[cfe-dev] [llvm-dev] RFC: Supported Optimizations attribute

Piotr Padlewski via cfe-dev cfe-dev at lists.llvm.org
Wed Dec 5 00:57:55 PST 2018

śr., 5 gru 2018 o 00:22 John McCall via cfe-dev <cfe-dev at lists.llvm.org>

> On 4 Dec 2018, at 17:50, Philip Reames wrote:
> Skimming along, apologies if I'm repeating something which already got
> said.
> If I understand this correctly, the basic problem we're trying to solve is
> to use a local hint (the invariant.group) to make a global assumption about
> other code which might exist elsewhere outside the function.  The attribute
> proposed can basically be phrased as describing a universe of functions
> within which our desired global property holds.  There's an ambiguity about
> what is allowed to be assumed about code outside that universe.
> I think it's important to note that we have a precedent of something
> similar to this in TBAA.  TBAA information coming from different modules
> has the same base problem.  We solve it by using the "root" of the TBAA
> tree as a scope descriptor, and essentially making two TBAA nodes from
> distinct roots incomparable.
> Can someone explain concisely why a similar scheme couldn't be used to
> solve this problem?
> TBAA is conservative in *two* ways:
> - It allows two accesses to alias if they have TBAA nodes with different
> roots.
> - It allows two accesses to alias if only one of them has a TBAA node.
> The second is what doesn't generalize: there are optimizations where you
> need to
> rely on transition points being explicitly identified. Looking at a
> function
> with no identified transition points, you don't know whether it actually
> doesn't
> transition or whether it was compiled without the transitions being
> explicitly
> marked. There's no way to extend the TBAA idea to make that work.
The other reason why similar scheme doesn't work for !invariant.group is
that we rely on a calls to launder/strip being present for some constructs
to preserve
information about invartianess of an object (like in the example from RFC).

> On 12/4/18 11:24 AM, John McCall via llvm-dev wrote:
> Note that IPO is generally permitted to partially inline or outline code,
> and so good-faith optimizations that e.g. require two instructions to be
> moved
> in tandem or not at all must use tokens to establish that unbreakable
> relationship.
> I think the way your framing this is dangerous.  We absolutely can not
> allow any annotation of this form to *weaken* the semantics of the existing
> IR.  We can and should impose a criteria that any extension of this variety
> strictly add information to the IR which might not have been previously
> inferred.  We can then design rules for how to preserve our new information
> as long as possible, but framing this in terms of disallowed
> transformations is really a non-starter.
> That's exactly what I was trying to convey here. Authors of good-faith
> optimizations need to design their representations so that transformations
> that know nothing about their optimizations but merely preserve semantics
> and well-formed IR structure will not break their representations. The only
> transforms that need to know about the existence of good-faith
> optimizations
> are interprocedural optimizations; furthermore, those optimizations don't
> need to know about any good-faith optimizations specifically, they just
> need
> to understand how to correctly update the supported_optimizations list.
> That is a very small burden on IPO that enables an interesting class of
> language-specific optimizations.
> John.
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