[M3devel] aliases/optimization

[Jay K]

Yep I just noticed, finally.

 - Jay

---------------------------------------
> From: hosking at cs.purdue.edu
> Date: Sun, 4 Jul 2010 10:47:50 -0400
> To: jay.krell at cornell.edu
> CC: m3devel at elegosoft.com
> Subject: Re: [M3devel] aliases/optimization
>
> I thought I already implemented one of those.
>
> On 4 Jul 2010, at 04:14, Jay K wrote:
>
> >
> > It appears this behavior is part of the C frontend, not the backend.
> > It appears..that maybe..we should provide a langhook get_alias_set that always returns 0.
> > The default is always -1.
> > Not clear to me.
> >
> > - Jay
> >
> > ----------------------------------------
> >> From: jay.krell at cornell.edu
> >> To: hosking at cs.purdue.edu
> >> CC: m3devel at elegosoft.com
> >> Subject: aliases/optimization
> >> Date: Sun, 4 Jul 2010 01:16:23 +0000
> >>
> >>
> >> aha you just reminded me of something that we need to remember a bit and apply soon.
> >>
> >>
> >> Depending on compilers, optimization, etc. gcc doesn't like:
> >>
> >>
> >> float f;
> >> int i;
> >> i = *(int*)&f;
> >>
> >> though I think that's perfectly reasonable..anyway the equivalent form of code that gcc is explicitly ok with is:
> >>
> >> float f;
> >>
> >> int i;
> >> union {
> >> float f;
> >> int i;
> >> } u;
> >> u.f = f;
> >> i = u.i;
> >>
> >> So, point being, we should try changing LOOPHOLE to compile like that.
> >> You know, cons up the union type on-demand, make a local, etc.
> >>
> >> If we are lucky, that might solve some of our problems.
> >> Not the PPC ones.
> >> But that I left some systematic use of volatile in, like for all floating point, or something.
> >> And maybe it'll fix some of the optimizations I disabled.
> >> It'd still leave "unit at a time" broken.
> >> Possibly in tree-nested we can remove any notion of the functions being nested and
> >> maybe that'll help..
> >>
> >> Search the for "gcc type punning"
> >> => wikipedia
> >> => link at the bottom
> >>
> >> http://gcc.gnu.org/onlinedocs/gcc-4.1.1/gcc/Optimize-Options.html#index-fstrict_002daliasing-542
> >>
> >> There is a subtlty there though..we'd have use member_ref on the union.
> >> They also give some pointer to what to do "for real". I can follow up, later.
> >>
> >>
> >> Disabling unit at a time is also lame.
> >>
> >>
> >> - Jay
> >>
> >> ----------------------------------------
> >>> From: hosking at cs.purdue.edu
> >>> Date: Sat, 3 Jul 2010 20:50:11 -0400
> >>> To: jay.krell at cornell.edu
> >>> CC: m3commit at elegosoft.com
> >>> Subject: Re: [M3commit] CVS Update: cm3
> >>>
> >>> I added it a long time back only because I saw failures with optimisation turned on. Something to do with the alias analysis (and lack of proper type information) as far as I recall.
> >>>
> >>>
> >>> On 3 Jul 2010, at 20:44, Jay K wrote:
> >>>
> >>>>
> >>>> Tony, just to be clear..you/I are disturbed by volatile, but it has also, I believe, like always been there.
> >>>> It has been gone only very briefly, and its non-use is probably limited for other reasons (how many people are
> >>>> using it, on how many platforms?).
> >>>>
> >>>>
> >>>> - Jay
> >>>>
> >>>> ----------------------------------------
> >>>>> From: jay.krell at cornell.edu
> >>>>> To: hosking at cs.purdue.edu; jkrell at elego.de
> >>>>> CC: m3commit at elegosoft.com
> >>>>> Subject: RE: [M3commit] CVS Update: cm3
> >>>>> Date: Sun, 4 Jul 2010 00:42:20 +0000
> >>>>>
> >>>>>
> >>>>> Not a multiprocessor.
> >>>>> Still interested in selective volatile?
> >>>>>
> >>>>>
> >>>>> This all bothers me too.
> >>>>> I don't want volatile. It makes the optimized code terrible.
> >>>>> But I don't want to debug any problem from removing it, beyond compilation failure.
> >>>>>
> >>>>>
> >>>>> I can try a few things.
> >>>>> This is all wierd. I swear I saw it hang several times.
> >>>>> I swear I'm trying to to change "too many" variables at a time. Yes, I know, 2 is too many.
> >>>>>
> >>>>>
> >>>>> Once I started getting version stamp mismatch, I resorted to using a cross built cm3.
> >>>>> Out of necessity sort of, but that causes more flucuation of variables.
> >>>>>
> >>>>> Let me try again with volatile and see if it is solid.
> >>>>> Then I'll try with only volatile stores.
> >>>>>
> >>>>> I've been trying optimized and unoptimized, and not kept good track of which when.
> >>>>>
> >>>>>
> >>>>> - Jay
> >>>>>
> >>>>>
> >>>>> ----------------------------------------
> >>>>>> From: hosking at cs.purdue.edu
> >>>>>> Date: Sat, 3 Jul 2010 20:36:20 -0400
> >>>>>> To: jkrell at elego.de
> >>>>>> CC: m3commit at elegosoft.com
> >>>>>> Subject: Re: [M3commit] CVS Update: cm3
> >>>>>>
> >>>>>> I am very disturbed that volatile is needed here. Can we selectively turn it on for thread-critical files like ThreadPThread and see if it fixes the problem. I wonder if the double-checked locking is broken for PPC memory model. Is this on a multi-processor?
> >>>>>>
> >>>>>> On 3 Jul 2010, at 12:57, Jay Krell wrote:
> >>>>>>
> >>>>>>> CVSROOT: /usr/cvs
> >>>>>>> Changes by: jkrell at birch. 10/07/03 12:57:09
> >>>>>>>
> >>>>>>> Modified files:
> >>>>>>> cm3/m3-sys/m3cc/gcc/gcc/m3cg/: parse.c
> >>>>>>>
> >>>>>>> Log message:
> >>>>>>> restore volatile for powerpc and powerpc64 platforms
> >>>>>>> This seems to fix PPC_LINUX hanging.
> >>>>>>> This needs further debugging, but I'm not eager.
> >>>>>>> This will also affect PPC_DARWIN, PPC64_DARWIN, PPC32_OPENBSD,
> >>>>>>> PPC32_NETBSD, PPC32_FREEBSD, etc., but these platforms are little used or
> >>>>>>> nonexistant.
> >>>>>>>
> >>>>>>> Having volatile like has been the very long standing situation though.
> >>>>>>> The result is that the optimizer does basically nothing.
> >>>>>>
> >>>>>
> >>>>
> >>>
> >>
> >
>