[RFC/RFA/sparc] problem with prologue analyzer

[Joel Brobecker <brobecker at adacore dot com>]

Hello,

Using break.exp, we have a function marker2 defined in break1.c as
follow (sic):

        int marker2 (a) int a; { return (1); }  /* set breakpoint 9 here */

Because the entire declaration is on one single line, the function
that skips prologue can not use the line number information from
debugging data (sparc32_skip_prologue()):

  /* This is the preferred method, find the end of the prologue by
     using the debugging information.  */
  if (find_pc_partial_function (start_pc, NULL, &func_start, &func_end))
    {
      sal = find_pc_line (func_start, 0);

      if (sal.end < func_end
          && start_pc <= sal.end)
        return sal.end;
    }

So sparc32_skip_prologue() fallsback to sparc_analyze_prologue().
Unfortunately, this function recognizes the prologue instructions
only up to the "save" instruction. But the prologue of a function
can contain store instructions that home the input registers into
their stack location. This is the case of our function marker2
above:

        (gdb) disass &marker2
        Dump of assembler code for function marker2:
        0x00010aa8 <marker2+0>: save  %sp, -112, %sp
        0x00010aac <marker2+4>: st  %i0, [ %fp + 0x44 ]
        0x00010ab0 <marker2+8>: mov  1, %g1
        0x00010ab4 <marker2+12>:        mov  %g1, %i0
        0x00010ab8 <marker2+16>:        nop 
        0x00010abc <marker2+20>:        ret 
        0x00010ac0 <marker2+24>:        restore 
        End of assembler dump.

A visible consequence of this problem is that GDB will insert
a breakpoint inside marker2 one instruction too earlier, and
hence just before parameter a has been homed. And that causes
the following FAIL in the GDB testsuite:

   (gdb) PASS: gdb.base/break.exp: run until file:function(1) breakpoint
   continue
   Continuing.
   720
   
   Breakpoint 2, 0x00010aac in marker2 (a=720) at break1.c:41
   41      int marker2 (a) int a; { return (1); }  /* set breakpoint 9 here */
   (gdb) FAIL: gdb.base/break.exp: run until quoted breakpoint

The value for parameter a is incorrect, it should be 43.

This test used to pass with 5.3. Doing a bit of archeology, I discovered
that the code analyzing problogues has been heavily rewritten at the end
of 2003, and that the piece of code that handles these store insns got
lost during one large code rewrite.

Assuming this was an accident, I put the code back more or less blindly.
I did exclude the part of the code that recognizes an instruction adding
and offset to sp, as I haven't seen evidences that this is needed, and
removed one if block that could only be executed in that case. But I'd
be happy to put the entire code back, if it is felt more appropriate.

2004-11-26  Joel Brobecker  <brobecker@gnat.com>

        * sparc-tdep.c (sparc_analyze_prologue): Recognize certain store
        instructions following the save instructions as part of the
        prologue.

Tested on sparc-solaris 2.8, with GCC (based on a 3.4.x backend).
Fixes:
  . break.exp: run until quoted breakpoint
    (the case I used to study the problem)
  . funcargs.exp: print *stp

Ok to apply?

Thanks,
-- 
Joel

Attachment: prologue.diff
Description: Text document