[cfe-dev] C++11 and enhacned devirtualization

[John McCall]

> On Jul 16, 2015, at 2:38 PM, Richard Smith <richard at metafoo.co.uk> wrote:
> On Thu, Jul 16, 2015 at 2:03 PM, John McCall <rjmccall at apple.com <mailto:rjmccall at apple.com>> wrote:
>> On Jul 16, 2015, at 11:46 AM, Richard Smith <richard at metafoo.co.uk <mailto:richard at metafoo.co.uk>> wrote:
>> On Thu, Jul 16, 2015 at 11:29 AM, John McCall <rjmccall at apple.com <mailto:rjmccall at apple.com>> wrote:
>> > On Jul 15, 2015, at 10:11 PM, Hal Finkel <hfinkel at anl.gov <mailto:hfinkel at anl.gov>> wrote:
>> >
>> > Hi everyone,
>> >
>> > C++11 added features that allow for certain parts of the class hierarchy to be closed, specifically the 'final' keyword and the semantics of anonymous namespaces, and I think we take advantage of these to enhance our ability to perform devirtualization. For example, given this situation:
>> >
>> > struct Base {
>> >  virtual void foo() = 0;
>> > };
>> >
>> > void external();
>> > struct Final final : Base {
>> >  void foo() {
>> >    external();
>> >  }
>> > };
>> >
>> > void dispatch(Base *B) {
>> >  B->foo();
>> > }
>> >
>> > void opportunity(Final *F) {
>> >  dispatch(F);
>> > }
>> >
>> > When we optimize this code, we do the expected thing and inline 'dispatch' into 'opportunity' but we don't devirtualize the call to foo(). The fact that we know what the vtable of F is at that callsite is not exploited. To a lesser extent, we can do similar things for final virtual methods, and derived classes in anonymous namespaces (because Clang could determine whether or not a class (or method) there is effectively final).
>> >
>> > One possibility might be to @llvm.assume to say something about what the vtable ptr of F might be/contain should it be needed later when we emit the initial IR for 'opportunity' (and then teach the optimizer to use that information), but I'm not at all sure that's the best solution. Thoughts?
>> 
>> The problem with any sort of @llvm.assume-encoded information about memory contents is that C++ does actually allow you to replace objects in memory, up to and including stuff like:
>> 
>> {
>>   MyClass c;
>> 
>>   // Reuse the storage temporarily.  UB to access the object through ‘c’ now.
>>   c.~MyClass();
>>   auto c2 = new (&c) MyOtherClass();
>> 
>>   // The storage has to contain a ‘MyClass’ when it goes out of scope.
>>   c2->~MyOtherClass();
>>   new (&c) MyClass();
>> }
>> 
>> The standard frontend devirtualization optimizations are permitted under a couple of different language rules, specifically that:
>> 1. If you access an object through an l-value of a type, it has to dynamically be an object of that type (potentially a subobject).
>> 2. Object replacement as above only “forwards” existing formal references under specific conditions, e.g. the dynamic type has to be the same, ‘const’ members have to have the same value, etc.  Using an unforwarded reference (like the name of the local variable ‘c’ above) doesn’t formally refer to a valid object and thus has undefined behavior.
>> 
>> You can apply those rules much more broadly than the frontend does, of course; but those are the language tools you get.
>> 
>> Right. Our current plan for modelling this is:
>> 
>> 1) Change the meaning of the existing !invariant.load metadata (or add another parallel metadata kind) so that it allows load-load forwarding (even if the memory is not known to be unmodified between the loads) if:
> 
> invariant.load currently allows the load to be reordered pretty aggressively, so I think you need a new metadata.
> 
> Our thoughts were:
> 1) The existing !invariant.load is redundant because it's exactly equivalent to a call to @llvm.invariant.start and a load.

No, that would not be arbitrarily hoistable.

> 2) The new semantics are a more strict form of the old semantics, so no special action is required to upgrade old IR.
> ... so changing the meaning of the existing metadata seemed preferable to adding a new, similar-but-not-quite-identical, form of the metadata. But either way seems fine.
>>   a) both loads have !invariant.load metadata with the same operand, and
>>   b) the pointer operands are the same SSA value (being must-alias is not sufficient)
>> 2) Add a new intrinsic "i8* @llvm.invariant.barrier(i8*)" that produces a new pointer that is different for the purpose of !invariant.load. (Some other optimizations are permitted to look through the barrier.)
>> 
>> In particular, "new (&c) MyOtherClass()" would be emitted as something like this:
>> 
>>   %1 = call @operator new(size, %c)
>>   %2 = call @llvm.invariant.barrier(%1)
>>   call @MyOtherClass::MyOtherClass(%2)
>>   %vptr = load %2
>>   %known.vptr = icmp eq %vptr, @MyOtherClass::vptr, !invariant.load !MyBaseClass.vptr
>>   call @llvm.assume(%known.vptr)
> 
> Hmm.  And all v-table loads have this invariant metadata?
> 
> That's the idea (but it's not essential that they do, we just lose optimization power if not).
>  
> I am concerned about mixing files with and without barriers.
> 
> I think we'd need to always generate the barrier (even at -O0, to support LTO between non-optimized and optimized code). I don't think we can support LTO between IR using the metadata and old IR that didn't contain the relevant barriers. How important is that use case?

Well, all current IR does not contain the barrier, so this would be a statement that current C++ IR will never be correctly LTO-able with future C++ IR.  That is generally something we try to avoid, yes.

> We were probably going to put this behind a -fstrict-something flag, at least to start off with, so we can create a transition period where we generate the barrier by default but don't generate the metadata if necessary.

If this goes into the function flags, perhaps there’s a way to prevent LTO between functions that disagree about the flag.

John.
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