[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

llvm-svn: 303362

1 files changed, 21 insertions, 2 deletions

diff --git a/llvm/lib/Analysis/MemorySSA.cpp b/llvm/lib/Analysis/MemorySSA.cpp
index 2480fe44d5c..e0e04a91410 100644
--- a/llvm/lib/Analysis/MemorySSA.cpp
+++ b/llvm/lib/Analysis/MemorySSA.cpp
@@ -1799,6 +1799,15 @@ bool MemorySSA::dominates(const MemoryAccess *Dominator,
 
 const static char LiveOnEntryStr[] = "liveOnEntry";
 
+void MemoryAccess::print(raw_ostream &OS) const {
+  switch (getValueID()) {
+  case MemoryPhiVal: return static_cast<const MemoryPhi *>(this)->print(OS);
+  case MemoryDefVal: return static_cast<const MemoryDef *>(this)->print(OS);
+  case MemoryUseVal: return static_cast<const MemoryUse *>(this)->print(OS);
+  }
+  llvm_unreachable("invalid value id");
+}
+
 void MemoryDef::print(raw_ostream &OS) const {
   MemoryAccess *UO = getDefiningAccess();
 
@@ -1836,8 +1845,6 @@ void MemoryPhi::print(raw_ostream &OS) const {
   OS << ')';
 }
 
-MemoryAccess::~MemoryAccess() {}
-
 void MemoryUse::print(raw_ostream &OS) const {
   MemoryAccess *UO = getDefiningAccess();
   OS << "MemoryUse(";
@@ -2054,3 +2061,15 @@ MemoryAccess *DoNothingMemorySSAWalker::getClobberingMemoryAccess(
   return StartingAccess;
 }
 } // namespace llvm
+
+void MemoryPhi::deleteMe(DerivedUser *Self) {
+  delete static_cast<MemoryPhi *>(Self);
+}
+
+void MemoryDef::deleteMe(DerivedUser *Self) {
+  delete static_cast<MemoryDef *>(Self);
+}
+
+void MemoryUse::deleteMe(DerivedUser *Self) {
+  delete static_cast<MemoryUse *>(Self);
+}