[RFA] i386-tdep.c: Add i386_skip_noop function

["Pierre Muller" <muller at ics dot u-strasbg dot fr>]

  I tried to conciliate Pedro's and Mark's remarks
in a new patch.
  As this patch only tries to catch no-op
instruction added on purpose at the start
of a function to allow hot-patching, 
I removed the 'check' variable of my previous patch
that allowed to check several different patterns
as in the optics of 'on purpose no-op', 
looking for several patterns is a non-sense.
  As I also directly read 2 bytes, I added 
a check that returns pc in case the call to read_memory_nobpt
fails.

ChangeLog entry:
2008-01-25  Pierre Muller  <muller@ics.u-strasbg.fr>

	* i386-tdep.c (i386_skip_noop): New function.
	(i386_analyze_prologue): Call i386_skip_noop function.


Index: gdb/i386-tdep.c
===================================================================
RCS file: /cvs/src/src/gdb/i386-tdep.c,v
retrieving revision 1.248
diff -u -p -r1.248 i386-tdep.c
--- gdb/i386-tdep.c	11 Jan 2008 13:20:00 -0000	1.248
+++ gdb/i386-tdep.c	25 Jan 2008 10:29:09 -0000
@@ -632,6 +632,41 @@ struct i386_insn i386_frame_setup_skip_i
   { 0 }
 };
 
+
+/* Check whether PC points to a no-op instruction.  */
+static CORE_ADDR
+i386_skip_noop (CORE_ADDR pc)
+{
+  gdb_byte op[2];
+
+  if (read_memory_nobpt (pc, (gdb_byte *) &op, 2) != 0)
+    return pc;
+/* Some Microsoft's system dll functions start with a
+   `mov %edi,%edi' instruction, which is effectively a two byte `nop'.
+   This instruction is used for hot patching support, together with 5
+   bytes of slack before the function.  Later, when hot-patching, the 2
+   byte op can be replaced with a relative jump to 5 bytes back.  The 5
+   bytes slack is large enough to hold a jump into anywhere in
+   the 32-bit address space.  
+   A two byte nop is used to be sure that no thread is executing
+   the instruction at byte 1 of the function, so the patching can be
+   performed atomically.  */
+
+/* 0x8b,0xff matches `mov %edi,%edi' */
+  if (op[0] == 0x8b && op[1] == 0xff)
+    {
+      return pc + 2;
+    }
+/* Here other patterns can be added if found.  */
+/* Quoted from Mark Kettenis:
+   "I've heard of a couple of code generation tools that do something
similar
+   as Microsoft and insert nop instructions at the start of a function to
be
+   patched up later.  So other targets could benefit from the same code.  
+   And calling this function unconditionally keeps the code simple."  */
+
+  return pc; 
+}
+
 /* Check whether PC points at a code that sets up a new stack frame.
    If so, it updates CACHE and returns the address of the first
    instruction after the sequence that sets up the frame or LIMIT,
@@ -817,6 +852,7 @@ static CORE_ADDR
 i386_analyze_prologue (CORE_ADDR pc, CORE_ADDR current_pc,
 		       struct i386_frame_cache *cache)
 {
+  pc = i386_skip_noop (pc);
   pc = i386_follow_jump (pc);
   pc = i386_analyze_struct_return (pc, current_pc, cache);
   pc = i386_skip_probe (pc);