[RFA] x86 - jump instruction after the prologue

[Jerome Guitton <guitton@act-europe.fr>]

[-- Attachment #1: Type: text/plain, Size: 1252 bytes --]

Hello,

For what I understand from the skip_prologue algorithm in i386-tdep.c,
there are cases when the first instruction of a function is a jump to
the prologue code, which is located somewhere else in the function
(e.g.  the end of the function).  The last instruction of the prologue
in this case is a branch to the "real" code.

To take this case into account, GDB applies two corrections:

C1: GDB tests if the first instruction of the function is a jump; if
so, GDB jumps to the target of the branch (i.e.  it follows the branch).

C2: GDB tests if the next instruction after the prologue is a jump; if
so, it considers that it is a branch back to the "real" beginning of
the program and follows the branch.

A problem appears if we are in the "usual" case and if the first instruction
of the "real" code is a branch instruction:

0x8049454 <_ada_b>:     push   %ebp
0x8049455 <_ada_b+1>:   mov    %esp,%ebp
0x8049457 <_ada_b+3>:   jmp    0x8049460 <_ada_b+12>
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

In this case, GDB will not apply C1 but will apply C2. That seems wrong to
me, I cannot see how the pair (not C1, C2) can be correct.

See a possible fix in attachment. Tested on x86-linux, no regression.

OK to apply?

-- 
Jerome

[-- Attachment #2: diff --]
[-- Type: text/plain, Size: 921 bytes --]

2004-04-19  Jerome Guitton  <guitton@gnat.com>

	* i386-tdep.c (i386_skip_prologue): follow the last jump only if the
	function begins with a branch instruction.

Index: i386-tdep.c
===================================================================
RCS file: /cvs/src/src/gdb/i386-tdep.c,v
retrieving revision 1.187
diff -u -p -r1.187 i386-tdep.c
--- i386-tdep.c	18 Apr 2004 18:38:04 -0000	1.187
+++ i386-tdep.c	19 Apr 2004 17:08:36 -0000
@@ -750,7 +750,15 @@ i386_skip_prologue (CORE_ADDR start_pc)
 	}
     }
 
-  return i386_follow_jump (pc);
+  /* If the first instruction of the function is a branch, then the
+     setup sequence is at the end of the function and the instruction
+     at pc is branch back to the start. In this case, follow the
+     jump.  */
+
+  if (i386_follow_jump (start_pc) != start_pc)
+    return i386_follow_jump (pc);
+  else
+    return pc;
 }
 
 /* This function is 64-bit safe.  */