[gdb/tdep] Fix gdb.base/large-frame.exp for aarch64

On aarch64, I run into:
...
FAIL: gdb.base/large-frame.exp: optimize=-O0: backtrace
...

The problem is that the architecture-specific prologue analyzer fails to
handle the first two insns in the prologue properly:
...
0000000000400610 <func>:
  400610:       d2880210        mov     x16, #0x4010
  400614:       cb3063ff        sub     sp, sp, x16
  400618:       a9007bfd        stp     x29, x30, [sp]
  40061c:       910003fd        mov     x29, sp
  400620:       910043a0        add     x0, x29, #0x10
  400624:       97fffff0        bl      4005e4 <blah>
...
so we get:
...
$ gdb -q -batch ./outputs/gdb.base/large-frame/large-frame-O0 -ex "b func"
Breakpoint 1 at 0x400614
...

Fix this by:
- fixing the support for the first insn to extract the immediate operand, and
- adding support for the second insn,
such that we have:
...
Breakpoint 1 at 0x400624
...
Note that we're overshooting by one insn (0x400620 is the first insn after the
prologue), but that's a pre-existing problem.

Tested on aarch64-linux.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29408

1 files changed, 21 insertions, 1 deletions

diff --git a/gdb/aarch64-tdep.c b/gdb/aarch64-tdep.c
index 8670197a88..f747ebda1a 100644
--- a/gdb/aarch64-tdep.c
+++ b/gdb/aarch64-tdep.c
@@ -340,6 +340,20 @@ aarch64_analyze_prologue (struct gdbarch *gdbarch,
 	  if (rn == AARCH64_SP_REGNUM && rd == AARCH64_FP_REGNUM)
 	    seen_stack_set = true;
 	}
+      else if (inst.opcode->iclass == addsub_ext
+	       && strcmp ("sub", inst.opcode->name) == 0)
+	{
+	  unsigned rd = inst.operands[0].reg.regno;
+	  unsigned rn = inst.operands[1].reg.regno;
+	  unsigned rm = inst.operands[2].reg.regno;
+
+	  gdb_assert (aarch64_num_of_operands (inst.opcode) == 3);
+	  gdb_assert (inst.operands[0].type == AARCH64_OPND_Rd_SP);
+	  gdb_assert (inst.operands[1].type == AARCH64_OPND_Rn_SP);
+	  gdb_assert (inst.operands[2].type == AARCH64_OPND_Rm_EXT);
+
+	  regs[rd] = pv_subtract (regs[rn], regs[rm]);
+	}
       else if (inst.opcode->iclass == pcreladdr
 	       && inst.operands[1].type == AARCH64_OPND_ADDR_ADRP)
 	{
@@ -370,14 +384,20 @@ aarch64_analyze_prologue (struct gdbarch *gdbarch,
 	}
       else if (inst.opcode->op == OP_MOVZ)
 	{
+	  unsigned rd = inst.operands[0].reg.regno;
+
+	  gdb_assert (aarch64_num_of_operands (inst.opcode) == 2);
 	  gdb_assert (inst.operands[0].type == AARCH64_OPND_Rd);
+	  gdb_assert (inst.operands[1].type == AARCH64_OPND_HALF);
+	  gdb_assert (inst.operands[1].shifter.kind == AARCH64_MOD_LSL);
 
 	  /* If this shows up before we set the stack, keep going.  Otherwise
 	     stop the analysis.  */
 	  if (seen_stack_set)
 	    break;
 
-	  regs[inst.operands[0].reg.regno] = pv_unknown ();
+	  regs[rd] = pv_constant (inst.operands[1].imm.value
+				  << inst.operands[1].shifter.amount);
 	}
       else if (inst.opcode->iclass == log_shift
 	       && strcmp (inst.opcode->name, "orr") == 0)