde_coproc_bits[];
   1,1,1,1,1,1,1,1,		\
   1,1,1,1,			\
   /* Specials.  */		\
-  1,1,1,1,1,1,1			\
+  1,1,1,1,1,1,1,1		\
 }
 
 /* 1 for registers not available across function calls.
@@ -886,7 +887,7 @@ extern const int arm_arch_cde_coproc_bits[];
   1,1,1,1,1,1,1,1,		\
   1,1,1,1,			\
   /* Specials.  */		\
-  1,1,1,1,1,1,1			\
+  1,1,1,1,1,1,1,1		\
 }
 
 #ifndef SUBTARGET_CONDITIONAL_REGISTER_USAGE
@@ -1062,10 +1063,10 @@ extern const int arm_arch_cde_coproc_bits[];
    && (LAST_VFP_REGNUM - (REGNUM) >= 2 * (N) - 1))
 
 /* The number of hard registers is 16 ARM + 1 CC + 1 SFP + 1 AFP
-   + 1 APSRQ + 1 APSRGE + 1 VPR.  */
+   + 1 APSRQ + 1 APSRGE + 1 VPR + 1 Pseudo register to save PAC.  */
 /* Intel Wireless MMX Technology registers add 16 + 4 more.  */
 /* VFP (VFP3) adds 32 (64) + 1 VFPCC.  */
-#define FIRST_PSEUDO_REGISTER   107
+#define FIRST_PSEUDO_REGISTER   108
 
 #define DBX_REGISTER_NUMBER(REGNO) arm_dbx_register_number (REGNO)
 
@@ -1248,12 +1249,15 @@ extern int arm_regs_in_sequence[];
   CC_REGNUM, VFPCC_REGNUM,			\
   FRAME_POINTER_REGNUM, ARG_POINTER_REGNUM,	\
   SP_REGNUM, PC_REGNUM, APSRQ_REGNUM,		\
-  APSRGE_REGNUM, VPR_REGNUM			\
+  APSRGE_REGNUM, VPR_REGNUM, RA_AUTH_CODE	\
 }
 
 #define IS_VPR_REGNUM(REGNUM) \
   ((REGNUM) == VPR_REGNUM)
 
+#define IS_PAC_Pseudo_REGNUM(REGNUM) \
+  ((REGNUM) == RA_AUTH_CODE)
+
 /* Use different register alloc ordering for Thumb.  */
 #define ADJUST_REG_ALLOC_ORDER arm_order_regs_for_local_alloc ()
 
@@ -1292,6 +1296,7 @@ enum reg_class
   SFP_REG,
   AFP_REG,
   VPR_REG,
+  PAC_REG,
   ALL_REGS,
   LIM_REG_CLASSES
 };
@@ -1321,6 +1326,7 @@ enum reg_class
   "SFP_REG",		\
   "AFP_REG",		\
   "VPR_REG",		\
+  "PAC_REG",		\
   "ALL_REGS"		\
 }
 
@@ -1349,6 +1355,7 @@ enum reg_class
   { 0x00000000, 0x00000000, 0x00000000, 0x00000040 }, /* SFP_REG */	\
   { 0x00000000, 0x00000000, 0x00000000, 0x00000080 }, /* AFP_REG */	\
   { 0x00000000, 0x00000000, 0x00000000, 0x00000400 }, /* VPR_REG.  */	\
+  { 0x00000000, 0x00000000, 0x00000000, 0x00000800 }, /* PAC_REG.  */	\
   { 0xFFFF7FFF, 0xFFFFFFFF, 0xFFFFFFFF, 0x0000000F }  /* ALL_REGS.  */	\
 }
 
diff --git a/gcc/config/arm/arm.c b/gcc/config/arm/arm.c
index a3e34747ba52840bd82f76df4a3f05704bd42ec1..966c49a3bf21de909dbab5abcac5d1d3b2af4262 100644
--- a/gcc/config/arm/arm.c
+++ b/gcc/config/arm/arm.c
@@ -22000,7 +22000,9 @@ emit_multi_reg_push (unsigned long mask, unsigned long dwarf_regs_mask)
     {
       if (mask & (1 << i))
 	{
-	  reg = gen_rtx_REG (SImode, i);
+	  rtx reg1 = reg = gen_rtx_REG (SImode, i);
+	  if (arm_pac_enabled_for_curr_function_p () && i == IP_REGNUM)
+	    reg1 = gen_rtx_REG (SImode, RA_AUTH_CODE);
 
 	  XVECEXP (par, 0, 0)
 	    = gen_rtx_SET (gen_frame_mem
@@ -22018,7 +22020,7 @@ emit_multi_reg_push (unsigned long mask, unsigned long dwarf_regs_mask)
 	  if (dwarf_regs_mask & (1 << i))
 	    {
 	      tmp = gen_rtx_SET (gen_frame_mem (SImode, stack_pointer_rtx),
-				 reg);
+				 reg1);
 	      RTX_FRAME_RELATED_P (tmp) = 1;
 	      XVECEXP (dwarf, 0, dwarf_par_index++) = tmp;
 	    }
@@ -23505,7 +23507,10 @@ arm_expand_prologue (void)
     saved_regs += arm_save_coproc_regs ();
 
   if (arm_pac_enabled_for_curr_function_p ())
-      emit_multi_reg_push (1 << IP_REGNUM, 1 << IP_REGNUM);
+    {
+      insn = emit_multi_reg_push (1 << IP_REGNUM, 1 << IP_REGNUM);
+      RTX_FRAME_RELATED_P (insn) = 1;
+    }
 
   if (frame_pointer_needed && TARGET_ARM)
     {
@@ -25435,6 +25440,9 @@ arm_regno_class (int regno)
   if (IS_VPR_REGNUM (regno))
     return VPR_REG;
 
+  if (IS_PAC_Pseudo_REGNUM (regno))
+    return PAC_REG;
+
   if (TARGET_THUMB1)
     {
       if (regno == STACK_POINTER_REGNUM)
@@ -29429,6 +29437,9 @@ arm_dbx_register_number (unsigned int regno)
   if (IS_IWMMXT_REGNUM (regno))
     return 112 + regno - FIRST_IWMMXT_REGNUM;
 
+  if (IS_PAC_Pseudo_REGNUM (regno))
+    return 143;
+
   return DWARF_FRAME_REGISTERS;
 }
 
@@ -29550,6 +29561,11 @@ arm_unwind_emit_sequence (FILE * out_file, rtx p)
       reg_size = 8;
       fprintf (out_file, "\t.vsave {");
     }
+  else if (IS_PAC_Pseudo_REGNUM (reg))
+    {
+      reg_size = 4;
+      fprintf (out_file, "\t.save {");
+    }
   else
     /* Unknown register type.  */
     gcc_unreachable ();
@@ -29579,6 +29595,8 @@ arm_unwind_emit_sequence (FILE * out_file, rtx p)
 	 double precision register names.  */
       if (IS_VFP_REGNUM (reg))
 	asm_fprintf (out_file, "d%d", (reg - FIRST_VFP_REGNUM) / 2);
+      else if (IS_PAC_Pseudo_REGNUM (reg))
+	asm_fprintf (asm_out_file, "ra_auth_code");
       else
 	asm_fprintf (out_file, "%r", reg);
 
@@ -30447,7 +30465,10 @@ arm_conditional_register_usage (void)
     }
 
   if (TARGET_HAVE_PACBTI)
-    call_used_regs[IP_REGNUM] = 1;
+    {
+      call_used_regs[IP_REGNUM] = 1;
+      fixed_regs[RA_AUTH_CODE] = 0;
+    }
 
   /* The Q and GE bits are only accessed via special ACLE patterns.  */
   CLEAR_HARD_REG_BIT (operand_reg_set, APSRQ_REGNUM);
diff --git a/gcc/config/arm/arm.md b/gcc/config/arm/arm.md
index 564170dc4500c2c5d961c9a19dd86a31416dc965..f27dc3f9041d8b32e7ad750c39143a6764156fe9 100644
--- a/gcc/config/arm/arm.md
+++ b/gcc/config/arm/arm.md
@@ -42,6 +42,7 @@
    (APSRQ_REGNUM    104)	; Q bit pseudo register
    (APSRGE_REGNUM   105)	; GE bits pseudo register
    (VPR_REGNUM      106)	; Vector Predication Register - MVE register.
+   (RA_AUTH_CODE    107)	; Pseudo register to save PAC.
   ]
 )
 ;; 3rd operand to select_dominance_cc_mode
diff --git a/gcc/testsuite/g++.target/arm/pac-1.C b/gcc/testsuite/g++.target/arm/pac-1.C
new file mode 100644
index 0000000000000000000000000000000000000000..d33c860f1e368581dd9c3552a7201c7932b8560f
--- /dev/null
+++ b/gcc/testsuite/g++.target/arm/pac-1.C
@@ -0,0 +1,32 @@
+/* Check that GCC does .save and .cfi_offset directives with RA_AUTH_CODE pseudo hard-register.  */
+/* { dg-do compile } */
+/* { dg-skip-if "avoid conflicting multilib options" { *-*-* } { "-marm" "-mcpu=*" } } */
+/* { dg-options "-O2 -march=armv8.1-m.main -mbranch-protection=pac-ret+leaf+bti -mthumb -mfloat-abi=soft --save-temps -g" } */
+
+__attribute__((noinline)) void
+fn1 (int a, int b, int c)
+{
+  if (a != b + c)
+    __builtin_abort ();
+  else
+    throw b+c;
+}
+
+int main ()
+{
+  int a = 120;
+  try
+    {
+      fn1 (a, 40, 80);
+    }
+  catch (int x)
+    {
+      if (x != a)
+        __builtin_abort ();
+      else
+	return 0;
+    }
+}
+
+/* { dg-final { scan-assembler "\.save \{ra_auth_code\}" } } */
+/* { dg-final { scan-assembler "\.cfi_offset 143, \-8" } } */