[PATCH] Partial fix for PR backtrace/1718

[Mark Kettenis <kettenis@chello.nl>]

This fixes the backtrace problem with Emacs that Eli reported.  It's a
partial fix since the prologue analyzer still doesn't notice that %ebx
gets saved on the stack, but that's not terribly important.  This
patch also doesn't handle all the other instructions that might end up
in the prologue.

Committed,

Mark

Index: ChangeLog
from  Mark Kettenis  <kettenis@gnu.org>

	Partial fix for PR backtrace/1718.
	* i386-tdep.c (i386_analyze_frame_setup): Handle more instructions
	that GCC migrates into the prolugue.  Don't handle any
	instructions that clobber %ebx.

Index: i386-tdep.c
===================================================================
RCS file: /cvs/src/src/gdb/i386-tdep.c,v
retrieving revision 1.197
diff -u -p -r1.197 i386-tdep.c
--- i386-tdep.c 18 Jun 2004 16:06:24 -0000 1.197
+++ i386-tdep.c 24 Jul 2004 12:53:37 -0000
@@ -503,20 +503,28 @@ i386_analyze_frame_setup (CORE_ADDR pc, 
 
       op = read_memory_unsigned_integer (pc + 1, 1);
 
-      /* Check for some special instructions that might be migrated
-	 by GCC into the prologue.  We check for
+      /* Check for some special instructions that might be migrated by
+	 GCC into the prologue.  At this point in the prologue, code
+	 should only touch the scratch registers %eax, %ecx and %edx,
+	 so we check for
+
+	    movl $XXX, %eax
+	    movl $XXX, %ecx
+	    movl $XXX, %edx
 
-	    xorl %ebx, %ebx
+	 These instructions have opcodes 0xb8, 0xb9 and 0xba.
+
+	 We also check for
+
+	    xorl %eax, %eax
 	    xorl %ecx, %ecx
 	    xorl %edx, %edx
-	    xorl %eax, %eax
 
 	 and the equivalent
 
-	    subl %ebx, %ebx
+	    subl %eax, %eax
 	    subl %ecx, %ecx
 	    subl %edx, %edx
-	    subl %eax, %eax
 
 	 Because of the symmetry, there are actually two ways to
 	 encode these instructions; with opcode bytes 0x29 and 0x2b
@@ -524,21 +532,35 @@ i386_analyze_frame_setup (CORE_ADDR pc, 
 
 	 Make sure we only skip these instructions if we later see the
 	 `movl %esp, %ebp' that actually sets up the frame.  */
-      while (op == 0x29 || op == 0x2b || op == 0x31 || op == 0x33)
+      while ((op >= 0xb8 && op <= 0xba)
+	     || op == 0x29 || op == 0x2b
+	     || op == 0x31 || op == 0x33)
 	{
-	  op = read_memory_unsigned_integer (pc + skip + 2, 1);
-	  switch (op)
+	  if (op >= 0xb8 && op <= 0xba)
+	    {
+	      /* Skip the `movl' instructions cited above.  */
+	      skip += 5;
+	    }
+	  else
 	    {
-	    case 0xdb:	/* %ebx */
-	    case 0xc9:	/* %ecx */
-	    case 0xd2:	/* %edx */
-	    case 0xc0:	/* %eax */
-	      skip += 2;
-	      break;
-	    default:
-	      return pc + 1;
+	      /* Skip the `subl' and `xorl' instructions cited above.  */
+	      op = read_memory_unsigned_integer (pc + skip + 2, 1);
+	      switch (op)
+		{
+		case 0xc0:	/* %eax */
+		case 0xc9:	/* %ecx */
+		case 0xd2:	/* %edx */
+		  skip += 2;
+		  break;
+		default:
+		  return pc + 1;
+		}
 	    }
 
+	  /* If that's all, return now.  */
+	  if (current_pc <= pc + skip + 1)
+	    return current_pc;
+
 	  op = read_memory_unsigned_integer (pc + skip + 1, 1);
 	}