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<div class="moz-cite-prefix">On 7/31/20 1:24 PM, Hal Finkel wrote:<br>
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<div class="moz-cite-prefix">On 7/31/20 12:43 PM, John McCall
wrote:<br>
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<blockquote type="cite" cite="mid:15AA8332-E764-4691-9C6F-90B4D5328906@apple.com">
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<p dir="auto">n 31 Jul 2020, at 7:35, Hal Finkel wrote:</p>
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<p dir="auto">On 7/29/20 9:00 PM, John McCall via cfe-dev
wrote:</p>
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<p dir="auto">On 29 Jul 2020, at 17:42, Richard Smith
wrote:<br>
<br>
On Wed, 29 Jul 2020 at 12:52, John McCall <a class="moz-txt-link-rfc2396E" href="mailto:rjmccall@apple.com" moz-do-not-send="true"><rjmccall@apple.com></a>
wrote:<br>
<br>
...<br>
<br>
I think concretely, the escape hatch doesn't stop
things from<br>
going wrong,<br>
because -- as you note -- even though we *could* have
made a copy,<br>
it's<br>
observable whether or not we *did* make a copy. For
example:<br>
<br>
I would say that it’s observable whether the parameter
variable has<br>
the same address as the argument. That doesn’t /have/
to be the same<br>
question as whether a copy was performed: we could
consider there to be<br>
a formal copy (or series of copies) that ultimately
creates /an/ object<br>
at the same address, but it’s not the /same/ object
and so pointers<br>
to the old object no longer validly pointer to it. But
I guess that<br>
would probably violate the lifetime rules, because it
would make accesses<br>
through old pointers UB when in fact they should at
worst access a valid<br>
object that’s just unrelated to the parameter object.<br>
</p>
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<p dir="auto">I think that it would be great to be able to
do this, but unfortunately, I think that the point that
you raise here is a key issue. Whether or not the copy
is performed is visible in the model, and so we can't
simply act as though there was a copy when optimizing.
Someone could easily have code that looks like:<br>
<br>
Foo DefaultX;<br>
<br>
...<br>
<br>
void something(Foo &A, Foo &B) {<br>
<br>
if (&A == &B) { ... }<br>
<br>
}<br>
<br>
void bar(Foo X) { something(X, DefaultX); }</p>
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<p dir="auto">This example isn’t really on point; a call
like <code style="background-color:#F7F7F7;
border-radius:3px; margin:0; padding:0 0.4em" bgcolor="#F7F7F7">bar(DefaultX)</code> obviously cannot
just pass the address of <code style="background-color:#F7F7F7; border-radius:3px;
margin:0; padding:0 0.4em" bgcolor="#F7F7F7">DefaultX</code>
as a by-value argument without first proving a lot of
stuff about how <code style="background-color:#F7F7F7;
border-radius:3px; margin:0; padding:0 0.4em" bgcolor="#F7F7F7">foo</code> uses both its parameter and
<code style="background-color:#F7F7F7; border-radius:3px;
margin:0; padding:0 0.4em" bgcolor="#F7F7F7">DefaultX</code>.
I think <code style="background-color:#F7F7F7;
border-radius:3px; margin:0; padding:0 0.4em" bgcolor="#F7F7F7">noalias</code> is actually a subset of
what would have to be proven there.</p>
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<p><br>
</p>
<p>Yes, I apologize. You're right: my pseudo-code missed the
point. So the record is clear, let me rephrase:</p>
Foo *DefaultX = nullptr;<br>
...<br>
Foo::Foo() { if (!DefaultX) DefaultX = this; }<br>
...<br>
void bar(Foo X) { something(X, *DefaultX); }<br>
...<br>
bar(Foo{});
<p>I think that's closer to what we're talking about.<br>
</p>
<p><br>
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<blockquote type="cite" cite="mid:15AA8332-E764-4691-9C6F-90B4D5328906@apple.com">
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<p dir="auto">In general, the standard is clear that you
cannot rely on escaping a pointer to/into a
trivially-copyable pr-value argument prior to the call and
then rely on that pointer pointing into the corresponding
parameter object. Implementations are <em>allowed</em> to
introduce copies. But it does seem like the current
wording would allow you to rely on that pointer pointing
into <em>some</em> valid object, at least until the end
of the caller’s full-expression. That means that, if we
don’t guarantee to do an actual copy of the argument, we
cannot make it UB to access the parameter variable through
pointers to the argument temporary, which is what marking
the parameter as <code style="background-color:#F7F7F7;
border-radius:3px; margin:0; padding:0 0.4em" bgcolor="#F7F7F7">noalias</code> would do.</p>
<p dir="auto">So I guess the remaining questions are:</p>
<ul>
<li>Is this something we can reasonably change in the
standard?</li>
</ul>
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<p><br>
</p>
<p>This is the part that I'm unclear about. What change would we
make? <br>
</p>
<p><br>
</p>
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<p><br>
</p>
<p>Also, maybe some extended use of the no_unique_address attribute
would help?<br>
</p>
<p> -Hal</p>
<p><br>
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<blockquote type="cite" cite="mid:9c6c15e9-6839-6191-d761-ac734c42e8bf@anl.gov">
<p> </p>
<p><br>
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<blockquote type="cite" cite="mid:15AA8332-E764-4691-9C6F-90B4D5328906@apple.com">
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<ul>
<li>Are we comfortable setting <code style="background-color:#F7F7F7; border-radius:3px;
margin:0; padding:0 0.4em" bgcolor="#F7F7F7">noalias</code>
in C if the only place that would break is with a C++
caller?</li>
</ul>
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<p><br>
</p>
<p>Out of curiosity, if you take C in combination with our
statement-expression extension implementation (<a class="moz-txt-link-freetext" href="https://gcc.gnu.org/onlinedocs/gcc/Statement-Exprs.html" moz-do-not-send="true">https://gcc.gnu.org/onlinedocs/gcc/Statement-Exprs.html</a>),
and notwithstanding the statement in the GCC manual about
returns by value (i.e., the part just before where it says,
"Therefore the this pointer observed by Foo is not the address
of a."), is there any relationship to this topic?</p>
<p>Thanks again,</p>
<p>Hal<br>
</p>
<p><br>
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<blockquote type="cite" cite="mid:15AA8332-E764-4691-9C6F-90B4D5328906@apple.com">
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<p dir="auto">John.</p>
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<p dir="auto">As Richard's example shows, the code doesn't
need to explicitly compare the addresses to detect the
copy either. Any code that reads/writes to the objects
can do it. A perhaps-more-realistic example might be:<br>
<br>
int Cnt = A.RefCnt; ++A.RefCnt; ++B.RefCnt; if (Cnt +
1 != A.RefCnt) { /* same object case */ }<br>
<br>
The best suggestion that I have so far is that we could
add an attribute like 'can_copy' indicating that the
optimizer can make a formal copy of the argument in the
callee and use that instead of the original pointer if
that seems useful. I can certainly imagine a
transformation such as LICM making use of such a thing
(although the cost modeling would probably need to be
fairly conservative).<br>
<br>
-Hal<br>
<br>
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<p dir="auto">...<br>
<br>
John.<br>
<br>
<br>
_______________________________________________<br>
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</blockquote>
<p dir="auto">-- <br>
Hal Finkel<br>
Lead, Compiler Technology and Programming Languages<br>
Leadership Computing Facility<br>
Argonne National Laboratory</p>
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<pre class="moz-signature" cols="72">--
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory</pre>
</blockquote>
<pre class="moz-signature" cols="72">--
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory</pre>
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