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<div class="moz-cite-prefix">On 06/18/2015 09:15 PM, Nick Lewycky
wrote:<br>
</div>
<blockquote
cite="mid:CADbEz-iDvXoia_8nHAnJNJnvWQuh1F6H0Wqy2WRq1ym6Sj8nkQ@mail.gmail.com"
type="cite">
<blockquote class="gmail_quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex">Currently in
AliasAnalysis we can model ModRef behaviour for functions which<br>
only access memory through their pointer arguments. However we
can't<br>
express this propery as a function attribute.<br>
<br>
For example, for intrinsics we can specify ReadWriteArgMem or
ReadArgMem<br>
attributes in tablegen definitions. But due to the lack of the
related function<br>
attributes on the llvm ir level, this intrinsics would be
modelled as if they<br>
were clobbering arbitrary memory.<br>
<br>
It feels very naturall to add new function attribute which can
cover such cases.<br>
<br>
I have a patch (<a moz-do-not-send="true"
href="https://urldefense.proofpoint.com/v2/url?u=http-3A__reviews.llvm.org_D10398&d=AwMFaQ&c=8hUWFZcy2Z-Za5rBPlktOQ&r=Mfk2qtn1LTDThVkh6-oGglNfMADXfJdty4_bhmuhMHA&m=6QD3kOt2shxvhEw0pkmPGL_NzzjWw6s3ZTzBo-rwkUs&s=MJrrF5wf5KOXiTUsfpoJMSDYvyM_nZ3L79ZSsA9iXaA&e="
rel="noreferrer" target="_blank">http://reviews.llvm.org/D10398</a>)
in which I added this<br>
attribute. Currently there is some discussion on how to name it
and how it should<br>
behave when defined together with other fucntion attributes.<br>
</blockquote>
<div><br>
</div>
<div>What does it mean? Can it only touch memory that is directly
referred to by an argument? Or if that argument points to
another pointer, can we follow it?</div>
</blockquote>
I just want to point out that the notion Igor is introducing as an
attribute is not a new one. It's a prexisting notion which is
already implemented within LLVM today; it's simply been restricted
to intrinsics. Here's the definition from Intrinsics.td:<br>
// IntrReadWriteArgMem - This intrinsic reads and writes only from
memory that <br>
// one of its arguments points to, but may access an unspecified
amount. The <br>
// reads and writes may be volatile, but except for this it has no
other side <br>
//
effects.
<br>
def IntrReadWriteArgMem : IntrinsicProperty; <br>
<br>
I did point out in the review that I think the notion of ReadsArgMem
is redundant given the existing notions of ReadWriteArgMem and
ReadOnly, but that's somewhat orthogonal. It's true of the existing
implementation, not just Igor's patch. It may be worth settling on
this to clarify naming (i.e. are we ever going to need an attribute
like reads_arg_mem? Or can we be more generic and use
accesses_arg_mem + readonly for the same purpose?), but I don't
think that really changes the semantics in major ways. <br>
<br>
Philip<br>
<br>
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