From 96ab8726a3c2676c614b4d38500478046f816ce2 Mon Sep 17 00:00:00 2001
From: Reid Kleckner <rnk@google.com>
Date: Thu, 18 May 2017 17:24:10 +0000
Subject: [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
---
 llvm/lib/AsmParser/LLParser.cpp | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

(limited to 'llvm/lib/AsmParser/LLParser.cpp')

diff --git a/llvm/lib/AsmParser/LLParser.cpp b/llvm/lib/AsmParser/LLParser.cpp
index d7602c83435..ff1ea44a18a 100644
--- a/llvm/lib/AsmParser/LLParser.cpp
+++ b/llvm/lib/AsmParser/LLParser.cpp
@@ -2502,7 +2502,7 @@ LLParser::PerFunctionState::~PerFunctionState() {
       continue;
     P.second.first->replaceAllUsesWith(
         UndefValue::get(P.second.first->getType()));
-    delete P.second.first;
+    P.second.first->deleteValue();
   }
 
   for (const auto &P : ForwardRefValIDs) {
@@ -2510,7 +2510,7 @@ LLParser::PerFunctionState::~PerFunctionState() {
       continue;
     P.second.first->replaceAllUsesWith(
         UndefValue::get(P.second.first->getType()));
-    delete P.second.first;
+    P.second.first->deleteValue();
   }
 }
 
@@ -2642,7 +2642,7 @@ bool LLParser::PerFunctionState::SetInstName(int NameID,
                        getTypeString(FI->second.first->getType()) + "'");
 
       Sentinel->replaceAllUsesWith(Inst);
-      delete Sentinel;
+      Sentinel->deleteValue();
       ForwardRefValIDs.erase(FI);
     }
 
@@ -2659,7 +2659,7 @@ bool LLParser::PerFunctionState::SetInstName(int NameID,
                      getTypeString(FI->second.first->getType()) + "'");
 
     Sentinel->replaceAllUsesWith(Inst);
-    delete Sentinel;
+    Sentinel->deleteValue();
     ForwardRefVals.erase(FI);
   }
 
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