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Re: [PATCH] Add atomic type qualifier

On 07/29/2013 12:06 PM, Joseph S. Myers wrote:
On Mon, 29 Jul 2013, Andrew MacLeod wrote:

Blick. What were they smoking the night before... I guess we'll probably need
to enhance the current atomic patterns in RTL...    We should be able to
figure out that its floating point and invoke the appropriate RTL pattern
during expansion rather than an existing one.    OR just frigging call
libatomic and let it deal with it. :-)  I guess there wouldnt be any other
fallback available. Actually, thats a mess... no way for the librtary to know
its floating point unless you tell it somehow with new entry points or
somesuch..  very lame.
Note that you only need *one* of the types to be floating-point for this
issue to apply.  If you have

_Atomic char c;
float f;

c /= f;

then all the same requirements apply; there may be exceptions to discard
not just from the division, but from the conversion of a float division
result to char.
yes, unfortunately.   but  gimplification should turn this to:
t0 = c
t1 =(float) t0
t2 = t0 / f
t3 = (char) t2
c = t3

so we simply see a floating point division bracketted by load and store from a TYPE_ATOMIC expression. so we'd should be able to recognize the need for the floating point extra stuff based on the type of the operation . and just wrap the whole blasted thing.

I must say, I'm not a fan :-)

I planned to do the work in gimplification... let the atomic decls through,
and during gimplification, loads or stores of an atomic decl would be
converted to the appropriate load or store builtin, and at the same time
recognize the  'decl = decl op value' expression and replace those as
appropriate with atomic_op_fetch operations.   I had discussed this at some
length with Lawrence crowl and Jeffrey Yasskin some time ago..   At
gimplification time we no longer know whether the original form was
decl op= val  or decl = decl op val;, but the decision was that it is ok to
recognize decl = decl op val and make that atomic.. it would still satisfy the
language requirements..
I think that's probably OK (though, is this a theorem of the formal
modelling work that has been done on the memory model?), but note it's not
I have no idea if its a theorem or not.
just a decl but an arbitrary pointer dereference (the address of the
lvalue is only evaluated once, no matter how many compare-and-exchange
operations are needed), and the operation may end up looking like

*ptr = (convert to type of *ptr) ((promote) *ptr op (promote) val)

rather than a simple decl = decl op val.  Or something more complicated if
I think gimplification takes care of that as well since all assignments have to be in the form DECL = VALUE op VALUE.. it constructs the sequence so that something like
*op_expr += 1

is properly transformed to
t0 = op_expr
  t1 = *t0
t2 = t1 + 1
*t0 = t2

With the TYPE_ATOMIC attribute set and rippled through the op_expr expression, we know that *t0 is atomic in nature, so t1 is an atomic load, *t0 is an atomic store, and looking back at t2 = t1 + 1 can see that this is an atomic += 1.

same thing with a normal load of an expression... the TYPE_ATOMIC gimple attribute *ought* to tell us everything we need.

the operation involves complex numbers - look at what gets for mixed real
/ complex arithmetic, for example.  Given

_Atomic _Complex float f;
double d;

f += d;

the atomicity is for the whole complex number (and so the
compare-and-exchange needs to work on the whole number) although only the
real part is modified by the addition.

complex I hadn't thought about until just now, I'll have to look. I know we can deal with parts on complex sometimes. Hopefully at gimplification time we still have the whole complex reference and if we just take care of that with the atomic builtins, we'll maintain the entire thing as we need.


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