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Re: change #9754: 5 x slowdown for perl -d

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From:
Dave Mitchell
Date:
January 19, 2003 15:57
Subject:
Re: change #9754: 5 x slowdown for perl -d
Message ID:
20030119235415.F24444@fdgroup.com
On Sun, Jan 19, 2003 at 11:00:51PM +0000, Nicholas Clark wrote:
> On Sun, Jan 19, 2003 at 10:24:58PM +0100, Rafael Garcia-Suarez wrote:
> > -    cop->op_private = (PL_hints & HINT_BYTE);
> > +    cop->op_private = (PL_hints & HINT_PRIVATE_MASK);
[snip]
> > The only thing that changes is the value of op_private for COPs.
> > This value is then tested in pp_dbstate, which replaces pp_nextstate
> > when run under the debugger.
>
> The two things I don't understand
> 
> 1: The above change seems to relate only to hints. Yet the profiling suggested
>    that restore_magic is now called about 23 million times, about 23 million
>    more than previously. How does changing hints increase the amount of
>    magic?

Because pp_dbstate() looks like this:

    PP(pp_dbstate)
    {
	PL_curcop = (COP*)PL_op;
	TAINT_NOT;		/* Each statement is presumed innocent */
	PL_stack_sp = PL_stack_base + cxstack[cxstack_ix].blk_oldsp;
	FREETMPS;

	if (PL_op->op_private || SvIV(PL_DBsingle)
	    || SvIV(PL_DBsignal) || SvIV(PL_DBtrace))
	{
	     .... call DB::DB ....


ie if op_private is non-zero, call DB:::DB for every single statement -
hence the zillions of extra function calls your profiling shows up.
Or to put it another way, putting hints in op_private is inadvertently
causing the debugger to single-step every statement.

> 2: What other things should break without this hints change?
>    So what sort of regression tests should we have for them?
> 
> > (Another fix is of course to stop storing hints in COP.op_private.)

I guess the other thing to do is to stop pp_dbstate() testing
op_private; or find out why it does it, and only test the right bit(s).

Dave.

-- 
The perl5 internals are a complete mess. It's like Jenga - to get the
perl5 tower taller and do something new you select a block somewhere in
the middle, with trepidation pull it out slowly, and then carefully
balance it somewhere new, hoping the whole edifice won't collapse as a
result.
		- Nicholas Clark.

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