[LLVMdev] Meaning of the nocapture attribute (possible bug?)
Richard Osborne
richard at xmos.com
Mon Oct 8 05:34:53 PDT 2012
Regarding the nocapture attribute the language ref says: "the callee
does not make any copies of the pointer that outlive the callee itself".
>From I inferred that it is OK for the callee to make a copy of the
pointer that doesn't outlive the call. However if I write some code that
does this the optimizers don't do what I'd expect. Consider the
following the example:
declare void @g(i32** %p, i32* %q) nounwind
define i32 @f(i32* noalias nocapture %p) nounwind {
entry:
%q = alloca i32*
call void @g(i32** %q, i32* %p) nounwind
store i32 0, i32* %p
%0 = load i32** %q
store i32 1, i32* %0
%1 = load i32* %p
ret i32 %1
}
I would expect it to be valid for g() to store the value of its second
argument to the object pointed to by its first argument. Because of this
I would expect a possible memory dependency between the last load (%1 =
load i32* %p) and the last store (store i32 1, i32* %0). However if I
run the example through opt -basicaa -gvn then the return instruction is
optimized to ret i32 0 suggesting basicaa doesn't think there is any
such dependency.
Is this a bug in the basic alias analysis pass or am I misunderstanding
the semantics of nocapture?
--
Richard Osborne | XMOS
http://www.xmos.com
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