Mark most PPC register classes to avoid write-after-write.

For processors with the G5-like instruction-grouping scheme, this helps avoid
early group termination due to a write-after-write dependency within the group.
It should also help on pipelined embedded cores.

On POWER7, over the test suite, this gives an average 0.5% speedup. The largest
speedups are:

SingleSource/Benchmarks/Stanford/Quicksort - 33%
MultiSource/Applications/d/make_dparser - 21%
MultiSource/Benchmarks/FreeBench/analyzer/analyzer - 12%
MultiSource/Benchmarks/MiBench/telecomm-FFT/telecomm-fft - 12%

Largest slowdowns:

SingleSource/Benchmarks/Stanford/Bubblesort - 23%
MultiSource/Benchmarks/Prolangs-C++/city/city - 21%
MultiSource/Benchmarks/BitBench/uuencode/uuencode - 16%
MultiSource/Benchmarks/mediabench/mpeg2/mpeg2dec/mpeg2decode - 13%

llvm-svn: 158719

llvm/lib/Target/PowerPC/PPCRegisterInfo.cpp

@@ -199,6 +199,20 @@ PPCRegisterInfo::getRegPressureLimit(const TargetRegisterClass *RC,
   }
 }
 
+bool
+PPCRegisterInfo::avoidWriteAfterWrite(const TargetRegisterClass *RC) const {
+  switch (RC->getID()) {
+  case PPC::G8RCRegClassID:
+  case PPC::GPRCRegClassID:
+  case PPC::F8RCRegClassID:
+  case PPC::F4RCRegClassID:
+  case PPC::VRRCRegClassID:
+    return true;
+  default:
+    return false;
+  }
+}
+
 //===----------------------------------------------------------------------===//
 // Stack Frame Processing methods
 //===----------------------------------------------------------------------===//

llvm/lib/Target/PowerPC/PPCRegisterInfo.h

@@ -47,6 +47,8 @@ public:
 
   BitVector getReservedRegs(const MachineFunction &MF) const;
 
+  virtual bool avoidWriteAfterWrite(const TargetRegisterClass *RC) const;
+
   /// requiresRegisterScavenging - We require a register scavenger.
   /// FIXME (64-bit): Should be inlined.
   bool requiresRegisterScavenging(const MachineFunction &MF) const;