[IR] De-virtualize ~Value to save a vptr

Summary:
Implements PR889

Removing the virtual table pointer from Value saves 1% of RSS when doing
LTO of llc on Linux. The impact on time was positive, but too noisy to
conclusively say that performance improved. Here is a link to the
spreadsheet with the original data:

https://docs.google.com/spreadsheets/d/1F4FHir0qYnV0MEp2sYYp_BuvnJgWlWPhWOwZ6LbW7W4/edit?usp=sharing

This change makes it invalid to directly delete a Value, User, or
Instruction pointer. Instead, such code can be rewritten to a null check
and a call Value::deleteValue(). Value objects tend to have their
lifetimes managed through iplist, so for the most part, this isn't a big
deal.  However, there are some places where LLVM deletes values, and
those places had to be migrated to deleteValue.  I have also created
llvm::unique_value, which has a custom deleter, so it can be used in
place of std::unique_ptr<Value>.

I had to add the "DerivedUser" Deleter escape hatch for MemorySSA, which
derives from User outside of lib/IR. Code in IR cannot include MemorySSA
headers or call the MemoryAccess object destructors without introducing
a circular dependency, so we need some level of indirection.
Unfortunately, no class derived from User may have any virtual methods,
because adding a virtual method would break User::getHungOffOperands(),
which assumes that it can find the use list immediately prior to the
User object. I've added a static_assert to the appropriate OperandTraits
templates to help people avoid this trap.

Reviewers: chandlerc, mehdi_amini, pete, dberlin, george.burgess.iv

Reviewed By: chandlerc

Subscribers: krytarowski, eraman, george.burgess.iv, mzolotukhin, Prazek, nlewycky, hans, inglorion, pcc, tejohnson, dberlin, llvm-commits

Differential Revision: https://reviews.llvm.org/D31261

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@303362 91177308-0d34-0410-b5e6-96231b3b80d8

2 files changed, 4 insertions, 4 deletions

diff --git a/lib/Bitcode/Reader/BitcodeReader.cpp b/lib/Bitcode/Reader/BitcodeReader.cpp
index 76298121566a..686c94687669 100644
--- a/lib/Bitcode/Reader/BitcodeReader.cpp
+++ b/lib/Bitcode/Reader/BitcodeReader.cpp
@@ -4489,11 +4489,11 @@ Error BitcodeReader::parseFunctionBody(Function *F) {
     // Add instruction to end of current BB.  If there is no current BB, reject
     // this file.
     if (!CurBB) {
-      delete I;
+      I->deleteValue();
       return error("Invalid instruction with no BB");
     }
     if (!OperandBundles.empty()) {
-      delete I;
+      I->deleteValue();
       return error("Operand bundles found with no consumer");
     }
     CurBB->getInstList().push_back(I);
diff --git a/lib/Bitcode/Reader/ValueList.cpp b/lib/Bitcode/Reader/ValueList.cpp
index d1a2a11bbfad..f2a3439a87be 100644
--- a/lib/Bitcode/Reader/ValueList.cpp
+++ b/lib/Bitcode/Reader/ValueList.cpp
@@ -73,7 +73,7 @@ void BitcodeReaderValueList::assignValue(Value *V, unsigned Idx) {
     // If there was a forward reference to this value, replace it.
     Value *PrevVal = OldV;
     OldV->replaceAllUsesWith(V);
-    delete PrevVal;
+    PrevVal->deleteValue();
   }
 }
 
@@ -194,6 +194,6 @@ void BitcodeReaderValueList::resolveConstantForwardRefs() {
 
     // Update all ValueHandles, they should be the only users at this point.
     Placeholder->replaceAllUsesWith(RealVal);
-    delete Placeholder;
+    Placeholder->deleteValue();
   }
 }