[LLVMdev] Anderson's analysis, getresult instruction on x86_64

Prakash Prabhu prakash.prabhu at gmail.com
Tue Aug 5 11:02:45 PDT 2008

Thanks for the replies, Daniel and Matthijs. I added some code to generate
the copy constraint, which I think (in the present form) is wrong (or at
best not field sensitive):

+void Andersens::visitGetResultInst(GetResultInst &GR) {
+       if (isa<PointerType>(GR.getType()))
+       {
+               // P1 = getresult P2 --> <Copy/P1/P2>
+               Constraints.push_back(Constraint(Constraint::Copy,
+       }

Take for example the getresult statement:

%mrv_gr = getresult { i64*, i64* } %74, 0 ; ; <i64*> [#uses=1]

The above code would add the constraint as Copy/%mrv_gr/%74 instead of
Copy/%mrv_gr/%74.0 (the actual field which is a pointer). Is there a way in
one can extract the Value* of the appropriate field of getresult using the
index (0 in this case) and the base structure (%74) ? (or is this what you
were refering to: "For aggregates in memory, the pointer to the part of the
aggregate can simply be used, but no such thing for first class values.",
Matthjis ?)

Since in all the programs on which I am running Andersen's analysis, the
only usage of getresult is in accessing non-pointers:

%mrv_gr205 = getresult { i64, i64 } %615, 1     ; <i64> [#uses=1]

, for now, it seems to be ok to just ignore such instructions for pointer
analysis (by the above code).

On Tue, Aug 5, 2008 at 3:59 AM, Matthijs Kooijman <matthijs at stdin.nl> wrote:

> Hi Prakash,
> I think this is highly related to PR2527 [1] and PR2451 [2]. I assume you
> are
> using LLVM 2.3, since current svn automatically replaces getresult with
> extractvalue instructions (but doesn't help for Andersen's).
> In either case, it seems that Andersen's currently has no ready made way to
> indicate a part of a first-class aggregate (or vector), which is the
> essence
> of the problem here.  For aggregates in memory, the pointer to the part of
> the
> aggregate can simply be used, but no such thing for first class values.
> I'm not really into how Andersen's works, so I won't be able to help you
> along
> with this any further, I'm afraid.
> Gr.
> Matthijs
> [1]: http://llvm.org/bugs/show_bug.cgi?id=2557
> [2]: http://llvm.org/bugs/show_bug.cgi?id=2451
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