[SMS,DDG] Correctly delete anti-dep edges for renaming

This letter contains second separate patch for ddg.c
It prevents removing some edges in data dependency graph
when renaming is allowed.

2011/10/12 Ayal Zaks <ayal.zaks@gmail.com>:
>>> The other bug happens only with -fmodulo-sched-allow-regmoves.  Here we
>>> eliminate some anti-dependence edges in data dependency graph in order to
>>> resolve them later by adding some register moves (renaming instructions).  But
>>> in situation as in example above compiler gives an ICE because it can't create
>>> a register move, when regR is hardware flag register.  So we have to know which
>>> register(s) cause anti-dependency in order to understand whether we can ignore
>>> it.  I can't find any easy way to gather this information, so I create my own
>>> structures to store this info and had implemented my own hooks for
>>> sched_analyze function.  This leads to more complex interconnection between
>>> ddg.c and modulo-sched.c.
>
> Well, having ddg.c's/create_ddg_dep_from_intra_loop_link() consult
> "Register sets from modulo scheduler structures" to determine if an
> anti-dep can be eliminated, seems awkward. One should be able to build
> a ddg independent of any modulo scheduler structures.

I think now I found a proper way to gather information about
the register(s) causing anti-dependency (see the patch attached).

> One simple solution is to keep all anti-dep edges of registers that
> are clobbered anywhere in the loop. This might be overly conservative,
> but perhaps not so if "On x86_64 it is clobbered by most of arithmetic
> instructions".
> If an anti-dep between a use and a dep of a clobber register is to be
> removed, a dependence should be created between the use and a
> clobbering instruction following the def. Hope this is clear.
>
> This too should be solved by a dedicated (separate) patch, for easier digestion.
>
> Presumably, the ddg already has all intra-iteration edges preventing
> motion of clobbering instructions within an iteration, and we are only
> missing inter-iteration deps or deps surfaced by eliminating
> anti-deps, right?
>
> You might consider introducing a new type of dependence for such
> edges, "Clobber", if it would help.
>

Ayal, I suppose here we have some misunderstanding.
This is not directly related to the problem of clobbers discussed previously.
Here I talk about the need to find out the register causing the
anti-dependence (for instance, we need to check whether it can be renamed).
Currently, SMS simply attempts to take the RHS of the second instruction (via
single_set()), and if it's a register, SMS assumes it's a register causing the
dependency. This breaks in a following scenario:

insn1: ... (read flags)
insn2: regA = regB - regC; update flags

Here, SMS would wrongly take regA as the dependency register, while it was in
fact the flags register. So, I rewrote this part in a different way and make
a separate patch.

--
Roman Zhuykov
zhroma@ispras.ru