llvm-6502/lib/CodeGen
Tim Northover 03eecdccff FastISel: constrain the RegClass of operands when emitting instructions.
ARM64 suffered multiple -verify-machineinstr failures (principally over the
xsp/xzr issue) because FastISel was completely ignoring which subset of the
general-purpose registers each instruction required.

More fixes are coming in ARM64 specific FastISel, but this should cover the
generic problems.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206283 91177308-0d34-0410-b5e6-96231b3b80d8
2014-04-15 13:59:49 +00:00
..
AsmPrinter Use unique_ptr for the result of Registry entries. 2014-04-15 05:53:26 +00:00
SelectionDAG FastISel: constrain the RegClass of operands when emitting instructions. 2014-04-15 13:59:49 +00:00
AggressiveAntiDepBreaker.cpp [C++11] More 'nullptr' conversion. In some cases just using a boolean check instead of comparing to nullptr. 2014-04-14 00:51:57 +00:00
AggressiveAntiDepBreaker.h [C++11] Add 'override' keyword to virtual methods that override their base class. 2014-03-07 09:26:03 +00:00
AllocationOrder.cpp
AllocationOrder.h
Analysis.cpp [C++11] More 'nullptr' conversion. In some cases just using a boolean check instead of comparing to nullptr. 2014-04-14 00:51:57 +00:00
AntiDepBreaker.h
BasicTargetTransformInfo.cpp Don't assert in BasicTTI::getMemoryOpCost for non-simple types 2014-04-14 05:59:09 +00:00
BranchFolding.cpp [C++11] More 'nullptr' conversion. In some cases just using a boolean check instead of comparing to nullptr. 2014-04-14 00:51:57 +00:00
BranchFolding.h
CalcSpillWeights.cpp [C++11] More 'nullptr' conversion. In some cases just using a boolean check instead of comparing to nullptr. 2014-04-14 00:51:57 +00:00
CallingConvLower.cpp [C++11] More 'nullptr' conversion. In some cases just using a boolean check instead of comparing to nullptr. 2014-04-14 00:51:57 +00:00
CMakeLists.txt [CodeGenPrepare] Move CodeGenPrepare into lib/CodeGen. 2014-02-22 00:07:45 +00:00
CodeGen.cpp [CodeGenPrepare] Move CodeGenPrepare into lib/CodeGen. 2014-02-22 00:07:45 +00:00
CodeGenPrepare.cpp Use unique_ptr to manage TypePromotionActions owned by TypePromotionTransaction. 2014-04-15 06:17:44 +00:00
CriticalAntiDepBreaker.cpp [C++11] More 'nullptr' conversion. In some cases just using a boolean check instead of comparing to nullptr. 2014-04-14 00:51:57 +00:00
CriticalAntiDepBreaker.h [C++11] Add 'override' keyword to virtual methods that override their base class. 2014-03-07 09:26:03 +00:00
DeadMachineInstructionElim.cpp [C++11] More 'nullptr' conversion. In some cases just using a boolean check instead of comparing to nullptr. 2014-04-14 00:51:57 +00:00
DFAPacketizer.cpp [C++11] More 'nullptr' conversion. In some cases just using a boolean check instead of comparing to nullptr. 2014-04-14 00:51:57 +00:00
DwarfEHPrepare.cpp [C++11] More 'nullptr' conversion. In some cases just using a boolean check instead of comparing to nullptr. 2014-04-14 00:51:57 +00:00
EarlyIfConversion.cpp [C++11] More 'nullptr' conversion. In some cases just using a boolean check instead of comparing to nullptr. 2014-04-14 00:51:57 +00:00
EdgeBundles.cpp
ErlangGC.cpp [C++11] Add 'override' keyword to virtual methods that override their base class. 2014-03-07 09:26:03 +00:00
ExecutionDepsFix.cpp [C++11] More 'nullptr' conversion. In some cases just using a boolean check instead of comparing to nullptr. 2014-04-14 00:51:57 +00:00
ExpandISelPseudos.cpp [C++11] Add 'override' keyword to virtual methods that override their base class. 2014-03-07 09:26:03 +00:00
ExpandPostRAPseudos.cpp [C++11] Add 'override' keyword to virtual methods that override their base class. 2014-03-07 09:26:03 +00:00
GCMetadata.cpp Use unique_ptr for the result of Registry entries. 2014-04-15 05:53:26 +00:00
GCMetadataPrinter.cpp
GCStrategy.cpp Use unique_ptr to manage ownership of GCFunctionInfos in GCStrategy 2014-04-15 06:07:26 +00:00
IfConversion.cpp [C++11] More 'nullptr' conversion. In some cases just using a boolean check instead of comparing to nullptr. 2014-04-14 00:51:57 +00:00
InlineSpiller.cpp [C++11] More 'nullptr' conversion. In some cases just using a boolean check instead of comparing to nullptr. 2014-04-14 00:51:57 +00:00
InterferenceCache.cpp [C++11] Replace llvm::tie with std::tie. 2014-03-02 13:30:33 +00:00
InterferenceCache.h The following patch' purpose is to reduce compile time for compilation of small 2014-02-06 09:23:24 +00:00
IntrinsicLowering.cpp [C++11] More 'nullptr' conversion. In some cases just using a boolean check instead of comparing to nullptr. 2014-04-14 00:51:57 +00:00
JITCodeEmitter.cpp
LatencyPriorityQueue.cpp [C++11] More 'nullptr' conversion. In some cases just using a boolean check instead of comparing to nullptr. 2014-04-14 00:51:57 +00:00
LexicalScopes.cpp [C++11] More 'nullptr' conversion. In some cases just using a boolean check instead of comparing to nullptr. 2014-04-14 00:51:57 +00:00
LiveDebugVariables.cpp [C++11] More 'nullptr' conversion. In some cases just using a boolean check instead of comparing to nullptr. 2014-04-14 00:51:57 +00:00
LiveDebugVariables.h [C++11] Add 'override' keyword to virtual methods that override their base class. 2014-03-07 09:26:03 +00:00
LiveInterval.cpp [C++11] More 'nullptr' conversion. In some cases just using a boolean check instead of comparing to nullptr. 2014-04-14 00:51:57 +00:00
LiveIntervalAnalysis.cpp [C++11] More 'nullptr' conversion. In some cases just using a boolean check instead of comparing to nullptr. 2014-04-14 00:51:57 +00:00
LiveIntervalUnion.cpp [C++11] More 'nullptr' conversion. In some cases just using a boolean check instead of comparing to nullptr. 2014-04-14 00:51:57 +00:00
LivePhysRegs.cpp Convert register liveness tracking to work on a sub-register level instead of just register units. 2013-12-14 06:52:56 +00:00
LiveRangeCalc.cpp [C++11] More 'nullptr' conversion. In some cases just using a boolean check instead of comparing to nullptr. 2014-04-14 00:51:57 +00:00
LiveRangeCalc.h
LiveRangeEdit.cpp [C++11] More 'nullptr' conversion. In some cases just using a boolean check instead of comparing to nullptr. 2014-04-14 00:51:57 +00:00
LiveRegMatrix.cpp Replace OwningPtr<T> with std::unique_ptr<T>. 2014-03-06 05:51:42 +00:00
LiveStackAnalysis.cpp
LiveVariables.cpp [C++11] More 'nullptr' conversion. In some cases just using a boolean check instead of comparing to nullptr. 2014-04-14 00:51:57 +00:00
LLVMBuild.txt
LLVMTargetMachine.cpp [C++11] More 'nullptr' conversion. In some cases just using a boolean check instead of comparing to nullptr. 2014-04-14 00:51:57 +00:00
LocalStackSlotAllocation.cpp Simplify resolveFrameIndex() signature. 2014-04-02 19:28:18 +00:00
MachineBasicBlock.cpp [C++11] More 'nullptr' conversion. In some cases just using a boolean check instead of comparing to nullptr. 2014-04-14 00:51:57 +00:00
MachineBlockFrequencyInfo.cpp blockfreq: Rename BlockFrequencyImpl to BlockFrequencyInfoImpl 2014-04-11 23:20:58 +00:00
MachineBlockPlacement.cpp [C++11] More 'nullptr' conversion. In some cases just using a boolean check instead of comparing to nullptr. 2014-04-14 00:51:57 +00:00
MachineBranchProbabilityInfo.cpp [C++11] More 'nullptr' conversion. In some cases just using a boolean check instead of comparing to nullptr. 2014-04-14 00:51:57 +00:00
MachineCodeEmitter.cpp
MachineCopyPropagation.cpp Disable each MachineFunctionPass for 'optnone' functions, unless that 2014-03-31 17:43:35 +00:00
MachineCSE.cpp Disable each MachineFunctionPass for 'optnone' functions, unless that 2014-03-31 17:43:35 +00:00
MachineDominators.cpp
MachineFunction.cpp Break PseudoSourceValue out of the Value hierarchy. It is now the root of its own tree containing FixedStackPseudoSourceValue (which you can use isa/dyn_cast on) and MipsCallEntry (which you can't). Anything that needs to use either a PseudoSourceValue* and Value* is strongly encouraged to use a MachinePointerInfo instead. 2014-04-15 07:22:52 +00:00
MachineFunctionAnalysis.cpp [C++11] More 'nullptr' conversion. In some cases just using a boolean check instead of comparing to nullptr. 2014-04-14 00:51:57 +00:00
MachineFunctionPass.cpp [stackprotector] Use analysis from the StackProtector pass for stack layout in PEI a nd LocalStackSlot passes. 2013-12-19 03:17:11 +00:00
MachineFunctionPrinterPass.cpp [C++11] Add 'override' keyword to virtual methods that override their base class. 2014-03-07 09:26:03 +00:00
MachineInstr.cpp Break PseudoSourceValue out of the Value hierarchy. It is now the root of its own tree containing FixedStackPseudoSourceValue (which you can use isa/dyn_cast on) and MipsCallEntry (which you can't). Anything that needs to use either a PseudoSourceValue* and Value* is strongly encouraged to use a MachinePointerInfo instead. 2014-04-15 07:22:52 +00:00
MachineInstrBundle.cpp [C++11] Add 'override' keyword to virtual methods that override their base class. 2014-03-07 09:26:03 +00:00
MachineLICM.cpp Break PseudoSourceValue out of the Value hierarchy. It is now the root of its own tree containing FixedStackPseudoSourceValue (which you can use isa/dyn_cast on) and MipsCallEntry (which you can't). Anything that needs to use either a PseudoSourceValue* and Value* is strongly encouraged to use a MachinePointerInfo instead. 2014-04-15 07:22:52 +00:00
MachineLoopInfo.cpp [C++11] Replace llvm::next and llvm::prior with std::next and std::prev. 2014-03-02 12:27:27 +00:00
MachineModuleInfo.cpp [C++11] More 'nullptr' conversion. In some cases just using a boolean check instead of comparing to nullptr. 2014-04-14 00:51:57 +00:00
MachineModuleInfoImpls.cpp
MachinePassRegistry.cpp [C++11] More 'nullptr' conversion. In some cases just using a boolean check instead of comparing to nullptr. 2014-04-14 00:51:57 +00:00
MachinePostDominators.cpp
MachineRegisterInfo.cpp [C++11] More 'nullptr' conversion. In some cases just using a boolean check instead of comparing to nullptr. 2014-04-14 00:51:57 +00:00
MachineScheduler.cpp [C++11] More 'nullptr' conversion. In some cases just using a boolean check instead of comparing to nullptr. 2014-04-14 00:51:57 +00:00
MachineSink.cpp [C++11] More 'nullptr' conversion. In some cases just using a boolean check instead of comparing to nullptr. 2014-04-14 00:51:57 +00:00
MachineSSAUpdater.cpp [C++11] More 'nullptr' conversion. In some cases just using a boolean check instead of comparing to nullptr. 2014-04-14 00:51:57 +00:00
MachineTraceMetrics.cpp [C++11] More 'nullptr' conversion. In some cases just using a boolean check instead of comparing to nullptr. 2014-04-14 00:51:57 +00:00
MachineVerifier.cpp [C++11] More 'nullptr' conversion. In some cases just using a boolean check instead of comparing to nullptr. 2014-04-14 00:51:57 +00:00
Makefile
OcamlGC.cpp
OptimizePHIs.cpp Disable each MachineFunctionPass for 'optnone' functions, unless that 2014-03-31 17:43:35 +00:00
Passes.cpp [C++11] More 'nullptr' conversion. In some cases just using a boolean check instead of comparing to nullptr. 2014-04-14 00:51:57 +00:00
PeepholeOptimizer.cpp [C++11] More 'nullptr' conversion. In some cases just using a boolean check instead of comparing to nullptr. 2014-04-14 00:51:57 +00:00
PHIElimination.cpp [C++11] More 'nullptr' conversion. In some cases just using a boolean check instead of comparing to nullptr. 2014-04-14 00:51:57 +00:00
PHIEliminationUtils.cpp Switch a number of loops in lib/CodeGen over to range-based for-loops, now that 2014-03-17 19:36:09 +00:00
PHIEliminationUtils.h
PostRASchedulerList.cpp [C++11] More 'nullptr' conversion. In some cases just using a boolean check instead of comparing to nullptr. 2014-04-14 00:51:57 +00:00
ProcessImplicitDefs.cpp Switch a number of loops in lib/CodeGen over to range-based for-loops, now that 2014-03-17 19:36:09 +00:00
PrologEpilogInserter.cpp [C++11] More 'nullptr' conversion. In some cases just using a boolean check instead of comparing to nullptr. 2014-04-14 00:51:57 +00:00
PrologEpilogInserter.h [C++11] Add 'override' keyword to virtual methods that override their base class. 2014-03-07 09:26:03 +00:00
PseudoSourceValue.cpp Break PseudoSourceValue out of the Value hierarchy. It is now the root of its own tree containing FixedStackPseudoSourceValue (which you can use isa/dyn_cast on) and MipsCallEntry (which you can't). Anything that needs to use either a PseudoSourceValue* and Value* is strongly encouraged to use a MachinePointerInfo instead. 2014-04-15 07:22:52 +00:00
README.txt
RegAllocBase.cpp [C++11] More 'nullptr' conversion. In some cases just using a boolean check instead of comparing to nullptr. 2014-04-14 00:51:57 +00:00
RegAllocBase.h Replace OwningPtr<T> with std::unique_ptr<T>. 2014-03-06 05:51:42 +00:00
RegAllocBasic.cpp [C++11] More 'nullptr' conversion. In some cases just using a boolean check instead of comparing to nullptr. 2014-04-14 00:51:57 +00:00
RegAllocFast.cpp [C++11] More 'nullptr' conversion. In some cases just using a boolean check instead of comparing to nullptr. 2014-04-14 00:51:57 +00:00
RegAllocGreedy.cpp [C++11] More 'nullptr' conversion. In some cases just using a boolean check instead of comparing to nullptr. 2014-04-14 00:51:57 +00:00
RegAllocPBQP.cpp [C++11] More 'nullptr' conversion. In some cases just using a boolean check instead of comparing to nullptr. 2014-04-14 00:51:57 +00:00
RegisterClassInfo.cpp [C++11] More 'nullptr' conversion. In some cases just using a boolean check instead of comparing to nullptr. 2014-04-14 00:51:57 +00:00
RegisterCoalescer.cpp [C++11] More 'nullptr' conversion. In some cases just using a boolean check instead of comparing to nullptr. 2014-04-14 00:51:57 +00:00
RegisterCoalescer.h
RegisterPressure.cpp [C++11] More 'nullptr' conversion. In some cases just using a boolean check instead of comparing to nullptr. 2014-04-14 00:51:57 +00:00
RegisterScavenging.cpp [C++11] More 'nullptr' conversion. In some cases just using a boolean check instead of comparing to nullptr. 2014-04-14 00:51:57 +00:00
ScheduleDAG.cpp [C++11] More 'nullptr' conversion. In some cases just using a boolean check instead of comparing to nullptr. 2014-04-14 00:51:57 +00:00
ScheduleDAGInstrs.cpp Break PseudoSourceValue out of the Value hierarchy. It is now the root of its own tree containing FixedStackPseudoSourceValue (which you can use isa/dyn_cast on) and MipsCallEntry (which you can't). Anything that needs to use either a PseudoSourceValue* and Value* is strongly encouraged to use a MachinePointerInfo instead. 2014-04-15 07:22:52 +00:00
ScheduleDAGPrinter.cpp Put the functionality for printing a value to a raw_ostream as an 2014-01-09 02:29:41 +00:00
ScoreboardHazardRecognizer.cpp [C++11] More 'nullptr' conversion. In some cases just using a boolean check instead of comparing to nullptr. 2014-04-14 00:51:57 +00:00
ShadowStackGC.cpp [C++11] More 'nullptr' conversion. In some cases just using a boolean check instead of comparing to nullptr. 2014-04-14 00:51:57 +00:00
SjLjEHPrepare.cpp [C++11] More 'nullptr' conversion. In some cases just using a boolean check instead of comparing to nullptr. 2014-04-14 00:51:57 +00:00
SlotIndexes.cpp [C++11] More 'nullptr' conversion. In some cases just using a boolean check instead of comparing to nullptr. 2014-04-14 00:51:57 +00:00
Spiller.cpp Phase 2 of the great MachineRegisterInfo cleanup. This time, we're changing 2014-03-13 23:12:04 +00:00
Spiller.h
SpillPlacement.cpp [C++11] More 'nullptr' conversion. In some cases just using a boolean check instead of comparing to nullptr. 2014-04-14 00:51:57 +00:00
SpillPlacement.h [C++11] Add 'override' keyword to virtual methods that override their base class. 2014-03-07 09:26:03 +00:00
SplitKit.cpp [C++11] More 'nullptr' conversion. In some cases just using a boolean check instead of comparing to nullptr. 2014-04-14 00:51:57 +00:00
SplitKit.h
StackColoring.cpp Break PseudoSourceValue out of the Value hierarchy. It is now the root of its own tree containing FixedStackPseudoSourceValue (which you can use isa/dyn_cast on) and MipsCallEntry (which you can't). Anything that needs to use either a PseudoSourceValue* and Value* is strongly encouraged to use a MachinePointerInfo instead. 2014-04-15 07:22:52 +00:00
StackMapLivenessAnalysis.cpp Minor change to StackMapLiveness DEBUG output. 2014-04-04 23:49:35 +00:00
StackMaps.cpp [Stackmaps] Update the stackmap format to use 64-bit relocations for the function address and properly align all entries. 2014-03-31 22:14:04 +00:00
StackProtector.cpp [C++11] More 'nullptr' conversion. In some cases just using a boolean check instead of comparing to nullptr. 2014-04-14 00:51:57 +00:00
StackSlotColoring.cpp Break PseudoSourceValue out of the Value hierarchy. It is now the root of its own tree containing FixedStackPseudoSourceValue (which you can use isa/dyn_cast on) and MipsCallEntry (which you can't). Anything that needs to use either a PseudoSourceValue* and Value* is strongly encouraged to use a MachinePointerInfo instead. 2014-04-15 07:22:52 +00:00
TailDuplication.cpp [C++11] More 'nullptr' conversion. In some cases just using a boolean check instead of comparing to nullptr. 2014-04-14 00:51:57 +00:00
TargetFrameLoweringImpl.cpp
TargetInstrInfo.cpp Break PseudoSourceValue out of the Value hierarchy. It is now the root of its own tree containing FixedStackPseudoSourceValue (which you can use isa/dyn_cast on) and MipsCallEntry (which you can't). Anything that needs to use either a PseudoSourceValue* and Value* is strongly encouraged to use a MachinePointerInfo instead. 2014-04-15 07:22:52 +00:00
TargetLoweringBase.cpp [C++11] More 'nullptr' conversion. In some cases just using a boolean check instead of comparing to nullptr. 2014-04-14 00:51:57 +00:00
TargetLoweringObjectFileImpl.cpp [C++11] More 'nullptr' conversion. In some cases just using a boolean check instead of comparing to nullptr. 2014-04-14 00:51:57 +00:00
TargetOptionsImpl.cpp Re-sort all of the includes with ./utils/sort_includes.py so that 2014-01-07 11:48:04 +00:00
TargetRegisterInfo.cpp [C++11] More 'nullptr' conversion. In some cases just using a boolean check instead of comparing to nullptr. 2014-04-14 00:51:57 +00:00
TargetSchedule.cpp
TwoAddressInstructionPass.cpp [C++11] More 'nullptr' conversion. In some cases just using a boolean check instead of comparing to nullptr. 2014-04-14 00:51:57 +00:00
UnreachableBlockElim.cpp [C++11] Add 'override' keyword to virtual methods that override their base class. 2014-03-07 09:26:03 +00:00
VirtRegMap.cpp Switch a number of loops in lib/CodeGen over to range-based for-loops, now that 2014-03-17 19:36:09 +00:00

//===---------------------------------------------------------------------===//

Common register allocation / spilling problem:

        mul lr, r4, lr
        str lr, [sp, #+52]
        ldr lr, [r1, #+32]
        sxth r3, r3
        ldr r4, [sp, #+52]
        mla r4, r3, lr, r4

can be:

        mul lr, r4, lr
        mov r4, lr
        str lr, [sp, #+52]
        ldr lr, [r1, #+32]
        sxth r3, r3
        mla r4, r3, lr, r4

and then "merge" mul and mov:

        mul r4, r4, lr
        str r4, [sp, #+52]
        ldr lr, [r1, #+32]
        sxth r3, r3
        mla r4, r3, lr, r4

It also increase the likelihood the store may become dead.

//===---------------------------------------------------------------------===//

bb27 ...
        ...
        %reg1037 = ADDri %reg1039, 1
        %reg1038 = ADDrs %reg1032, %reg1039, %NOREG, 10
    Successors according to CFG: 0x8b03bf0 (#5)

bb76 (0x8b03bf0, LLVM BB @0x8b032d0, ID#5):
    Predecessors according to CFG: 0x8b0c5f0 (#3) 0x8b0a7c0 (#4)
        %reg1039 = PHI %reg1070, mbb<bb76.outer,0x8b0c5f0>, %reg1037, mbb<bb27,0x8b0a7c0>

Note ADDri is not a two-address instruction. However, its result %reg1037 is an
operand of the PHI node in bb76 and its operand %reg1039 is the result of the
PHI node. We should treat it as a two-address code and make sure the ADDri is
scheduled after any node that reads %reg1039.

//===---------------------------------------------------------------------===//

Use local info (i.e. register scavenger) to assign it a free register to allow
reuse:
        ldr r3, [sp, #+4]
        add r3, r3, #3
        ldr r2, [sp, #+8]
        add r2, r2, #2
        ldr r1, [sp, #+4]  <==
        add r1, r1, #1
        ldr r0, [sp, #+4]
        add r0, r0, #2

//===---------------------------------------------------------------------===//

LLVM aggressively lift CSE out of loop. Sometimes this can be negative side-
effects:

R1 = X + 4
R2 = X + 7
R3 = X + 15

loop:
load [i + R1]
...
load [i + R2]
...
load [i + R3]

Suppose there is high register pressure, R1, R2, R3, can be spilled. We need
to implement proper re-materialization to handle this:

R1 = X + 4
R2 = X + 7
R3 = X + 15

loop:
R1 = X + 4  @ re-materialized
load [i + R1]
...
R2 = X + 7 @ re-materialized
load [i + R2]
...
R3 = X + 15 @ re-materialized
load [i + R3]

Furthermore, with re-association, we can enable sharing:

R1 = X + 4
R2 = X + 7
R3 = X + 15

loop:
T = i + X
load [T + 4]
...
load [T + 7]
...
load [T + 15]
//===---------------------------------------------------------------------===//

It's not always a good idea to choose rematerialization over spilling. If all
the load / store instructions would be folded then spilling is cheaper because
it won't require new live intervals / registers. See 2003-05-31-LongShifts for
an example.

//===---------------------------------------------------------------------===//

With a copying garbage collector, derived pointers must not be retained across
collector safe points; the collector could move the objects and invalidate the
derived pointer. This is bad enough in the first place, but safe points can
crop up unpredictably. Consider:

        %array = load { i32, [0 x %obj] }** %array_addr
        %nth_el = getelementptr { i32, [0 x %obj] }* %array, i32 0, i32 %n
        %old = load %obj** %nth_el
        %z = div i64 %x, %y
        store %obj* %new, %obj** %nth_el

If the i64 division is lowered to a libcall, then a safe point will (must)
appear for the call site. If a collection occurs, %array and %nth_el no longer
point into the correct object.

The fix for this is to copy address calculations so that dependent pointers
are never live across safe point boundaries. But the loads cannot be copied
like this if there was an intervening store, so may be hard to get right.

Only a concurrent mutator can trigger a collection at the libcall safe point.
So single-threaded programs do not have this requirement, even with a copying
collector. Still, LLVM optimizations would probably undo a front-end's careful
work.

//===---------------------------------------------------------------------===//

The ocaml frametable structure supports liveness information. It would be good
to support it.

//===---------------------------------------------------------------------===//

The FIXME in ComputeCommonTailLength in BranchFolding.cpp needs to be
revisited. The check is there to work around a misuse of directives in inline
assembly.

//===---------------------------------------------------------------------===//

It would be good to detect collector/target compatibility instead of silently
doing the wrong thing.

//===---------------------------------------------------------------------===//

It would be really nice to be able to write patterns in .td files for copies,
which would eliminate a bunch of explicit predicates on them (e.g. no side 
effects).  Once this is in place, it would be even better to have tblgen 
synthesize the various copy insertion/inspection methods in TargetInstrInfo.

//===---------------------------------------------------------------------===//

Stack coloring improvements:

1. Do proper LiveStackAnalysis on all stack objects including those which are
   not spill slots.
2. Reorder objects to fill in gaps between objects.
   e.g. 4, 1, <gap>, 4, 1, 1, 1, <gap>, 4 => 4, 1, 1, 1, 1, 4, 4

//===---------------------------------------------------------------------===//

The scheduler should be able to sort nearby instructions by their address. For
example, in an expanded memset sequence it's not uncommon to see code like this:

  movl $0, 4(%rdi)
  movl $0, 8(%rdi)
  movl $0, 12(%rdi)
  movl $0, 0(%rdi)

Each of the stores is independent, and the scheduler is currently making an
arbitrary decision about the order.

//===---------------------------------------------------------------------===//

Another opportunitiy in this code is that the $0 could be moved to a register:

  movl $0, 4(%rdi)
  movl $0, 8(%rdi)
  movl $0, 12(%rdi)
  movl $0, 0(%rdi)

This would save substantial code size, especially for longer sequences like
this. It would be easy to have a rule telling isel to avoid matching MOV32mi
if the immediate has more than some fixed number of uses. It's more involved
to teach the register allocator how to do late folding to recover from
excessive register pressure.