gas: aarch64: add experimental support for SCFI

[Changes from V1]
- Factored out the ginsn creation functionality from tc-aarch64.c into
tc-aarch64-ginsn.c.
- The switch case in aarch64_ginsn_new now is based on iclass rather
than (earlier) opcode.
- Rename aarch64_ginsn_jump / aarch64_ginsn_jump_cond to
aarch64_ginsn_branch_uncond / aarch64_ginsn_branch_cond respectively.
- Explicitly whitelist irg insn.
- Other minor code comment and readability fixes.
[End of changes from V1]

For synthesizing CFI (SCFI) for hand-written asm, the SCFI machinery in
GAS works on the generic GAS insns (ginsns).  This patch adds support in
the aarch64 backend to create ginsns for a subset of the supported
machine instructions.  The subset includes the minimal necessary
instructions to ensure SCFI correctness:

- Any potential register saves and unsaves.  Hence, process
instructions belonging to a variety of iclasses involving str, ldr,
stp, ldp.
- Any change of flow instructions.  This includes all conditional and
unconditional branches, call (bl, blr, etc.) and return.
- Most importantly, any instruction that could affect the two registers of
interest: REG_SP, REG_FP.  This set includes all pre-indexed and
post-indexed memory operations, with writeback, on the stack.  This
set must also include other instructions (e.g., arithmetic insns) where
the destination register is one of the afore-mentioned registers.

Apart from creating ginsn, another key responsibility of the backend is
to make sure there are safeguards in place to detect and alert if an
instruction of interest may have been skipped.  This is done via
aarch64_ginsn_unhandled () (similar to the x86 backend).  This function
, hence, is also intended to alert when future ISA changes may otherwise
render SCFI results incorrect, because of missing ginsns for the newly
added machine instructions.

The current SCFI machinery does not currently synthesize the
PAC-related, aarch64-specific CFI directives: .cfi_b_key_frame.  The
support for this is planned for near future.

SCFI is enabled for ELF targets only.

gas/
	* config/tc-aarch64-ginsn.c: New file.
	* gas/config/tc-aarch64.c (md_assemble): Include
	tc-aarch64-ginsn.c file.  Invoke aarch64_ginsn_new.
	* gas/config/tc-aarch64.h (TARGET_USE_GINSN): Define for SCFI
	enablement.
	(TARGET_USE_SCFI): Likewise.
	(SCFI_MAX_REG_ID): New definition.
	(REG_FP): Likewise.
	(REG_LR): Likewise.
	(REG_SP): Likewise.
	(SCFI_INIT_CFA_OFFSET): Likewise.
	(SCFI_CALLEE_SAVED_REG_P): Likewise.
	(aarch64_scfi_callee_saved_p): New declaration.

3 files changed, 778 insertions, 0 deletions

diff --git a/gas/config/tc-aarch64-ginsn.c b/gas/config/tc-aarch64-ginsn.c
new file mode 100644
index 0000000..923d210
--- /dev/null
+++ b/gas/config/tc-aarch64-ginsn.c
@@ -0,0 +1,743 @@
+/* tc-aarch64-ginsn.c -- Ginsn generation for the AArch64 ISA
+
+   Copyright (C) 2024 Free Software Foundation, Inc.
+   Contributed by ARM Ltd.
+
+   This file is part of GAS.
+
+   GAS 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.
+
+   GAS 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; see the file COPYING3. If not,
+   see <http://www.gnu.org/licenses/>.  */
+
+/* This file contains the implementation of the ginsn creation for aarch64
+   instructions.  Most functions will read the aarch64_instruction inst
+   object, but none should need to modify it.  */
+
+#ifdef OBJ_ELF
+
+/* DWARF register number for R1.  Used as dummy value when WZR.  */
+#define GINSN_DW2_REGNUM_R1_DUMMY 1
+
+/* Return whether the given register number is a callee-saved register for
+   SCFI purposes.  SCFI always tracks SP, FP and LR additionally.  */
+
+bool
+aarch64_scfi_callee_saved_p (uint32_t dw2reg_num)
+{
+  if (dw2reg_num == REG_SP /* x31.  */
+      || dw2reg_num == REG_FP /* x29.  */
+      || dw2reg_num == REG_LR /* x30.  */
+      || (dw2reg_num >= 19 && dw2reg_num <= 28) /* x19 - x28.  */)
+    return true;
+
+  return false;
+}
+
+/* Get the DWARF register number for the given OPND.
+   Whether 31 is used to encode WZR or SP is specified via SP_ALLOWED_P.
+
+   The caller must decide the value of SP_ALLOWED_P based on the instruction
+   encoding.  */
+
+static uint32_t
+ginsn_dw2_regnum (aarch64_opnd_info *opnd, bool sp_allowed_p)
+{
+  /* Use a dummy register value in case of WZR, else this will be an
+     incorrect dependency on REG_SP.  */
+  if (!sp_allowed_p && opnd->reg.regno == REG_SP)
+    return GINSN_DW2_REGNUM_R1_DUMMY;
+  /* For registers of our interest (callee-saved regs, SP, FP, LR),
+     DWARF register number is the same as AArch64 register number.  */
+  return opnd->reg.regno;
+}
+
+/* Generate ginsn for addsub instructions with immediate opnd.  */
+
+static ginsnS *
+aarch64_ginsn_addsub_imm (const symbolS *insn_end_sym)
+{
+  ginsnS *ginsn = NULL;
+  bool add_p, sub_p;
+  int32_t src_imm = 0;
+  uint32_t dst_reg, opnd_reg;
+  aarch64_opnd_info *dst, *opnd;
+  ginsnS *(*ginsn_func) (const symbolS *, bool,
+			 enum ginsn_src_type, unsigned int, offsetT,
+			 enum ginsn_src_type, unsigned int, offsetT,
+			 enum ginsn_dst_type, unsigned int, offsetT);
+
+  aarch64_inst *base = &inst.base;
+  const aarch64_opcode *opcode = base->opcode;
+
+  add_p = opcode->flags & F_ARITH_ADD;
+  sub_p = opcode->flags & F_ARITH_SUB;
+  gas_assert (add_p || sub_p);
+  ginsn_func = add_p ? ginsn_new_add : ginsn_new_sub;
+
+  gas_assert (aarch64_num_of_operands (opcode) == 3);
+  dst = &base->operands[0];
+  opnd = &base->operands[1];
+
+  dst_reg = ginsn_dw2_regnum (dst, true);
+
+  if (aarch64_gas_internal_fixup_p () && inst.reloc.exp.X_op == O_constant)
+    src_imm = inst.reloc.exp.X_add_number;
+  /* For any other relocation type, e.g., in add reg, reg, symbol, skip now
+     and handle via aarch64_ginsn_unhandled () code path.  */
+  else if (inst.reloc.type != BFD_RELOC_UNUSED)
+    return ginsn;
+  /* FIXME - verify the understanding and remove assert.  */
+  else
+    gas_assert (0);
+
+  opnd_reg = ginsn_dw2_regnum (opnd, true);
+
+  ginsn = ginsn_func (insn_end_sym, true,
+		      GINSN_SRC_REG, opnd_reg, 0,
+		      GINSN_SRC_IMM, 0, src_imm,
+		      GINSN_DST_REG, dst_reg, 0);
+  ginsn_set_where (ginsn);
+
+  return ginsn;
+}
+
+/* Generate ginsn for addsub instructions with reg opnd.  */
+
+static ginsnS *
+aarch64_ginsn_addsub_reg (const symbolS *insn_end_sym)
+{
+  ginsnS *ginsn = NULL;
+  bool add_p, sub_p;
+  uint32_t dst_reg, src1_reg, src2_reg;
+  aarch64_opnd_info *dst, *src1, *src2;
+  ginsnS *(*ginsn_func) (const symbolS *, bool,
+			 enum ginsn_src_type, unsigned int, offsetT,
+			 enum ginsn_src_type, unsigned int, offsetT,
+			 enum ginsn_dst_type, unsigned int, offsetT);
+
+  aarch64_inst *base = &inst.base;
+  const aarch64_opcode *opcode = base->opcode;
+
+  add_p = opcode->flags & F_ARITH_ADD;
+  sub_p = opcode->flags & F_ARITH_SUB;
+  gas_assert (add_p || sub_p);
+  ginsn_func = add_p ? ginsn_new_add : ginsn_new_sub;
+
+  gas_assert (aarch64_num_of_operands (opcode) == 3);
+  dst = &base->operands[0];
+  src1 = &base->operands[1];
+  src2 = &base->operands[2];
+
+  dst_reg = ginsn_dw2_regnum (dst, true);
+  src1_reg = ginsn_dw2_regnum (src1, true);
+  src2_reg = ginsn_dw2_regnum (src2, false);
+
+  ginsn = ginsn_func (insn_end_sym, true,
+		      GINSN_SRC_REG, src1_reg, 0,
+		      GINSN_SRC_REG, src2_reg, 0,
+		      GINSN_DST_REG, dst_reg, 0);
+  ginsn_set_where (ginsn);
+
+  return ginsn;
+}
+
+/* Generate ginsn for the load pair and store pair instructions.  */
+
+static ginsnS *
+aarch64_ginsn_ldstp (const symbolS *insn_end_sym)
+{
+  ginsnS *ginsn = NULL;
+  ginsnS *ginsn_ind = NULL;
+  ginsnS *ginsn_mem1 = NULL;
+  ginsnS *ginsn_mem2 = NULL;
+  uint32_t opnd_reg, addr_reg;
+  int32_t offset, mem_offset;
+  unsigned int width = 8;
+  bool store_p = false;
+
+  aarch64_opnd_info *opnd1, *opnd2, *addr;
+  aarch64_inst *base = &inst.base;
+  const aarch64_opcode *opcode = base->opcode;
+
+  /* This function is for handling ldp / stp ops only.  */
+  gas_assert (opcode->iclass == ldstpair_indexed
+	      || opcode->iclass == ldstpair_off);
+  gas_assert (aarch64_num_of_operands (opcode) == 3);
+
+  opnd1 = &base->operands[0];
+  opnd2 = &base->operands[1];
+  addr = &base->operands[2];
+  store_p = opcode->flags & F_LDST_STORE;
+
+  addr_reg = ginsn_dw2_regnum (addr, true);
+  gas_assert (!addr->addr.offset.is_reg);
+  mem_offset = addr->addr.offset.imm;
+  if (opnd1->qualifier == AARCH64_OPND_QLF_W)
+    width = 4;
+
+  /* Handle address calculation.  */
+  if ((addr->addr.preind || addr->addr.postind) && addr->addr.writeback)
+    {
+      /* Pre-indexed store, e.g., stp x29, x30, [sp, -128]!
+	 Pre-indexed addressing is like offset addressing, except that
+	 the base pointer is updated as a result of the instruction.
+
+	 Post-indexed store, e.g., stp     x29, x30, [sp],128
+	 Post-index addressing is useful for popping off the stack.  The
+	 instruction loads the value from the location pointed at by the stack
+	 pointer, and then moves the stack pointer on to the next full location
+	 in the stack.  */
+      ginsn_ind = ginsn_new_add (insn_end_sym, false,
+				 GINSN_SRC_REG, addr_reg, 0,
+				 GINSN_SRC_IMM, 0, mem_offset,
+				 GINSN_DST_REG, addr_reg, 0);
+      ginsn_set_where (ginsn_ind);
+    }
+
+  /* Save / restore of floating point register or WZR is not of interest for
+     SCFI.  However, the address processing component may have updated the
+     stack pointer.  At least, emit that ginsn and return.  PS: opnd_reg will
+     be GINSN_DW2_REGNUM_R1_DUMMY by now;  Check explicitly the regno for WZR.
+     Also note, TBD_GINSN_GEN_NOT_SCFI.  */
+  if (aarch64_get_operand_class (opnd1->type) != AARCH64_OPND_CLASS_INT_REG
+      || opnd1->reg.regno == REG_SP /* WZR.  */)
+    return ginsn_ind;
+
+  /* With post-index addressing, the value is loaded from the
+     address in the base pointer, and then the pointer is updated.
+     With pre-index addressing, the addr computation has already
+     been explicitly done.  */
+  offset = mem_offset;
+  if ((addr->addr.postind || addr->addr.preind) && addr->addr.writeback)
+    offset = 0;
+
+  opnd_reg = ginsn_dw2_regnum (opnd1, false);
+  if (store_p)
+    {
+      ginsn_mem1 = ginsn_new_store (insn_end_sym, false,
+				    GINSN_SRC_REG, opnd_reg,
+				    GINSN_DST_INDIRECT, addr_reg, offset);
+      ginsn_set_where (ginsn_mem1);
+
+      opnd_reg = ginsn_dw2_regnum (opnd2, false);
+      ginsn_mem2 = ginsn_new_store (insn_end_sym, false,
+				    GINSN_SRC_REG, opnd_reg,
+				    GINSN_DST_INDIRECT, addr_reg, offset + width);
+      ginsn_set_where (ginsn_mem2);
+    }
+  else
+    {
+      opnd_reg = ginsn_dw2_regnum (opnd1, false);
+      ginsn_mem1 = ginsn_new_load (insn_end_sym, false,
+				   GINSN_SRC_INDIRECT, addr_reg, offset,
+				   GINSN_DST_REG, opnd_reg);
+      ginsn_set_where (ginsn_mem1);
+
+      opnd_reg = ginsn_dw2_regnum (opnd2, false);
+      ginsn_mem2 = ginsn_new_load (insn_end_sym, false,
+				   GINSN_SRC_INDIRECT, addr_reg, offset + width,
+				   GINSN_DST_REG, opnd_reg);
+      ginsn_set_where (ginsn_mem2);
+    }
+
+  /* Link the list of ginsns created.  */
+  if (addr->addr.preind && addr->addr.writeback)
+    gas_assert (!ginsn_link_next (ginsn_ind, ginsn_mem1));
+
+  gas_assert (!ginsn_link_next (ginsn_mem1, ginsn_mem2));
+
+  if (addr->addr.postind && addr->addr.writeback)
+    gas_assert (!ginsn_link_next (ginsn_mem2, ginsn_ind));
+
+  /* Make note of the first instruction in the list.  */
+  ginsn = (addr->addr.preind && addr->addr.writeback) ? ginsn_ind : ginsn_mem1;
+  return ginsn;
+}
+
+/* Generate ginsn for load and store instructions.  */
+
+static ginsnS *
+aarch64_ginsn_ldstr (const symbolS *insn_end_sym)
+{
+  ginsnS *ginsn = NULL;
+  ginsnS *ginsn_ind = NULL;
+  ginsnS *ginsn_mem = NULL;
+  uint32_t opnd_reg, addr_reg;
+  int32_t offset = 0;
+  int32_t mem_offset = 0;
+  bool store_p = false;
+
+  aarch64_opnd_info *opnd1, *addr;
+  aarch64_inst *base = &inst.base;
+  const aarch64_opcode *opcode = base->opcode;
+
+  /* This function is for handling ldr, str ops only.  */
+  gas_assert (opcode->iclass == ldst_imm9 || opcode->iclass == ldst_pos);
+  gas_assert (aarch64_num_of_operands (opcode) == 2);
+
+  opnd1 = &base->operands[0];
+  addr = &base->operands[1];
+  store_p = opcode->flags & F_LDST_STORE;
+
+  addr_reg = ginsn_dw2_regnum (addr, true);
+
+  /* STR <Xt>, [<Xn|SP>, (<Wm>|<Xm>){, <extend> {<amount>}}].
+     LDR <Xt>, [<Xn|SP>], #<simm>.  */
+  opnd_reg = ginsn_dw2_regnum (opnd1, false);
+
+  if (aarch64_gas_internal_fixup_p () && inst.reloc.exp.X_op == O_constant)
+    mem_offset = inst.reloc.exp.X_add_number;
+  else
+    {
+      gas_assert (!addr->addr.offset.is_reg);
+      mem_offset = addr->addr.offset.imm;
+    }
+
+  /* Handle address calculation.  */
+  if ((addr->addr.preind || addr->addr.postind) && addr->addr.writeback)
+    {
+      ginsn_ind = ginsn_new_add (insn_end_sym, false,
+				 GINSN_SRC_REG, addr_reg, 0,
+				 GINSN_SRC_IMM, 0, mem_offset,
+				 GINSN_DST_REG, addr_reg, 0);
+      ginsn_set_where (ginsn_ind);
+    }
+
+  /* Save / restore of floating point register or WZR is not of interest for
+     SCFI.  However, the address processing component may have updated the
+     stack pointer.  At least, emit that ginsn and return.  PS: opnd_reg will
+     be GINSN_DW2_REGNUM_R1_DUMMY by now;  Check explicitly the regno for WZR.
+     Also note, TBD_GINSN_GEN_NOT_SCFI.  */
+  if (aarch64_get_operand_class (opnd1->type) != AARCH64_OPND_CLASS_INT_REG
+      || opnd1->reg.regno == REG_SP /* WZR.  */)
+    return ginsn_ind;
+
+  /* With post-index addressing, the value is loaded from the
+     address in the base pointer, and then the pointer is updated.
+     With pre-index addressing, the addr computation has already
+     been explicitly done.  */
+  offset = mem_offset;
+  if ((addr->addr.postind || addr->addr.preind) && addr->addr.writeback)
+    offset = 0;
+
+  if (store_p)
+    ginsn_mem = ginsn_new_store (insn_end_sym, false,
+				 GINSN_SRC_REG, opnd_reg,
+				 GINSN_DST_INDIRECT, addr_reg, offset);
+  else
+    ginsn_mem = ginsn_new_load (insn_end_sym, false,
+				GINSN_SRC_INDIRECT, addr_reg, offset,
+				GINSN_DST_REG, opnd_reg);
+  ginsn_set_where (ginsn_mem);
+
+  if (addr->addr.preind && addr->addr.writeback)
+    gas_assert (!ginsn_link_next (ginsn_ind, ginsn_mem));
+  else if (addr->addr.postind && addr->addr.writeback)
+    gas_assert (!ginsn_link_next (ginsn_mem, ginsn_ind));
+
+  /* Make note of the first instruction in the list.  */
+  ginsn = (addr->addr.preind && addr->addr.writeback) ? ginsn_ind : ginsn_mem;
+
+  return ginsn;
+}
+
+/* Generate ginsn for unconditional branch instructions.  */
+
+static ginsnS *
+aarch64_ginsn_branch_uncond (const symbolS *insn_end_sym)
+{
+  ginsnS *ginsn = NULL;
+  const symbolS *src_symbol = NULL;
+  enum ginsn_src_type src_type = GINSN_SRC_UNKNOWN;
+  uint32_t src_value = 0;
+
+  aarch64_inst *base = &inst.base;
+  const aarch64_opcode *opcode = base->opcode;
+
+  if (opcode->iclass == branch_imm)
+    {
+      /* b or bl. opcode 0x14000000 or 0x94000000.  */
+      gas_assert (inst.reloc.type == BFD_RELOC_AARCH64_CALL26
+		  || inst.reloc.type == BFD_RELOC_AARCH64_JUMP26);
+      src_symbol = inst.reloc.exp.X_add_symbol;
+      src_type = GINSN_SRC_SYMBOL;
+    }
+  else if (aarch64_num_of_operands (opcode) >= 1)
+    {
+      gas_assert (opcode->iclass == branch_reg);
+      /* Some insns (e.g., braa, blraa etc.) may have > 1 operands.  For
+	 current SCFI implementation, it suffices however to simply pass
+	 the information about the first source.  Although, strictly speaking,
+	 (if reg) the source info is currently of no material use either.  */
+      src_type = GINSN_SRC_REG;
+      src_value = ginsn_dw2_regnum (&base->operands[0], false);
+    }
+
+  if (opcode->flags & F_BRANCH_CALL)
+    {
+      gas_assert (src_type != GINSN_SRC_UNKNOWN);
+      ginsn = ginsn_new_call (insn_end_sym, true,
+			      src_type, src_value, src_symbol);
+    }
+  else if (opcode->flags & F_BRANCH_RET)
+    /* TBD_GINSN_REPRESENTATION_LIMIT.  The following function to create a
+       GINSN_TYPE_RETURN does not allow src info ATM.  */
+    ginsn = ginsn_new_return (insn_end_sym, true);
+  else
+    ginsn = ginsn_new_jump (insn_end_sym, true,
+			    src_type, src_value, src_symbol);
+
+  ginsn_set_where (ginsn);
+
+  return ginsn;
+}
+
+/* Generate ginsn for conditional branch instructions.  */
+
+static ginsnS *
+aarch64_ginsn_branch_cond (const symbolS *insn_end_sym)
+{
+  ginsnS *ginsn;
+  const symbolS *src_symbol;
+  enum ginsn_src_type src_type;
+
+  gas_assert (inst.reloc.type == BFD_RELOC_AARCH64_BRANCH19
+	      || inst.reloc.type == BFD_RELOC_AARCH64_TSTBR14);
+
+  src_symbol = inst.reloc.exp.X_add_symbol;
+  src_type = GINSN_SRC_SYMBOL;
+  ginsn = ginsn_new_jump_cond (insn_end_sym, true, src_type, 0, src_symbol);
+  ginsn_set_where (ginsn);
+
+  return ginsn;
+}
+
+/* Generate ginsn for mov instructions.  */
+
+static ginsnS *
+aarch64_ginsn_mov (const symbolS *insn_end_sym)
+{
+  ginsnS *ginsn = NULL;
+  uint32_t src_reg = 0, dst_reg;
+  aarch64_opnd_info *src, *dst;
+  offsetT src_imm = 0;
+  enum ginsn_src_type src_type;
+
+  aarch64_inst *base = &inst.base;
+  const aarch64_opcode *opcode = base->opcode;
+
+  gas_assert (aarch64_num_of_operands (opcode) == 2);
+
+  dst = &base->operands[0];
+  src = &base->operands[1];
+
+  dst_reg = ginsn_dw2_regnum (dst, true);
+
+  if (src->type == AARCH64_OPND_IMM_MOV
+      && aarch64_gas_internal_fixup_p () && inst.reloc.exp.X_op == O_constant)
+    {
+      src_imm = inst.reloc.exp.X_add_number;
+      src_type = GINSN_SRC_IMM;
+    }
+  else
+    {
+      /* mov   x27, sp.  */
+      src_reg = ginsn_dw2_regnum (src, true);
+      src_type = GINSN_SRC_REG;
+    }
+
+  ginsn = ginsn_new_mov (insn_end_sym, false,
+			 src_type, src_reg, src_imm,
+			 GINSN_DST_REG, dst_reg, 0);
+  ginsn_set_where (ginsn);
+
+  return ginsn;
+}
+
+/* Check if an instruction is whitelisted.
+
+   An instruction is a candidate for whitelisting if not generating ginsn for
+   it, does not affect SCFI correctness.
+
+   TBD_GINSN_GEN_NOT_SCFI. This function assumes GINSN_GEN_SCFI is in effect.
+   When other ginsn_gen_mode are added, this will need fixing.   */
+
+static bool
+aarch64_ginsn_safe_to_skip_p (void)
+{
+  bool skip_p = false;
+  aarch64_opnd_info *opnd = NULL;
+  aarch64_opnd_info *addr = NULL;
+  unsigned int dw2_regnum;
+  uint32_t opnd_reg;
+  int num_opnds = 0;
+
+  aarch64_inst *base = &inst.base;
+  const aarch64_opcode *opcode = base->opcode;
+
+  switch (opcode->iclass)
+    {
+    case ldst_regoff:
+      /* It is not expected to have reg offset based ld/st ops to be used
+	 for reg save and restore operations.  Warn the user though.  */
+      opnd = &base->operands[0];
+      opnd_reg = ginsn_dw2_regnum (opnd, false);
+      if (aarch64_scfi_callee_saved_p (opnd_reg))
+	{
+	  skip_p = true;
+	  as_warn ("SCFI: ignored probable save/restore op with reg offset");
+	}
+      break;
+
+    case ldstpair_off:
+      /* Load/store FP register pair (offset) ...  */
+    case ldst_pos:
+      /* ... and load/store FP register (unsigned immediate) must be
+	 whitelisted.  This is because the addr register may be SP-based, but
+	 there cannot be writeback with pre- or post- indexing.  */
+      opnd = &base->operands[0];
+      if (aarch64_get_operand_class (opnd->type) != AARCH64_OPND_CLASS_INT_REG)
+	{
+	  num_opnds = aarch64_num_of_operands (opcode);
+	  addr = &base->operands[num_opnds - 1];
+	  /* There cannot be a writeback here.  Confirm? FIXME.
+	     if not true, need to generate addr gen opcodes at the least.  */
+	  gas_assert (!addr->addr.writeback);
+	  skip_p = true;
+	}
+      break;
+
+    case dp_2src:
+      /* irg insn needs to be explicitly whitelisted.  This is because the
+	 dest is Rd_SP, but irg insn affects the tag only.  To detect irg
+	 insn, avoid an opcode-based check, however.  */
+      opnd = &base->operands[0];
+      if (opnd->type == AARCH64_OPND_Rd_SP)
+	{
+	  dw2_regnum = ginsn_dw2_regnum (opnd, true);
+	  if (dw2_regnum == REG_SP)
+	    skip_p = true;
+	}
+      break;
+
+    default:
+      break;
+    }
+
+  return skip_p;
+}
+
+#define AARCH64_GINSN_UNHANDLED_NONE        0
+#define AARCH64_GINSN_UNHANDLED_DEST_REG    1
+#define AARCH64_GINSN_UNHANDLED_CFG         2
+#define AARCH64_GINSN_UNHANDLED_STACKOP     3
+#define AARCH64_GINSN_UNHANDLED_UNEXPECTED  4
+
+/* Check the input insn for its impact on the correctness of the synthesized
+   CFI.  Returns an error code to the caller.  */
+
+static int
+aarch64_ginsn_unhandled (void)
+{
+  int err = AARCH64_GINSN_UNHANDLED_NONE;
+  aarch64_inst *base = &inst.base;
+  const aarch64_opcode *opcode = base->opcode;
+  aarch64_opnd_info *dest = &base->operands[0];
+  int num_opnds = aarch64_num_of_operands (opcode);
+  aarch64_opnd_info *addr;
+  unsigned int dw2_regnum;
+  uint32_t addr_reg;
+  bool sp_allowed_p = false;
+
+  /* All change of flow instructions are important for SCFI.  */
+  if (opcode->iclass == condbranch
+      || opcode->iclass == compbranch
+      || opcode->iclass == testbranch
+      || opcode->iclass == branch_imm
+      || opcode->iclass == branch_reg)
+    err = AARCH64_GINSN_UNHANDLED_CFG;
+  /* Also, any memory instructions that may involve an update to the stack
+     pointer.  Some classes can be skipped altogether though, as they cannot
+     be used to push or pop stack because of disallowed writeback:
+       - ldst_unscaled, ldst_regoff, ldst_unpriv, ldstexcl, loadlit,
+     ldstnapair_offs.  FIXME double-check.   */
+  else if (opcode->iclass == ldstpair_off
+	   || opcode->iclass == ldstpair_indexed
+	   || opcode->iclass == ldst_imm9
+	   || opcode->iclass == ldst_imm10
+	   || opcode->iclass == ldst_pos)
+    {
+      addr = &base->operands[num_opnds - 1];
+      addr_reg = ginsn_dw2_regnum (addr, true);
+      /* For all skipped memory operations, check if an update to REG_SP or
+	 REG_FP is involved.  */
+      if ((addr_reg == REG_SP || addr_reg == REG_FP)
+	  && (addr->addr.postind || addr->addr.preind) && addr->addr.writeback)
+	err = AARCH64_GINSN_UNHANDLED_STACKOP;
+    }
+  /* Finally, also check if the missed instructions are affecting REG_SP or
+     REG_FP.  */
+  else if (dest && (dest->type == AARCH64_OPND_Rd
+		    || dest->type == AARCH64_OPND_Rd_SP))
+    {
+      sp_allowed_p = (dest->type == AARCH64_OPND_Rd_SP) ? true : false;
+      dw2_regnum = ginsn_dw2_regnum (dest, sp_allowed_p);
+
+      if (dw2_regnum == REG_SP || dw2_regnum == REG_FP)
+	err = AARCH64_GINSN_UNHANDLED_DEST_REG;
+    }
+
+  return err;
+}
+
+/* Generate one or more generic GAS instructions, a.k.a, ginsns for the
+   current machine instruction.
+
+   Returns the head of linked list of ginsn(s) added, if success; Returns NULL
+   if failure.
+
+   The input ginsn_gen_mode GMODE determines the set of minimal necessary
+   ginsns necessary for correctness of any passes applicable for that mode.
+   For supporting the GINSN_GEN_SCFI generation mode, following is the list of
+   machine instructions that must be translated into the corresponding ginsns
+   to ensure correctness of SCFI:
+     - All instructions affecting the two registers that could potentially
+       be used as the base register for CFA tracking.  For SCFI, the base
+       register for CFA tracking is limited to REG_SP and REG_FP only.
+     - All change of flow instructions: conditional and unconditional
+       branches, call and return from functions.
+     - All instructions that can potentially be a register save / restore
+       operations.
+     - All instructions that may update the stack pointer: pre-indexed and
+     post-indexed stack operations with writeback.
+
+   The function currently supports GINSN_GEN_SCFI ginsn generation mode only.
+   To support other generation modes will require work on this target-specific
+   process of creation of ginsns:
+     - Some of such places are tagged with TBD_GINSN_GEN_NOT_SCFI to serve as
+       possible starting points.
+     - Also note that ginsn representation may need enhancements.  Specifically,
+       note some TBD_GINSN_INFO_LOSS and TBD_GINSN_REPRESENTATION_LIMIT markers.
+   */
+
+static ginsnS *
+aarch64_ginsn_new (const symbolS *insn_end_sym, enum ginsn_gen_mode gmode)
+{
+  int err = 0;
+  ginsnS *ginsn = NULL;
+  unsigned int dw2_regnum;
+  aarch64_opnd_info *dest = NULL;
+  bool sp_allowed_p = false;
+  aarch64_inst *base = &inst.base;
+  const aarch64_opcode *opcode = base->opcode;
+
+  /* Currently supports generation of selected ginsns, sufficient for
+     the use-case of SCFI only.  To remove this condition will require
+     work on this target-specific process of creation of ginsns.  Some
+     of such places are tagged with TBD_GINSN_GEN_NOT_SCFI to serve as
+     examples.  */
+  if (gmode != GINSN_GEN_SCFI)
+    return ginsn;
+
+  switch (opcode->iclass)
+    {
+    case addsub_ext:
+      /* TBD_GINSN_GEN_NOT_SCFI: other insns are not of interest for SCFI.  */
+      if (opcode->flags & F_ARITH_ADD || opcode->flags & F_ARITH_SUB)
+	ginsn = aarch64_ginsn_addsub_reg (insn_end_sym);
+      break;
+
+    case addsub_imm:
+      if (opcode->flags & F_ARITH_MOV)
+	ginsn = aarch64_ginsn_mov (insn_end_sym);
+      else if (opcode->flags & F_ARITH_ADD || opcode->flags & F_ARITH_SUB)
+	ginsn = aarch64_ginsn_addsub_imm (insn_end_sym);
+      break;
+
+    case movewide:
+      ginsn = aarch64_ginsn_mov (insn_end_sym);
+      break;
+
+    case ldst_imm9:
+    case ldst_pos:
+      ginsn = aarch64_ginsn_ldstr (insn_end_sym);
+      break;
+
+    case ldstpair_indexed:
+    case ldstpair_off:
+      ginsn = aarch64_ginsn_ldstp (insn_end_sym);
+      break;
+
+    case branch_imm:
+    case branch_reg:
+      ginsn = aarch64_ginsn_branch_uncond (insn_end_sym);
+      break;
+
+    case compbranch:
+      /* Although cbz/cbnz has an additional operand and are functionally
+	 distinct from conditional branches, it is fine to use the same ginsn
+	 type for both from the perspective of SCFI.  */
+    case testbranch:
+    case condbranch:
+      ginsn = aarch64_ginsn_branch_cond (insn_end_sym);
+      break;
+
+    default:
+      /* TBD_GINSN_GEN_NOT_SCFI: Skip all other opcodes uninteresting for
+	 GINSN_GEN_SCFI mode.  */
+      break;
+    }
+
+  if (!ginsn && !aarch64_ginsn_safe_to_skip_p ())
+    {
+      /* For all unhandled insns, check that they no not impact SCFI
+	 correctness.  */
+      err = aarch64_ginsn_unhandled ();
+      switch (err)
+	{
+	case AARCH64_GINSN_UNHANDLED_NONE:
+	  break;
+	case AARCH64_GINSN_UNHANDLED_DEST_REG:
+	  /* Not all writes to REG_FP are harmful in context of SCFI.  Simply
+	     generate a GINSN_TYPE_OTHER with destination set to the
+	     appropriate register.  The SCFI machinery will bail out if this
+	     ginsn affects SCFI correctness.  */
+	  dest = &base->operands[0];
+	  sp_allowed_p = (dest->type == AARCH64_OPND_Rd_SP) ? true : false;
+	  dw2_regnum = ginsn_dw2_regnum (dest, sp_allowed_p);
+	  ginsn = ginsn_new_other (insn_end_sym, true,
+				   GINSN_SRC_IMM, 0,
+				   GINSN_SRC_IMM, 0,
+				   GINSN_DST_REG, dw2_regnum);
+	  ginsn_set_where (ginsn);
+	  break;
+	case AARCH64_GINSN_UNHANDLED_CFG:
+	case AARCH64_GINSN_UNHANDLED_STACKOP:
+	  as_bad (_("SCFI: unhandled op %#x may cause incorrect CFI"),
+		  opcode->opcode);
+	  break;
+	case AARCH64_GINSN_UNHANDLED_UNEXPECTED:
+	  as_bad (_("SCFI: unexpected op %#x may cause incorrect CFI"),
+		  opcode->opcode);
+	  break;
+	default:
+	  abort ();
+	  break;
+	}
+    }
+
+  return ginsn;
+}
+
+#endif /* OBJ_ELF */
+
diff --git a/gas/config/tc-aarch64.c b/gas/config/tc-aarch64.c
index fec17c4..94b6d74 100644
--- a/gas/config/tc-aarch64.c
+++ b/gas/config/tc-aarch64.c
@@ -33,6 +33,7 @@
 #include "dw2gencfi.h"
 #include "sframe.h"
 #include "gen-sframe.h"
+#include "scfi.h"
 #endif
 
 #include "dw2gencfi.h"
@@ -8452,6 +8453,10 @@ dump_opcode_operands (const aarch64_opcode *opcode)
 }
 #endif /* DEBUG_AARCH64 */
 
+#ifdef OBJ_ELF
+# include "tc-aarch64-ginsn.c"
+#endif
+
 /* This is the guts of the machine-dependent assembler.  STR points to a
    machine dependent instruction.  This function is supposed to emit
    the frags/bytes it assembles to.  */
@@ -8589,6 +8594,16 @@ md_assemble (char *str)
 	      output_inst (copy);
 	    }
 
+#ifdef OBJ_ELF
+	  if (flag_synth_cfi)
+	    {
+	      ginsnS *ginsn;
+	      ginsn = aarch64_ginsn_new (symbol_temp_new_now (),
+					 frch_ginsn_gen_mode ());
+	      frch_ginsn_data_append (ginsn);
+	    }
+#endif
+
 	  /* Issue non-fatal messages if any.  */
 	  output_operand_error_report (str, true);
 	  return;
diff --git a/gas/config/tc-aarch64.h b/gas/config/tc-aarch64.h
index 1b8bada..97147bd 100644
--- a/gas/config/tc-aarch64.h
+++ b/gas/config/tc-aarch64.h
@@ -263,6 +263,26 @@ extern void aarch64_after_parse_args (void);
 
 #ifdef OBJ_ELF
 
+#define TARGET_USE_GINSN 1
+/* Allow GAS to synthesize DWARF CFI for hand-written asm.
+   PS: TARGET_USE_CFIPOP is a pre-condition.  */
+#define TARGET_USE_SCFI 1
+/* Identify the maximum DWARF register number of all the registers being
+   tracked for SCFI.  This is the last DWARF register number of the set
+   of SP, FP, and all callee-saved registers.  For Aarch64, this means 31.  */
+# define SCFI_MAX_REG_ID 31
+/* Identify the DWARF register number of the frame-pointer register.  */
+# define REG_FP 29
+/* Identify the DWARF register number of the link register.  */
+# define REG_LR 30
+/* Identify the DWARF register number of the stack-pointer register.  */
+# define REG_SP 31
+
+#define SCFI_INIT_CFA_OFFSET 0
+
+#define SCFI_CALLEE_SAVED_REG_P(dw2reg)  aarch64_scfi_callee_saved_p (dw2reg)
+extern bool aarch64_scfi_callee_saved_p (uint32_t dw2reg_num);
+
 /* Whether SFrame stack trace info is supported.  */
 extern bool aarch64_support_sframe_p (void);
 #define support_sframe_p aarch64_support_sframe_p