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

Philip Reames via cfe-dev cfe-dev at lists.llvm.org
Tue Dec 4 14:50:21 PST 2018


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?

On 12/4/18 11:24 AM, John McCall via llvm-dev wrote:
>
> On 4 Dec 2018, at 13:16, Sanjoy Das wrote:
>
>     On Mon, Dec 3, 2018 at 11:49 PM John McCall jmccall at apple.com
>     <mailto:jmccall at apple.com> wrote:
>
>         Piotr's proposal unfortunately doesn't give us a good name for
>         the class
>         of optimizations that require being listed in
>         supported_optimizations.
>         In earlier discussions I called them "brittle", but I can
>         understand why
>         nobody wants to call their optimization that, so let's call them
>         "good-faith optimizations" instead since they rely on the good
>         faith of
>         all the participating code.
>
>         Every optimization has to know how to maintain the structural
>         rules of
>         LLVM IR; that's what makes them structural rules. We don't
>         want the set of
>         structural rules to substantially change because such-and-such
>         good-faith
>         optimization is in effect because that would require arbitrary
>         transforms
>         to check the supported_optimizations list before they knew
>         which rules to
>         follow. Instead, the burden is on the optimization designer to
>         pick IR
>         constructs that won't be messed up by an arbitrary transform
>         with no special
>         knowledge of the optimization. The only thing the optimization
>         designer
>         can rely on is this:
>
>         other transforms will preserve the apparent semantics of the
>         function and
>         other transforms will maintain the standard structural rules
>         of LLVM IR.
>
>     Ok. Just to make sure we're on the same page, if this was all there
>     is we would not need this attribute right? All LLVM optimizations do
>     need to preserve semantics and structural properties anyway?
>
> We need this attribute because interprocedural optimizations otherwise
> break good-faith optimizations, so yes, my suummary here is missing some
> qualification (that I included in the next paragraph, but with a slightly
> different spin). So let me restate this.
>
> The designer of a good-faith optimization can rely on this:
>
>   * other transforms will preserve the apparent semantics of the function,
>   * other transforms will maintain the standard structural rules of
>     LLVM IR, and
>   * interprocedural transforms will honor supported_optimizations as
>     mentioned in Piotr's proposal --- and, in particular, will
>     intersect the supported_optimizations list whenever moving code
>     into a function.
>
> 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.
>
>         So the defining property of a good-faith optimization is that:
>         - there are rules which participating functions are expected
>         to follow on
>         pain of undefined behavior but which LLVM IR doesn't require
>         every function
>         to follow, and
>         - those rules will be preserved by any transform that doesn't
>         move code
>         between functions and which preserves the apparent function
>         semantics
>         and maintains the standard structural rules of LLVM IR.
>
>     In other words, certain things are UB in functions tagged with
>     supported_optimizations that are not UB otherwise? This breaks code
>     hoisting transformations right? I.e.
>     isSafeToSpeculativelyExecute(Inst) will have to return false if Inst
>     is in a function with a non-empty supported_optimizations?
>
> Good question. I would consider that to be an unacceptable intrusion:
> intraprocedural transforms should never have to be aware of
> supported_optimizations (unless they're implementing a good-faith
> optimization, of course) and interprocedural transforms should only have
> to be aware of supported_optimizations in the narrow sense outlined
> by Piotr. If something about the optimization's representation in IR
> is unsafe to speculate, it should be made impossible to speculate for
> standard semantic/structural reasons, like having apparently arbitrary
> side-effects.
>
> I think the right way to formalize this is to say that, while the
> good-faith optimization may impose additional UB rules on the function,
> it must guarantee that transforms that are well-behaved as described
> above --- i.e. that preserve standard structure and semantics and which,
> if interprocedural, appropriately honor supported_optimizations --- will
> never introduce new UB.
>
> John.
>
>
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