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

[Arthur O'Dwyer via llvm-dev]

On Tue, Dec 4, 2018 at 2:24 PM John McCall via cfe-dev <
cfe-dev at lists.llvm.org> 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 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.
>
> [...] 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.
>
Peanut gallery says: I don't fully understand the use-case or the domain,
but this description sounds unworkable to me.

AIUI, there are two players here: the "brittle optimization" (which relies
on some invariant), and the "transform" (which has the power to break that
invariant).

The only two mathematically workable scenarios are:
(A) The brittle optimization's invariant is "impossible to [break] for
standard semantic/structural reasons." Therefore no transform ever needs to
know anything about it. The result is a "robust" optimization, and no need
for the supported-optimizations flagset.
(B) The brittle optimization's invariant is, in fact, brittle. Any
transform that doesn't explicitly preserve the invariant *can and will*
break the invariant. Therefore, every transform must have its own whitelist
of "invariants I know I don't break." Any flag in the
supported-optimizations flagset which is not whitelisted by a given
transform *must* be cleared when that transform is applied to the code.
(Because, by definition, a transform that doesn't explicitly preserve the
brittle invariant must be assumed to break it.)

my $.02,
–Arthur
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