r359960 - Reduce amount of work ODR hashing does.
David Blaikie via cfe-commits
cfe-commits at lists.llvm.org
Mon May 6 16:39:43 PDT 2019
On Mon, May 6, 2019 at 4:24 PM Richard Trieu <rtrieu at google.com> wrote:
>
> There was no cycle for this crash.
Oh, yeah, didn't mean to imply there were - but that a system designed
to prevent cycles might also be used/help prevent redundant work like
this.
> What happened is that an exponential runtime is reduced to a linear runtime. Without this revision, ODR hashing would have worked if the machine had enough memory and the user waited long enough.
>
> void foo(int a, int b) {}
> When computing the ODR hash for function foo, it will visit the type int twice, once per parameter. In general, re-visiting types shouldn't be a problem, and in most cases, should be pretty fast.
It does mean some potentially problematic worst-case situations where
non-trivial types are mentioned more than once (eg: if, instead of
int, it was a complex struct type - it wouldn't cycle, but it would do
all that work twice (or many more times if it appears in more places
in the entity being hashed)
> class S {
> void bar(S* s);
> };
> There's actually two ways to visit the Decl behind S, ODR::AddCXXRecordDecl and ODR::AddDecl. When computing the ODR hash of S, ODR::AddCXXRecordDecl is used for a deep dive into the AST of S. When reaching S another way, (via FunctionDecl bar, parameter s, PointerType S*, RecordType S), then the CXXRecordDecl gets processed through ODR::AddDecl, which only processes enough information to identify S, but not any of its deeper details. This allows self-reference without introducing cycles.
Ah, OK - specifically to break the cycle.
So the ODR hash of the function "void f(S*)" doesn't hash the
implementation of S, (it uses AddDecl, not AddCXXRecordDecl)? But if
it were "void f(S)" it would hash S? What about a member function that
takes a parameter by value? ("struct S { void bar(S); }")
> I think it would be possible to add some checks in debug mode to catch cycles. I'm not sure it can detect redundant work as the function foo example above shows that visiting the same types over multiple times is expected.
Both for efficiency and to avoid these cycles, it might be worthwhile
to consider a different way to resolve this issue
The reason these ideas come to my mind is that DWARF has a type hash
that works in a different way to avoid cycles and redundant work.
http://dwarfstd.org/doc/DWARF5.pdf - 7.32, Type Signature Computation.
It works by assigning every type a number when it's first encountered
(even before its contents are hashed), and if it's ever encountered
again, hash the number again rather than going back into hashing the
implementation.
This way no type is hashed more than once, avoiding cycles and redundant work.
>
>
>
> From: David Blaikie <dblaikie at gmail.com>
> Date: Sat, May 4, 2019 at 9:06 AM
> To: Richard Trieu
> Cc: cfe-commits
>
>> Does the ODR hashing have some sort of cycle breaking infrastructure -
>> so that if the same type is seen more than once (eg: classes have
>> members that have pointers back to the outer class type, etc) they
>> don't cause indefinite cycles? Should that infrastructure have caught
>> these cases & avoided the redundant work?
>>
>> I'm curious to understand better how these things work/overlap/or don't.
>>
>> On Fri, May 3, 2019 at 9:20 PM Richard Trieu via cfe-commits
>> <cfe-commits at lists.llvm.org> wrote:
>> >
>> > Author: rtrieu
>> > Date: Fri May 3 21:22:33 2019
>> > New Revision: 359960
>> >
>> > URL: http://llvm.org/viewvc/llvm-project?rev=359960&view=rev
>> > Log:
>> > Reduce amount of work ODR hashing does.
>> >
>> > When a FunctionProtoType is in the original type in a DecayedType, the decayed
>> > type is a PointerType which points back the original FunctionProtoType. The
>> > visitor for ODRHashing will attempt to process both Type's, doing double work.
>> > By chaining together multiple DecayedType's and FunctionProtoType's, this would
>> > result in 2^N Type's visited only N DecayedType's and N FunctionProtoType's
>> > exsit. Another bug where VisitDecayedType and VisitAdjustedType did
>> > redundant work doubled the work at each level, giving 4^N Type's visited. This
>> > patch removed the double work and detects when a FunctionProtoType decays to
>> > itself to only check the Type once. This lowers the exponential runtime to
>> > linear runtime. Fixes https://bugs.llvm.org/show_bug.cgi?id=41625
>> >
>> > Modified:
>> > cfe/trunk/lib/AST/ODRHash.cpp
>> > cfe/trunk/test/Modules/odr_hash.cpp
>> >
>> > Modified: cfe/trunk/lib/AST/ODRHash.cpp
>> > URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/AST/ODRHash.cpp?rev=359960&r1=359959&r2=359960&view=diff
>> > ==============================================================================
>> > --- cfe/trunk/lib/AST/ODRHash.cpp (original)
>> > +++ cfe/trunk/lib/AST/ODRHash.cpp Fri May 3 21:22:33 2019
>> > @@ -703,14 +703,36 @@ public:
>> > void VisitType(const Type *T) {}
>> >
>> > void VisitAdjustedType(const AdjustedType *T) {
>> > - AddQualType(T->getOriginalType());
>> > - AddQualType(T->getAdjustedType());
>> > + QualType Original = T->getOriginalType();
>> > + QualType Adjusted = T->getAdjustedType();
>> > +
>> > + // The original type and pointee type can be the same, as in the case of
>> > + // function pointers decaying to themselves. Set a bool and only process
>> > + // the type once, to prevent doubling the work.
>> > + SplitQualType split = Adjusted.split();
>> > + if (auto Pointer = dyn_cast<PointerType>(split.Ty)) {
>> > + if (Pointer->getPointeeType() == Original) {
>> > + Hash.AddBoolean(true);
>> > + ID.AddInteger(split.Quals.getAsOpaqueValue());
>> > + AddQualType(Original);
>> > + VisitType(T);
>> > + return;
>> > + }
>> > + }
>> > +
>> > + // The original type and pointee type are different, such as in the case
>> > + // of a array decaying to an element pointer. Set a bool to false and
>> > + // process both types.
>> > + Hash.AddBoolean(false);
>> > + AddQualType(Original);
>> > + AddQualType(Adjusted);
>> > +
>> > VisitType(T);
>> > }
>> >
>> > void VisitDecayedType(const DecayedType *T) {
>> > - AddQualType(T->getDecayedType());
>> > - AddQualType(T->getPointeeType());
>> > + // getDecayedType and getPointeeType are derived from getAdjustedType
>> > + // and don't need to be separately processed.
>> > VisitAdjustedType(T);
>> > }
>> >
>> >
>> > Modified: cfe/trunk/test/Modules/odr_hash.cpp
>> > URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/test/Modules/odr_hash.cpp?rev=359960&r1=359959&r2=359960&view=diff
>> > ==============================================================================
>> > --- cfe/trunk/test/Modules/odr_hash.cpp (original)
>> > +++ cfe/trunk/test/Modules/odr_hash.cpp Fri May 3 21:22:33 2019
>> > @@ -4587,6 +4587,43 @@ int num = bar();
>> > #endif
>> > }
>> >
>> > +namespace FunctionProtoTypeDecay {
>> > +#if defined(FIRST)
>> > +struct S1 {
>> > + struct X {};
>> > + using Y = X(X());
>> > +};
>> > +#elif defined(SECOND)
>> > +struct S1 {
>> > + struct X {};
>> > + using Y = X(X(X()));
>> > +};
>> > +#else
>> > +S1 s1;
>> > +// expected-error at first.h:* {{'FunctionProtoTypeDecay::S1::Y' from module 'FirstModule' is not present in definition of 'FunctionProtoTypeDecay::S1' in module 'SecondModule'}}
>> > +// expected-note at second.h:* {{declaration of 'Y' does not match}}
>> > +#endif
>> > +
>> > +#if defined(FIRST)
>> > +struct S2 {
>> > + struct X {};
>> > + using Y =
>> > + X(X(X(X(X(X(X(X(X(X(X(X(X(X(X(X(
>> > + X(X(X(X(X(X(X(X(X(X(X(X(X(X(X(X(
>> > + X(X(X(X(X(X(X(X(X(X(X(X(X(X(X(X(
>> > + X(X(X(X(X(X(X(X(X(X(X(X(X(X(X(X(
>> > + ))))))))))))))))
>> > + ))))))))))))))))
>> > + ))))))))))))))))
>> > + ))))))))))))))));
>> > +};
>> > +#elif defined(SECOND)
>> > +#else
>> > +S2 s2;
>> > +#endif
>> > +
>> > +}
>> > +
>> > // Keep macros contained to one file.
>> > #ifdef FIRST
>> > #undef FIRST
>> >
>> >
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