[LLVMdev] question about enabling cfl-aa and collecting a57 numbers

[Daniel Berlin]

On Sat Jan 17 2015 at 12:03:33 AM Hal Finkel <hfinkel at anl.gov> wrote:

> Hi Danny,
>
>    // Add TypeBasedAliasAnalysis before BasicAliasAnalysis so that
>    // BasicAliasAnalysis wins if they disagree. This is intended to help
>    // support "obvious" type-punning idioms.
> -  if (UseCFLAA)
> -    addPass(createCFLAliasAnalysisPass());
>    addPass(createTypeBasedAliasAnalysisPass());
>    addPass(createScopedNoAliasAAPass());
> +  if (UseCFLAA)
> +    addPass(createCFLAliasAnalysisPass());
>    addPass(createBasicAliasAnalysisPass());
>
> Do we really want to change the order here? I had originally placed it
> after the metadata-based passes thinking that the compile-time would be
> better (guessing that the metadata queries would be faster than the CFL
> queries, so if the metadata could quickly return a NoAlias, then we'd cut
> out unecessary CFL queries). Perhaps this is an irrelevant effect, but we
> should have some documented rationale.
>

Yeah, this was a mistake (Chandler had suggested it was right earlier, but
we were both wrong :P)


>
>  ; CHECK: PartialAlias:  i16* %bigbase0, i8* %phi
> -define i8 @test0(i8* %base, i1 %x) {
> +define i8 @test0(i1 %x) {
>  entry:
> +  %base = alloca i8, align 4
>    %baseplusone = getelementptr i8* %base, i64 1
>    br i1 %x, label %red, label %green
>  red:
> @@ -25,8 +26,9 @@ green:
>  }
>
> why should this return PartialAlias? %ohi does partially overlap, so this
> correct, but what happens when the overlap is partial or control dependent?

So, after talking with some people offline, they convinced me in SSA form,
 the name would change in these situations, and the only situations you can
get into trouble is with things "based on pointer arguments" (because you
have no idea what their initial state is),  or "globals" (because they are
not in SSA form)
I could not come up with a case otherwise
But i'm welcome to hear if you think this is wrong.

FWIW: I bootstrapped/tested the compiler with an assert that triggered if
CFL-AA was going to return PartialAlias and BasicAA would have returned
NoAlias, and it did not trigger with this change.

(It would have triggered before this set of changes)

Not proof of course, but it at least tells me it's not "obviously wrong" :)



thought you had concluded that CFL should return only NoAlias or MayAlias?


> Thanks again,
> Hal
>
> ----- Original Message -----
> > From: "Daniel Berlin" <dberlin at dberlin.org>
> > To: "Nick Lewycky" <nlewycky at google.com>
> > Cc: "Jiangning Liu" <Jiangning.Liu at arm.com>, "George Burgess IV" <
> george.burgess.iv at gmail.com>, "LLVM Developers
> > Mailing List" <llvmdev at cs.uiuc.edu>
> > Sent: Friday, January 16, 2015 2:22:23 PM
> > Subject: Re: [LLVMdev] question about enabling cfl-aa and collecting
> a57      numbers
> >
> >
> >
> > Okay, overnight i ran a ton of tests on this patch, and it seems
> > right.
> > Nick, Hal, can you review it?
> >
> >
> > I've reattached it for simplicity
> >
> >
> >
> >
> > On Thu, Jan 15, 2015 at 3:05 PM, Daniel Berlin < dberlin at dberlin.org
> > > wrote:
> >
> >
> >
> >
> >
> >
> >
> > On Thu, Jan 15, 2015 at 1:26 PM, Nick Lewycky < nlewycky at google.com >
> > wrote:
> >
> >
> >
> >
> >
> > On 15 January 2015 at 13:10, Daniel Berlin < dberlin at dberlin.org >
> > wrote:
> >
> >
> >
> > Yes.
> > I've attached an updated patch that does the following:
> >
> >
> > 1. Fixes the partialalias of globals/arguments
> > 2. Enables partialalias for cases where nothing has been unified to a
> > global/argument
> > 3. Fixes that select was unifying the condition to the other pieces
> > (the condition does not need to be processed :P). This was causing
> > unnecessary aliasing.
> >
> >
> > Consider this:
> >
> >
> >
> > void *p = ...;
> > uintptr_t i = p;
> > uintptr_t j = 0;
> > for (int a = 0; a < sizeof(uintptr_t); ++a) {
> > j = i >> (sizeof(uintptr_t) - a - 1) ? 1 : 0;
> > j <<= 1;
> > }
> > void *q = j;
> >
> >
> > alias(p, q) isn't NoAlias. (Okay, it kinda is in C++, but not in the
> > equivalent LLVM IR. Please don't make me rewrite my example in LLVM
> > IR.)
> >
> >
> > Agreed :)
> >
> >
> > But after chatting with you, i think we both agree that this change
> > does not affect.
> > I probably should not have said "the condition does not need to be
> > processed". It would be more accurate to say "the reference to a
> > condition in a select instruction, by itself, does not cause
> > aliasing"
> >
> >
> > What happens now is:
> >
> >
> > given %4 = select %1, %2, %3
> >
> >
> > we do
> > aliasset(%4) += %1
> > aliasset(%4) += %2
> > aliasset(%4) += %3
> >
> > The first one is unnecessary.
> > There can be no alias caused simply because it is referenced in
> > condition of the select.
> >
> >
> >
> >
> >
> > We still need to process what %1 refers to (and we do).
> >
> >
> >
> >
> > To make this empirical, in your example, we get the right answer in
> > CFL-AA.
> >
> >
> > Interestingly, i'll point out that basic-aa says:
> >
> > NoAlias: i8* %p, i8** %q
> > NoAlias: i8** %p.addr, i8** %q
> >
> >
> > (I translated your case as best i can :P)
> >
> >
> > So you may want to implement it for real if you think it's supposed
> > to be handled right in basic-aa, because I don't believe it is :)
> >
> >
> >
> > _______________________________________________
> > LLVM Developers mailing list
> > LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
> >
>
> --
> Hal Finkel
> Assistant Computational Scientist
> Leadership Computing Facility
> Argonne National Laboratory
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20150117/88a3d4b4/attachment.html>