On 07/19/2011 04:13 AM, Georg-Johann Lay wrote:
Vladimir Makarov wrote:
On 07/18/2011 04:14 PM, Richard Henderson wrote:
On 07/18/2011 12:15 PM, Georg-Johann Lay wrote:
However, what you've done is try very hard to work around reload
doing the Right Thing with constant spilling, namely re-generate
the constant rather than spill and restore it. I cannot believe
that's the right way to proceed.
You mean that with a clobber reload will rematerialize the constant
if there are not enough registers instead of spilling it?
And with an ordinary move like
(set (reg) (const_int))
reload will not see that it can reaterialize it and spill it to
the stack?
Well, it certainly didn't use to.
Vlad, do you know what the current state of the register allocator
is wrt this sort of rematerialization?
Rematerialization is done in reload. So it is the same as it was before
IRA.
Reload should definitely rematerialize constants and I saw it many times
when I analyzed the code generated by IRA+reload.
It will also do rematerialization of 'a hard reg + const'.
If I understand you correctly, a pre-reload split has no disadvantage
compared to a post-reload split? As oulined in
http://gcc.gnu.org/ml/gcc-patches/2011-07/msg01422.html
It is hard to me to say it. Only the experiment on several targets can
show it and efforts of achieving this.
GCC is an old compiler. More generally speaking, I believe in classic
approach that at this stage (1st insn scheduling, RA) RTL insns should
be as close as possible to real target insns.
The splitting mechanism is mostly a consequence of an outdated method of
partial code selection (combiner). Moving to the classical approach is
a huge job and might be too difficult to implement it.
The first split is pre-reload and generates a pseudo to load the constant.
The second split is post-reload and uses a clobber reg.
The question is what's best if the constant is used more than once, e.g. used
again in similar insn.
Again it is hard to say definitely for me. To remove second constant
load, reload has inheritance mechanism. In post-reload passes, there is
gcse for this.
reload pass can combine such reloads for one insn. But again i think
you should try if you wan to know, reload is too complicated to
definitely affirm something.
Is one approach better than the other? Or does it not
matter at all?
The whole patch is here:
http://gcc.gnu.org/ml/gcc-patches/2011-07/msg01411.html
Johann
