[M3devel] aliases/optimization

[Tony Hosking]

Yes, that's right!

On 3 Jul 2010, at 21:16, Jay K wrote:

> 
> 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.
>>>>> 
>>>> 
>>> 
>> 
>