[patch, nios2] update trap/break handling in prologue analyzer

[Sandra Loosemore <sandra at codesourcery dot com>]

This patch updates the nios2 prologue analyzer to handle some recent-ish 
GCC code generation changes:

* In GCC r223083, the instruction emitted when a stack overflow is 
detected and in other trap situations was changed from "break" to 
"trap".  This patch tweaks the prologue analyzer to recognize both the 
old and new stack checking sequences.

* In GCC r225787, the stack overflow checking logic was changed so that 
now there is only a single check being emitted instead of one after each 
stack adjustment.  This doesn't require code changes to GDB, but I 
tidied up the comments that explain what can appear in the prologue.

* It seemed to me to be an oversight that "trap" and "break" 
instructions not related to stack overflow checking don't indicate that 
we've advanced past the end of the prologue, as do control transfer 
instructions.  GCC only generates these from calls to __builtin_trap() 
or in situations where it has detected some other trap situation with 
undefined behavior, like a null pointer dereference, which shouldn't 
appear in prologue code.

OK to commit?

-Sandra

2015-07-29  Sandra Loosemore  <sandra@codesourcery.com>

	gdb/
	* nios2-tdep.c (nios2_analyze_prologue): Update comments to
	reflect how current GCC emits stack overflow checks.  Match
	both trap and break instructions for backward compatbility.
	Disallow other trap and break instructions in the prologue.

diff --git a/gdb/nios2-tdep.c b/gdb/nios2-tdep.c
index 1968a88..1c5dcde 100644
--- a/gdb/nios2-tdep.c
+++ b/gdb/nios2-tdep.c
@@ -697,22 +697,29 @@ nios2_stack_frame_destroyed_p (struct gdbarch *gdbarch, CORE_ADDR pc)
 	  stw    sp, constant(rx)
 	  mov    sp, rx
 
-     5) A frame pointer save, which can be either a MOV or ADDI.
+     4) A frame pointer save, which can be either a MOV or ADDI.
 
-     6) A further stack pointer adjustment.  This is normally included
-        adjustment in step 4 unless the total adjustment is too large
+     5) A further stack pointer adjustment.  This is normally included
+        adjustment in step 3 unless the total adjustment is too large
 	to be done in one step.
 
      7) A stack overflow check, which can take either of these forms:
 	  bgeu   sp, rx, +8
-	  break  3
+	  trap  3
 	or
 	  bltu   sp, rx, .Lstack_overflow
 	  ...
 	.Lstack_overflow:
-	  break  3
-        If present, this is inserted after the stack pointer adjustments
-	for steps 3, 4, and 6.
+	  trap  3
+	  
+	Older versions of GCC emitted "break 3" instead of "trap 3" here,
+	so we check for both cases.
+
+	Older GCC versions emitted stack overflow checks after the SP
+	adjustments in both steps 3 and 4.  Starting with GCC 6, there is
+	at most one overflow check, which is placed before the first
+	stack adjustment for R2 CDX and after the first stack adjustment
+	otherwise.
 
     The prologue instructions may be combined or interleaved with other
     instructions.
@@ -995,14 +1002,15 @@ nios2_analyze_prologue (struct gdbarch *gdbarch, const CORE_ADDR start_pc,
 	  else if (cond == branch_geu)
 	    {
 	      /* BGEU sp, rx, +8
-		 BREAK 3
+		 TRAP 3  (or BREAK 3)
 		 This instruction sequence is used in stack checking;
 		 we can ignore it.  */
 	      unsigned int next_insn;
 	      const struct nios2_opcode *next_op
 		= nios2_fetch_insn (gdbarch, pc, &next_insn);
 	      if (next_op != NULL
-		  && nios2_match_break (next_insn, op, mach, &uimm))
+		  && (nios2_match_trap (next_insn, op, mach, &uimm)
+		      || nios2_match_break (next_insn, op, mach, &uimm)))
 		pc += next_op->size;
 	      else
 		break;
@@ -1010,13 +1018,14 @@ nios2_analyze_prologue (struct gdbarch *gdbarch, const CORE_ADDR start_pc,
 	  else if (cond == branch_ltu)
 	    {
 	      /* BLTU sp, rx, .Lstackoverflow
-		 If the location branched to holds a BREAK 3 instruction
-		 then this is also stack overflow detection.  */
+		 If the location branched to holds a TRAP or BREAK
+		 instruction then this is also stack overflow detection.  */
 	      unsigned int next_insn;
 	      const struct nios2_opcode *next_op
 		= nios2_fetch_insn (gdbarch, pc + imm, &next_insn);
 	      if (next_op != NULL
-		  && nios2_match_break (next_insn, op, mach, &uimm))
+		  && (nios2_match_trap (next_insn, op, mach, &uimm)
+		      || nios2_match_break (next_insn, op, mach, &uimm)))
 		;
 	      else
 		break;
@@ -1025,11 +1034,13 @@ nios2_analyze_prologue (struct gdbarch *gdbarch, const CORE_ADDR start_pc,
 	    break;
 	}
 
-      /* All other calls or jumps (including returns) terminate 
+      /* All other calls, jumps, returns, TRAPs, or BREAKs terminate
 	 the prologue.  */
       else if (nios2_match_callr (insn, op, mach, &ra)
 	       || nios2_match_jmpr (insn, op, mach, &ra)
-	       || nios2_match_jmpi (insn, op, mach, &uimm))
+	       || nios2_match_jmpi (insn, op, mach, &uimm)
+	       || nios2_match_trap (insn, op, mach, &uimm)
+	       || nios2_match_break (insn, op, mach, &uimm))
 	break;
     }