> Log:
> Teach BasicAA about noalias function parameters. Passes all of DejaGNU and
> test-suite.
I just grepped through LLVM's test-suite and didn't find any uses of the
restrict keyword though...
> // Pointing at a discernible object?
> if (O1) {
> + // Check for noalias attribute
> + if (isa<Argument>(O1)) {
> + const Argument *Arg = cast<Argument>(O1);
> + const Function *Func = Arg->getParent();
> + const ParamAttrsList *Attr = Func->getFunctionType()->getParamAttrs();
> + if (Attr) {
> + unsigned Idx = 1;
> + for (Function::const_arg_iterator I = Func->arg_begin(),
> + E = Func->arg_end(); I != E; ++I, ++Idx) {
> + if (&(*I) == Arg &&
> + Attr->paramHasAttr(Idx, ParamAttr::NoAlias))
> + return NoAlias;
> + }
> + }
> + }
This logic is more aggressive than what C's restrict qualifier allows
in two ways.
For this testcase:
define void @foo(i32* noalias %p, i32* noalias %q, i32 %i, i32 %j) {
%pi = getelementptr i32* %p, i32 %i
%qi = getelementptr i32* %q, i32 %i
%pj = getelementptr i32* %p, i32 %j
%qj = getelementptr i32* %q, i32 %j
store i32 0, i32* %p
store i32 0, i32* %pi
store i32 0, i32* %pj
store i32 0, i32* %q
store i32 0, i32* %qi
store i32 0, i32* %qj
ret void
}
basicaa is now saying this:
15 no alias responses (100.0%)
0 may alias responses (0.0%)
0 must alias responses (0.0%)
The noalias logic should consider that a noalias pointer still aliases
itself. In this example, %p, %pi, and %pj should all MayAlias each other,
and %q, %qi, and %qj should all MayAlias each other.
Second, since you're mapping C's restrict qualifier on parameters to this
noalias attribute, it's necessary to consider cases like this:
declare i32* @unclear(i32* %a)
define void @foo(i32* noalias %x) {
%y = call i32* @unclear(i32* %x)
store i32 0, i32* %x
store i32 0, i32* %y
ret void
}
Right now basicaa is saying NoAlias for %x and %y. The answer should be
MayAlias, because %y may legally be "based on" %x. There's nothing in the
definition of restrict that says that "based on" relationships must be
obvious to a compiler.
So for an alias query where one of the two pointers is noalias and the
other is not, and the non-noalias one doesn't have a base object that
basicaa can find, the answer should be MayAlias. A more advanced pass
could try to prove that the non-noalias pointer couldn't possibly be
"based on" the noalias one by examining all its uses, though the specific
example here would defy such analysis.
> + // Check for noalias atrribute independently from above logic
> + if (isa<Argument>(O2)) {
> + const Argument *Arg = cast<Argument>(O2);
> + const Function *Func = Arg->getParent();
> + const ParamAttrsList *Attr =
> Func->getFunctionType()->getParamAttrs();
> + if (Attr) {
> + unsigned Idx = 1;
> + for (Function::const_arg_iterator I = Func->arg_begin(),
> + E = Func->arg_end(); I != E; ++I, ++Idx) {
> + if (&(*I) == Arg &&
> + Attr->paramHasAttr(Idx, ParamAttr::NoAlias))
> + return NoAlias;
> + }
> + }
> + }
Same as above.
Dan
--
Dan Gohman, Cray Inc.
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