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Re: [-ftree-cselim desoptimizes SH code


Hi,

On Sat, 26 Sep 2009, Ian Lance Taylor wrote:

> I believe this transformation is invalid under the C++0x memory model.

Well, if we choose to support this less than thrilling memory model then 
we possibly have to disable the pass (or limit it) for C++0x, as well as 
the vectorizer (and probably some other passes).  No reason that other 
languages have to suffer from the c++ committees decision to ignore 
volatile and barriers to the disfavor of optimizing compilers.


Ciao,
Michael.

P.S: Btw. it's really funny (if it weren't so sad) to read e.g. 
http://www.hpl.hp.com/personal/Hans_Boehm/misc_slides/c++mm.pdf .  It 
shows some nice examples what isn't possible with that crappy memory model 
anymore.  Without any words on how bad an idea that is.  And without 
really fixing the problem they try to solve (hardware reordering as 
explained later in the same slides).  Horrific.  I'm not into committees 
so much, but why they rejected the idea of marking the shared data as - 
well - shared (which would immediately remove all performance problems), 
is completely beyond me.  They must have rejected it, as it's so obvious 
that somebody there simply must have proposed it.  That something like 
this can get into a standard ... baffling.


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