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<p>I've stumbled across a case related to the nofree attribute where
we seem to have inconsistent interpretations of the attribute
semantic in tree. I'd like some input from others as to what the
"right" semantic should be.</p>
<p>The basic question is does the presence of nofree prevent the
callee from allocating and freeing memory entirely within it's
dynamic scope? At first, it seems obvious that it does, but that
turns out to be a bit inconsistent with other attributes and leads
to some surprising results. <br>
</p>
<p>For reference in the following discussion, here is the current
wording for the nofree function attribute in LangRef:</p>
<blockquote>
<p>"This function attribute indicates that the function does not,
directly or
indirectly, call a memory-deallocation function (free, for
example). As a
result, uncaptured pointers that are known to be dereferenceable
prior to a
call to a function with the <code class="docutils literal
notranslate"><span class="pre">nofree</span></code> attribute
are still known to be
dereferenceable after the call (the capturing condition is
necessary in
environments where the function might communicate the pointer to
another thread
which then deallocates the memory)."</p>
</blockquote>
<p>For discussion purposes, please assume the concurrency case has
been separately proven. That's not the point I'm getting at here.<br>
</p>
<p>The two possible semantics as I see them are:</p>
<p><b>Option 1</b> - nofree implies no call to free, period</p>
<p>This is the one that to me seems most consistent with the current
wording, but it prevents the callee from allocating storage and
freeing it entirely within it's scope. This is, for instance, a
reasonable thing a target might want to do when lowering large
allocs. This requires transforms to be careful in stripping the
attribute, but isn't entirely horrible.<br>
</p>
<p>The more surprising bit is that it means we can not infer nofree
from readonly or readnone. Why? Because both are specified only
in terms of memory effects visible to the caller. As a result, a
readnone function can allocate storage, write to it, and still be
readonly. Our current inference rules for readnone and readonly
do exploit this flexibility.</p>
<p>The optimizer does currently assume that readonly implies
nofree. (See the accessor on Function) Removing this
substantially weakens our ability to infer nofree when faced with
a function declaration which hasn't been explicitly annotated for
nofree. We can get most of this back by adding appropriate
annotations to intrinsics, but not all. <br>
</p>
<p><b>Option 2</b> - nofree applies to memory visible to the caller</p>
<p>In this case, we'd add wording to the nofree definition analogous
to that in the readonly/readnone specification. (There's a
subtlety about the precise definition of visible here, but for the
moment, let's hand wave in the same way we do for the other
attributes.)</p>
<p>This allows us to infer nofree from readonly, but essentially
cripples our ability to drive transformations within an annotated
function. We'd have to restrict all transforms and inference to
cases where we can prove that the object being affected is visible
to the caller. <br>
</p>
<p>The benefit is that this makes it slightly easier to infer nofree
in some cases. The main impact of this is improving ability to
reason about dereferenceability for uncaptured objects over calls
to functions for which we inferred nofree. <br>
</p>
<p>The downside of this is that we essentially loose all ability to
reason about nofree in a context free manner. For a specific
example of the impact of this, it means we can't infer
dereferenceability for an object allocated in F, and returned
(e.g. not freed), in the scope of F.</p>
<p>This breaks hoisting and vectorization improvements (e.g.
unconditional loads instead of predicated ones) I've checked in
over the last few months, and makes the ongoing deref redefinition
work substantially harder. <a class="moz-txt-link-freetext" href="https://reviews.llvm.org/D100141">https://reviews.llvm.org/D100141</a> shows
what this looks like code wise.<br>
</p>
<p><b>My Take</b><br>
</p>
<p>At first, I was strongly convinced that option 1 was the right
choice. So much so in fact that I nearly didn't bother to post
this question. However, after giving it more thought, I've come
to distrust my own response a bit. I definitely have a conflict
of interest here. Option 2 requires me to effectively cripple
several recent optimizer enhancements, and maybe even revert some
code which becomes effectively useless. It also makes a project
I'm currently working on (deref redef) substantially harder. <br>
</p>
<p>On the other hand, the inconsistency with readonly and readnone
is surprising. I can see an argument for that being the right
overall approach long term.</p>
<p>So essentially, this email is me asking for a sanity check. Do
folks think option 1 is the right option? Or am I forcing it to
be the right option because it makes things easier for me?</p>
<p>Philip<br>
</p>
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