Merge commit 'c723eb1aef817d47feec620933ee1ec6005cdd14'

This merges r159618 from upstream into master. It goes with clang rev
af50aab0c317462129d73ae8000c6394c718598d

Conflicts:
	include/llvm/CodeGen/LexicalScopes.h
	include/llvm/Target/TargetOptions.h
	lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
	lib/Target/ARM/ARMBaseInstrInfo.cpp
	lib/Target/ARM/ARMTargetMachine.cpp
	lib/Target/ARM/ARMTargetObjectFile.cpp
	lib/Target/Mips/MCTargetDesc/MipsAsmBackend.cpp
	lib/Target/Mips/MipsISelDAGToDAG.cpp
	lib/Target/Mips/MipsInstrFPU.td
	lib/Target/Mips/MipsMCInstLower.cpp
	lib/Target/Mips/MipsTargetMachine.cpp
	lib/Target/TargetMachine.cpp
	lib/Target/X86/X86ISelLowering.cpp
	lib/Target/X86/X86RegisterInfo.cpp
	lib/Target/X86/X86TargetObjectFile.cpp
	lib/Target/X86/X86TargetObjectFile.h
	tools/llc/llc.cpp

(tools/llc/llc.cpp is from a merged version of r160532 because it was a bit
hairy and I didn't want to redo it.)

1 files changed, 11 insertions, 14 deletions

diff --git a/lib/CodeGen/MachineBlockPlacement.cpp b/lib/CodeGen/MachineBlockPlacement.cpp
index 9ca0ad2e24..5a15f92a18 100644
--- a/lib/CodeGen/MachineBlockPlacement.cpp
+++ b/lib/CodeGen/MachineBlockPlacement.cpp
@@ -63,17 +63,13 @@ namespace {
 ///
 /// This is the datastructure representing a chain of consecutive blocks that
 /// are profitable to layout together in order to maximize fallthrough
-/// probabilities. We also can use a block chain to represent a sequence of
-/// basic blocks which have some external (correctness) requirement for
-/// sequential layout.
+/// probabilities and code locality. We also can use a block chain to represent
+/// a sequence of basic blocks which have some external (correctness)
+/// requirement for sequential layout.
 ///
-/// Eventually, the block chains will form a directed graph over the function.
-/// We provide an SCC-supporting-iterator in order to quicky build and walk the
-/// SCCs of block chains within a function.
-///
-/// The block chains also have support for calculating and caching probability
-/// information related to the chain itself versus other chains. This is used
-/// for ranking during the final layout of block chains.
+/// Chains can be built around a single basic block and can be merged to grow
+/// them. They participate in a block-to-chain mapping, which is updated
+/// automatically as chains are merged together.
 class BlockChain {
   /// \brief The sequence of blocks belonging to this chain.
   ///
@@ -179,10 +175,11 @@ class MachineBlockPlacement : public MachineFunctionPass {
 
   /// \brief Allocator and owner of BlockChain structures.
   ///
-  /// We build BlockChains lazily by merging together high probability BB
-  /// sequences according to the "Algo2" in the paper mentioned at the top of
-  /// the file. To reduce malloc traffic, we allocate them using this slab-like
-  /// allocator, and destroy them after the pass completes.
+  /// We build BlockChains lazily while processing the loop structure of
+  /// a function. To reduce malloc traffic, we allocate them using this
+  /// slab-like allocator, and destroy them after the pass completes. An
+  /// important guarantee is that this allocator produces stable pointers to
+  /// the chains.
   SpecificBumpPtrAllocator<BlockChain> ChainAllocator;
 
   /// \brief Function wide BasicBlock to BlockChain mapping.