This patch adds "NoAlias" to ParamAttr namespace's list of Attributes
This patch adds "NoAlias" to ParamAttr namespace's list of Attributes. No
use of it is yet made.

Assembly support for "NoAlias" attribute (1).

Assembly support for "NoAlias" attribute (2).

all 3 patches look good, plz apply. In the future, please submit proposed patches to llvm-commits.
Thanks!

-Chris

Information from Chris, this clearly explains why we need the intrinsic
llvm.noalis and its behavior:

Consider if you have something like this:

void foo(restrict *P1, restrict *P2) {
alias P1, P2 ?
}

In this case, obviously P1 and P2 don't alias. Now consider this:

void bar(void *P1, void *P2) {
foo(P1, P2);
}

Imagine if we inlined the function, we'd get this:

void bar(void *P1, void *P2) {
alias P1, P2 ?
}

Now we have to assume the pointer alias, we lost information. I'm
suggesting the intrinsics be defined to just return their pointer, so
the inline can create them. This would allow the inliner to produce:

void bar(void *P1, void *P2) {
P1tmp = llvm.noalias(P1)
P2tmp = llvm.noalias(P2)
alias P1tmp, P2tmp ?
}

which we can now decide don't alias.

The problem with this approach is that the intrinsics would block
optimizations, specifically stuff like SROA and mem2reg, which should
be updated to handle these intrinsics.

These intrinsics could also be used to translate code like this:

restrict int *G;

void bar() { *G; }

into ->

void bar() {
%tmp = load int** @​G
%tmp2 = call llvm.noalias int* tmp
}

Thanks, Chris :)

happy to help :)

BasicAA has been taught about noalias function arguments.

http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20070730/052392.html

It appears BasicAliasAnalysis.cpp does have an AddressMightEscape routine, which could be used to improve the analysis of queries involving a noalias pointer and a pointer with an unknown base. If the noalias pointer doesn't escape, then there is no alias. They key is to make sure that the logic for finding a base object always succeeds for expressions on which AddressMightEscape returns false.

Acomplia
Acomplia

The content of attachment 1352 has been deleted by
Tanya Lattner tonic@nondot.org
without providing any reason.

The token used to delete this attachment was generated at 2008-01-25 11:29:20.

We now support the noalias attribute, and have another PR to track the work for TBAA, closing as fixed.

mentioned in issue llvm/llvm-bugzilla-archive#1582