[polly] r303362 - [IR] De-virtualize ~Value to save a vptr

[Reid Kleckner via llvm-commits]

Author: rnk
Date: Thu May 18 12:24:10 2017
New Revision: 303362

URL: http://llvm.org/viewvc/llvm-project?rev=303362&view=rev
Log:
[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

Modified:
    polly/trunk/lib/CodeGen/BlockGenerators.cpp

Modified: polly/trunk/lib/CodeGen/BlockGenerators.cpp
URL: http://llvm.org/viewvc/llvm-project/polly/trunk/lib/CodeGen/BlockGenerators.cpp?rev=303362&r1=303361&r2=303362&view=diff
==============================================================================
--- polly/trunk/lib/CodeGen/BlockGenerators.cpp (original)
+++ polly/trunk/lib/CodeGen/BlockGenerators.cpp Thu May 18 12:24:10 2017
@@ -227,7 +227,7 @@ void BlockGenerator::copyInstScalar(Scop
     if (!NewOperand) {
       assert(!isa<StoreInst>(NewInst) &&
              "Store instructions are always needed!");
-      delete NewInst;
+      NewInst->deleteValue();
       return;
     }