Re: haifa-scheduler marks instructions having TRUE dependencies as an ANTI dependencies.

[Richard Biener <richard dot guenther at gmail dot com>]

On Thu, Jul 4, 2013 at 4:25 PM, Viktor Pobedin <viktor.pobedin@gmail.com> wrote:
> Hi all,
>
> It seems that sometimes haifa-scheduler assigns ANTI dependency for the
> instructions having TRUE dependency.
>
> I observed it happening in case of basic block as following:
>         <32 memory load/store rtx>
>         rtx_1: store to memory
>         rtx_2: load from memory
>
> I1 and i2 will have ANTI dependency instead of TRUE dependency.
>
> Once the sizes of pending_read_insns and pending_write_insns is bigger than
> MAX_PENDING_LIST_LENGTH (32) the sched_analyze_1 does flush_pending_lists
> thus putting current insn (rtx_1 in this example) to the
> last_pending_memory_flush list. During the analysis of the next rtx (rtx_2
> in the example) sched_analyze_2 marks rtx_1 and rtx_2  as having ANTI
> dependence. This is done in sched-deps.c:2652 by the following code:
>                     for (u = deps->last_pending_memory_flush; u; u = XEXP
> (u, 1))
>                       add_dependence (insn, XEXP (u, 0), REG_DEP_ANTI);
>
>
> I have few questions with this regard:
> 1.  Is my understanding correct?
> 2.  If it is what is the reason for this behavior?

Possibly a bug.

> 3.  What should I do in the cost_adjust hook if I want to distinguish
> between two cases (TRUE and ANTI dependency). In my target the cost for
> anti-dependency is 0, and cost for the true dependency 1 or bigger.
>
> BR, Viktor.
>