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Re: [PATCH] gdb: Don't skip prologue for explicit line breakpoints in assembler
* Pedro Alves <palves@redhat.com> [2019-06-21 14:43:26 +0100]:
> On 6/21/19 12:23 AM, Andrew Burgess wrote:
>
> > I spent some more time trying to find a path that would call both
> > 'decode_digits_list_mode' and then 'skip_prologue_sal', but I still
> > can't find one.
>
> But won't that change affect any code path that ends up in
> skip_prologue_sal with explicit_line set?
[ Disclaimer: In the below I'll take about 'in current testing we
never do X'. I understand that this doesn't mean GDB will
_never_ do X as our testing doesn't guarantee to hit every
possible code path, it's more an invitation for people to
suggest how me might create a test that does do X. ]
Indeed. My claim is that in the current testing we never get to
skip_prologue_sal with explicit_line set. My patch means we do now
enter skip_prologue_sal with explicit_line set, and I find that the
existing check that uses explicit_line means GDB doesn't behave as I'd
like.
Given that in HEAD explicit_line only seems to be set when decoding a
line spec in 'list_mode', my current belief is that explicit_line is
never set in a condition where the flag will then be checked. In
other words, I think the explicit_line is currently useless.
>
> E.g.:
>
> /* Helper function for break_command_1 and disassemble_command. */
>
> void
> resolve_sal_pc (struct symtab_and_line *sal)
> {
> CORE_ADDR pc;
>
> if (sal->pc == 0 && sal->symtab != NULL)
> {
> if (!find_line_pc (sal->symtab, sal->line, &pc))
> error (_("No line %d in file \"%s\"."),
> sal->line, symtab_to_filename_for_display (sal->symtab));
> sal->pc = pc;
>
> /* If this SAL corresponds to a breakpoint inserted using a line
> number, then skip the function prologue if necessary. */
> if (sal->explicit_line)
> skip_prologue_sal (sal);
> }
>
> Is that path unreachable today?
In current testing we enter this code block once, by accident.
The test 'gdb.dwarf2/dw2-objfile-overlap.exp' enters the block because
we load a symbol file at address 0. The check '(sal->pc == 0 &&
sal->symtab != NULL)' is trying to find SALs where the 'pc' has not
been set, in our case the 'pc' has been set; to zero. When we then
call 'find_line_pc' and then 'sal->pc = pc', we reset the 'pc' to zero
again.
In this one case the explicit_line flag is false, so skip_prologue_sal
is never called.
As an aside how would you feel about a patch that made the 'pc' field
of symtab_and_line private, and updated all users to use getter/setter
methods? I already did this in order to add a 'is_pc_initialised?'
type method to symtab_and_line. When I add this and change the above
code to say this:
void
resolve_sal_pc (struct symtab_and_line *sal)
{
CORE_ADDR pc;
if (sal->symtab != NULL && !sal->pc_p ())
{
// ... etc ...
then we no longer enter this block at all during the current testing.
>
>
> >
> > Looking back at how the explicit_line flag was originally used when
> > it was added in commit ed0616c6b78a0966, things have changed quite a
> > bit in the 10+ years since. There were some tests added along with
> > the explicit_line flag (gdb.cp/mb-*.exp) and these all pass both in
> > master and in my patched branch.
> >
> > My current thinking is that the explicit_line flag was no longer doing
> > anything useful in master, but if someone disagrees I'd love to
> > understand more about this.
>
> I seem to recall that GDB didn't use to update a breakpoint's line
> number to advance to the next line number that includes some actual
> compiled code. Like if you set a breakpoint at line 10 below:
>
> 10 // just a comment
> 11 i++;
>
> you end up with a breakpoint at line 11. Maybe it's old code
> related to that.
I wonder if what you meant to say here is the breakpoint is placed at
the address of line 11, but is recorded as being at line 10. This
actually would line up with what the explicit line flag was doing if
the explicit line flag was being set.
The problem seems to be that when the explicit_line flag was first
added there was just function for decoding linespec line numbers
'decode_all_digits'. At some point in time this split into
decode_digits_ordinary and decode_digits_list_mode, when this happened
the explicit_line flag was only ever being set in one path.
I suspect that if the behaviour you discussed above ever existed, then
it was before the split in how digits were decoded.
I'm running out of time to investigate this today, but when I get some
more time I'll dig a little more on this line of enquiry to see if I
can confirm or deny the above theory.
>
> Maybe I'm misremembering.
>
> In any case, if you change this, then you also should change
> the function's entry comment:
>
> /* Adjust SAL to the first instruction past the function prologue.
> If the PC was explicitly specified, the SAL is not changed.
> If the line number was explicitly specified, at most the SAL's PC
> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> is updated. If SAL is already past the prologue, then do nothing. */
> ^^^^^^^^^^
Would this be OK? (I'm not pushing anything until the above questions
are resolved):
diff --git a/gdb/symtab.c b/gdb/symtab.c
index c10e6b3e358..6e7e32fb4d8 100644
--- a/gdb/symtab.c
+++ b/gdb/symtab.c
@@ -3673,8 +3673,10 @@ skip_prologue_using_lineinfo (CORE_ADDR func_addr, struct symtab *symtab)
/* Adjust SAL to the first instruction past the function prologue.
If the PC was explicitly specified, the SAL is not changed.
- If the line number was explicitly specified, at most the SAL's PC
- is updated. If SAL is already past the prologue, then do nothing. */
+ If the line number was explicitly specified then the SAL can still be
+ updated, unless the language for SAL is assembler, in which case the SAL
+ will be left unchanged.
+ If SAL is already past the prologue, then do nothing. */
void
skip_prologue_sal (struct symtab_and_line *sal)
Thanks,
Andrew
>
> void
> skip_prologue_sal (struct symtab_and_line *sal)
> {
>
> Thanks,
> Pedro Alves