[llvm-dev] RFC: Supported Optimizations attribute
Philip Reames via llvm-dev
llvm-dev at lists.llvm.org
Wed Dec 5 09:19:46 PST 2018
On 12/4/18 3:21 PM, John McCall wrote:
> 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
> with no identified transition points, you don't know whether it
> actually doesn't
> transition or whether it was compiled without the transitions being
> marked. There's no way to extend the TBAA idea to make that work.
> 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
> 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
> 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
> transforms that need to know about the existence of good-faith
> 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.
1) My comment was on *framing*, not substance. I'm not debating the
*semantics* you've proposed (here), just the way they're described. The
way they're described here is very likely to lead to problematic
2) Reading back through your description again, it really sounds like
you've reinvented the rules for metadata with an alternate framing. The
only part which is possibly new is the IPO rules you want to apply.
Worth noting is that we already have existing support for metadata on
both instructions and functions.
If we frame all of this as being *metadata*, then your
supported_optimization attribute reduces to the need to define an
intersect rule for metadata on functions during inlining and IPO. Note
that we already have precedence for conservative-by-default handling at
the instruction level, so extending that to the function scope seems
-------------- next part --------------
An HTML attachment was scrubbed...
More information about the llvm-dev