[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
diff --git a/llvm/lib/IR/Value.cpp b/llvm/lib/IR/Value.cpp
index 02b40c9..51a7d42 100644
--- a/llvm/lib/IR/Value.cpp
+++ b/llvm/lib/IR/Value.cpp
@@ -20,6 +20,7 @@
#include "llvm/IR/Constants.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/DerivedTypes.h"
+#include "llvm/IR/DerivedUser.h"
#include "llvm/IR/GetElementPtrTypeIterator.h"
#include "llvm/IR/InstrTypes.h"
#include "llvm/IR/Instructions.h"
@@ -59,7 +60,7 @@
(SubclassID < ConstantFirstVal || SubclassID > ConstantLastVal))
assert((VTy->isFirstClassType() || VTy->isVoidTy()) &&
"Cannot create non-first-class values except for constants!");
- static_assert(sizeof(Value) == 3 * sizeof(void *) + 2 * sizeof(unsigned),
+ static_assert(sizeof(Value) == 2 * sizeof(void *) + 2 * sizeof(unsigned),
"Value too big");
}
@@ -89,6 +90,32 @@
destroyValueName();
}
+void Value::deleteValue() {
+ switch (getValueID()) {
+#define HANDLE_VALUE(Name) \
+ case Value::Name##Val: \
+ delete static_cast<Name *>(this); \
+ break;
+#define HANDLE_MEMORY_VALUE(Name) \
+ case Value::Name##Val: \
+ static_cast<DerivedUser *>(this)->DeleteValue( \
+ static_cast<DerivedUser *>(this)); \
+ break;
+#define HANDLE_INSTRUCTION(Name) /* nothing */
+#include "llvm/IR/Value.def"
+
+#define HANDLE_INST(N, OPC, CLASS) \
+ case Value::InstructionVal + Instruction::OPC: \
+ delete static_cast<CLASS *>(this); \
+ break;
+#define HANDLE_USER_INST(N, OPC, CLASS)
+#include "llvm/IR/Instruction.def"
+
+ default:
+ llvm_unreachable("attempting to delete unknown value kind");
+ }
+}
+
void Value::destroyValueName() {
ValueName *Name = getValueName();
if (Name)