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Re: [RFA] skip_prologue_sal and sal expansion
- From: Jerome Guitton <guitton at adacore dot com>
- To: gdb-patches at sourceware dot org
- Date: Mon, 15 Jun 2009 12:48:51 +0200
- Subject: Re: [RFA] skip_prologue_sal and sal expansion
- References: <20090602162133.GA20678@adacore.com>
Ping
Jerome Guitton (guitton@adacore.com):
>
> A couple of cleanups in breakpoint.c. Let me give some background
> first; consider the following program:
>
> int counter = 42;
>
> inline void
> callee ()
> {
> counter = 0; /* set breakpoint in an inlined function. */
> }
>
> void
> caller ()
> {
> callee ();
> }
>
> int
> main ()
> {
> caller ();
> callee ();
> return counter;
> }
>
>
>
> When callee is inlined, we have three occurence for the line
> "counter = 0;": inlined in caller, inlined in main, and not inlined.
> When a breakpoint is set on this line, GDB sets a breakpoint on 3
> locations.
>
> (gdb) l p.c:6
> 1 int counter = 42;
> 2
> 3 inline void
> 4 callee ()
> 5 {
> 6 counter = 0;
> 7 }
> 8
> 9 void
> 10 caller ()
> (gdb) b 6
> Breakpoint 1 at 0x1800074: file p.c, line 6. (3 locations)
>
>
> I have recently hit a bug in an assembler which was optimizing out the
> prologue line info; it was making GDB think that the line
> "counter = 0;" was a part of callee's prologue. And this pointed me to
> something strange in GDB.
>
> After having used this bogus assembler to generate my program, if I try
> to set a breakpoint at line "counter = 0;", I end up with only one
> occurence instead of three:
>
> (gdb) b 6
> Breakpoint 1 at 0x1800074: file p.c, line 6.
>
> The problem was in skip_prologue_sal defined in breakpoint.c. When it
> actually skips a prologue, it does not assure that the other sal's
> fields (explicit_pc and explicit_line) are left unchanged. In my case,
> it was accidently changing explicit_line from 1 to 0. This change
> disabled the line sal expansion, and in consequence we ended up with
> the breakpoint set in only one location. I think that it's a bug in
> skip_prologue_sal, this function should not change mess with these
> fields.
>
> Now, if I change skip_prologue_sal to copy explicit_line and
> explicit_pc, the line expansion is done; but we should make sure that
> prologue is skipped similarly, otherwise we get an assertion failure
> when the address returned by resolve_sal_pc cannot be found after
> line sal expansion:
>
> (gdb) break p.c:6
> ../../src/gdb/breakpoint.c:5113: internal-error: expand_line_sal_maybe:
> Assertion `found' failed.
>
>
> Patch attached, tested on x86-linux. OK to apply?
>
>
> 2009-06-02 Jerome Guitton <guitton@adacore.com>
>
> * breakpoint.c (expand_line_sal_maybe): When explicit_line,
> skip prologue on each sals.
> (skip_prologue_sal): Return explicit_line and explicit_pc
> unmodified.
> Index: breakpoint.c
> ===================================================================
> --- breakpoint.c (revision 148760)
> +++ breakpoint.c (working copy)
> @@ -207,6 +207,9 @@ static void disable_trace_command (char
>
> static void trace_pass_command (char *, int);
>
> +static void skip_prologue_sal (struct symtab_and_line *sal);
> +
> +
> /* Flag indicating that a command has proceeded the inferior past the
> current breakpoint. */
>
> @@ -5412,6 +5415,15 @@ expand_line_sal_maybe (struct symtab_and
> }
> }
> }
> + else
> + {
> + for (i = 0; i < expanded.nelts; ++i)
> + {
> + /* If this SAL corresponds to a breakpoint inserted using a
> + line number, then skip the function prologue if necessary. */
> + skip_prologue_sal (&expanded.sals[i]);
> + }
> + }
>
>
> if (expanded.nelts <= 1)
> @@ -5896,7 +5908,8 @@ set_breakpoint (char *address, char *con
>
> /* Adjust SAL to the first instruction past the function prologue.
> The end of the prologue is determined using the line table from
> - the debugging information.
> + the debugging information. explicit_pc and explicit_line are
> + not modified.
>
> If SAL is already past the prologue, then do nothing. */
>
> @@ -5911,7 +5924,11 @@ skip_prologue_sal (struct symtab_and_lin
>
> start_sal = find_function_start_sal (sym, 1);
> if (sal->pc < start_sal.pc)
> - *sal = start_sal;
> + {
> + start_sal.explicit_line = sal->explicit_line;
> + start_sal.explicit_pc = sal->explicit_pc;
> + *sal = start_sal;
> + }
> }
>
> /* Helper function for break_command_1 and disassemble_command. */