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-------- Original Message --------
Subject: Re: C++0x Memory model and gcc
Date: Fri, 07 May 2010 10:37:40 -0400
From: Andrew MacLeod <[email protected]>
To: Ian Lance Taylor <[email protected]>
References: <[email protected]>
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Ian Lance Taylor wrote:
Andrew MacLeod <[email protected]> writes:
They are independent as far as dependencies within this compilation unit.
The problem is if thread number 2 is performing
a.j = val
b.i = val2
now there are data races on both A and B if we load/store full words
and the struct was something like: struct {
char i;
char j;
} a, b;
The store to B is particularly unpleasant since you may lose one of
the 2 stores. The load data race on A is only in the territory of
hardware or software race detectors.
In this exmaple, if we do a word access to a, then we are running past
yes, well that's not what I was getting at. Add a short to the struct
to pad it out to a word or access the variables via a short load and
store... and do the appropriate masking. I was just trying not to
make the example any bigger than need be :-P
So the only potential problem here is if we have two small variables
where one is aligned and the other is not. This is an unusual
situation because small variables are not normally aligned. We can
avoid trouble by forcing an alignment to a word boundary after every
aligned variable.
Or so it seems to me.
The problem is when you load or store a larger unit than the actual
object you are loading or storing. Its not specifically an alignment
thing.
I don't know how many architectures still do this, but we need to
disable those that do.
Andrew
