[llvm] r225401 - IR: Add MDNode::isDistinct()

[Duncan P. N. Exon Smith]

> On 2015-Jan-12, at 12:59, David Blaikie <dblaikie at gmail.com> wrote:
> 
> 
> 
> On Mon, Jan 12, 2015 at 11:16 AM, Duncan P. N. Exon Smith <dexonsmith at apple.com> wrote:
> 
> > On 2015-Jan-12, at 11:07, David Blaikie <dblaikie at gmail.com> wrote:
> >
> >
> >
> > On Wed, Jan 7, 2015 at 1:35 PM, Duncan P. N. Exon Smith <dexonsmith at apple.com> wrote:
> > Author: dexonsmith
> > Date: Wed Jan  7 15:35:38 2015
> > New Revision: 225401
> >
> > URL: http://llvm.org/viewvc/llvm-project?rev=225401&view=rev
> > Log:
> > IR: Add MDNode::isDistinct()
> >
> > Add API to indicate whether an `MDNode` is distinct.  A distinct node is
> > not stored in the MDNode uniquing tables, and will never be returned by
> > `MDNode::get()`.
> >
> > Although distinct nodes are only currently created by uniquing
> > collisions (when operands change),
> >
> > I don't quite understand how changing an operand relates to uniquing collisions - once the operand has changed would we not want the uniquing to happen again? (eg: if changing an operand made this node equal to some other node instead?)
> >
> 
> It's impossible without RAUW.
> 
> Even when doing that, it seems strange that it would prevent uniquing - but again, I guess I'm missing something.
> 
> I would sort of expect:
> 
> {1, 2}
> 1 = {a}
> 2 = {b}
> 
> RAUW a -> b
> 
> {2, 2}
> 2 = {b}
> 
> it sort of seems strange

It is strange :/.

>  to me that the path (directly constructing, versus constructing then RAUWing) to create the value produces a different graph/node/etc?
> 
> - David

It's unavoidable though.  Concrete example:

    @g1 = global i32 0
    @g2 = global i32 0

    !0 = !{i32* @g1}
    !1 = !{i32* @g2}
    !2 = !{!0, !1}

`@g1->replaceAllUsesWith(@g2)` calls `!0->handleChangedOperand(0, @g2)`.

In there, we call `MDNode::getIfExists(Context, @g2)` and get back `!1`.
This is a uniquing collision.

`!0` isn't tracking its users, so it doesn't know about `!2`.  We'd really
like to call `!0->replaceAllUsesWith(!1)`, but we can't.

Instead, we're left with this:

    @g1 = global i32 0
    @g2 = global i32 0

    !0 = distinct !{i32* @g2}
    !1 = !{i32* @g2}
    !2 = !{!0, !1}

>  
> 
> > PR22111 will allow these nodes to be
> > explicitly created.
> >
> > Modified:
> >     llvm/trunk/include/llvm/IR/Metadata.h
> >     llvm/trunk/unittests/IR/MetadataTest.cpp
> >
> > Modified: llvm/trunk/include/llvm/IR/Metadata.h
> > URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/IR/Metadata.h?rev=225401&r1=225400&r2=225401&view=diff
> > ==============================================================================
> > --- llvm/trunk/include/llvm/IR/Metadata.h (original)
> > +++ llvm/trunk/include/llvm/IR/Metadata.h Wed Jan  7 15:35:38 2015
> > @@ -642,6 +642,12 @@ public:
> >    /// \brief Check if node is fully resolved.
> >    bool isResolved() const;
> >
> > +  /// \brief Check if node is distinct.
> > +  ///
> > +  /// Distinct nodes are not uniqued, and will not be returned by \a
> > +  /// MDNode::get().
> > +  bool isDistinct() const { return IsDistinctInContext; }
> > +
> >  protected:
> >    /// \brief Set an operand.
> >    ///
> >
> > Modified: llvm/trunk/unittests/IR/MetadataTest.cpp
> > URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/unittests/IR/MetadataTest.cpp?rev=225401&r1=225400&r2=225401&view=diff
> > ==============================================================================
> > --- llvm/trunk/unittests/IR/MetadataTest.cpp (original)
> > +++ llvm/trunk/unittests/IR/MetadataTest.cpp Wed Jan  7 15:35:38 2015
> > @@ -222,6 +222,33 @@ TEST_F(MDNodeTest, NullOperand) {
> >    EXPECT_EQ(N, NullOp);
> >  }
> >
> > +TEST_F(MDNodeTest, DistinctOnUniquingCollision) {
> > +  // !{}
> > +  MDNode *Empty = MDNode::get(Context, None);
> > +  ASSERT_TRUE(Empty->isResolved());
> > +  EXPECT_FALSE(Empty->isDistinct());
> > +
> > +  // !{!{}}
> > +  Metadata *Wrapped1Ops[] = {Empty};
> > +  MDNode *Wrapped1 = MDNode::get(Context, Wrapped1Ops);
> > +  ASSERT_EQ(Empty, Wrapped1->getOperand(0));
> > +  ASSERT_TRUE(Wrapped1->isResolved());
> > +  EXPECT_FALSE(Wrapped1->isDistinct());
> > +
> > +  // !{!{!{}}}
> > +  Metadata *Wrapped2Ops[] = {Wrapped1};
> > +  MDNode *Wrapped2 = MDNode::get(Context, Wrapped2Ops);
> > +  ASSERT_EQ(Wrapped1, Wrapped2->getOperand(0));
> > +  ASSERT_TRUE(Wrapped2->isResolved());
> > +  EXPECT_FALSE(Wrapped2->isDistinct());
> > +
> > +  // !{!{!{}}} => !{!{}}
> > +  Wrapped2->replaceOperandWith(0, Empty);
> > +  ASSERT_EQ(Empty, Wrapped2->getOperand(0));
> > +  EXPECT_TRUE(Wrapped2->isDistinct());
> > +  EXPECT_FALSE(Wrapped1->isDistinct());
> > +}
> > +
> >  typedef MetadataTest MetadataAsValueTest;
> >
> >  TEST_F(MetadataAsValueTest, MDNode) {
> >
> >
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