[2/3 patch, nios2] clean up prologue/epilogue detection code, v2

[Sandra Loosemore <sandra at codesourcery dot com>]

This part of the revised Nios II prologue/epilogue refactoring patch set 
fixes the epilogue detection code to deal with the multiple stack 
adjustments that GCC may now emit in some cases.  OK to commit on top of 
part 1?

-Sandra

2014-11-24  Sandra Loosemore  <sandra@codesourcery.com>

	gdb/
	* nios2-tdep.c (nios2_in_epilogue_p): Handle multiple stack
	adjustments.

diff -u b/gdb/nios2-tdep.c b/gdb/nios2-tdep.c
--- b/gdb/nios2-tdep.c
+++ b/gdb/nios2-tdep.c
@@ -589,26 +589,42 @@
 		     CORE_ADDR start_pc)
 {
   unsigned long mach = gdbarch_bfd_arch_info (gdbarch)->mach;
+  /* Maximum number of possibly-epilogue instructions to check.
+     Note that this number should not be too large, else we can
+     potentially end up iterating through unmapped memory.  */
+  int ninsns, max_insns = 5;
   unsigned int insn;
   const struct nios2_opcode *op = NULL;
   unsigned int uimm;
   int imm;
   int ra, rb, rc;
   enum branch_condition cond;
+  CORE_ADDR pc;
 
   /* There has to be a previous instruction in the function.  */
-  if (current_pc > start_pc)
-    {
-      int ok = 0;
+  if (current_pc <= start_pc)
+    return 0;
 
-      /* Check whether the previous instruction was a stack adjustment.
-	 Possible instructions here include:
+  /* Find the previous instruction before current_pc.
+     For the moment we will assume that all instructions are the
+     same size here.  */
+  pc = current_pc - NIOS2_OPCODE_SIZE;
+
+  /* Beginning with the previous instruction we just located, check whether
+     we are in a sequence of at least one stack adjustment instruction.
+     Possible instructions here include:
 	 ADDI sp, sp, n
 	 ADD sp, sp, rn
 	 LDW sp, n(sp)  */
-      op = nios2_fetch_insn (gdbarch, current_pc - NIOS2_OPCODE_SIZE, &insn);
+  for (ninsns = 0; ninsns < max_insns; ninsns++)
+    {
+      int ok = 0;
+
+      /* Fetch the insn at pc.  */
+      op = nios2_fetch_insn (gdbarch, pc, &insn);
       if (op == NULL)
 	return 0;
+      pc += op->size;
 
       /* Was it a stack adjustment?  */
       if (nios2_match_addi (insn, op, mach, &ra, &rb, &imm))
@@ -618,18 +634,27 @@
       else if (nios2_match_ldw (insn, op, mach, &ra, &rb, &imm))
 	ok = (rb == NIOS2_SP_REGNUM);
       if (!ok)
-	return 0;
-
-      /* Then check if it's followed by a return or a tail call.  */
-      op = nios2_fetch_insn (gdbarch, current_pc, &insn);
-      if (op == NULL)
-       return 0;
-      if (nios2_match_jmpr (insn, op, mach, &ra)
-         || nios2_match_jmpi (insn, op, mach, &uimm)
-         || (nios2_match_branch (insn, op, mach, &ra, &rb, &imm, &cond)
-             && cond == branch_none))
-       return 1;
+	break;
     }
+
+  /* No stack adjustments found.  */
+  if (ninsns == 0)
+    return 0;
+
+  /* We found more stack adjustments than we expect GCC to be generating.
+     Since it looks like a stack unwind might be in progress tell GDB to
+     treat it as such.  */
+  if (ninsns == max_insns)
+    return 1;
+
+  /* The next instruction following the stack adjustments must be a
+     return, jump, or unconditional branch.  */
+  if (nios2_match_jmpr (insn, op, mach, &ra)
+      || nios2_match_jmpi (insn, op, mach, &uimm)
+      || (nios2_match_branch (insn, op, mach, &ra, &rb, &imm, &cond)
+	  && cond == branch_none))
+    return 1;
+
   return 0;
 }