[AVR] Disable register coalescing to the PTRDISPREGS class

If we would allow register coalescing on PTRDISPREGS class then register
allocator can lock Z register to some virtual register. Larger instructions
requiring a memory acces then fail during the register allocation phase since
there is no available register to hold a pointer if Y register was already
taken for a stack frame. This patch prevents it by keeping Z register
spillable. It does it by not allowing coalescer to lock it.

Original discussion on https://github.com/avr-rust/rust/issues/128.

llvm-svn: 362298

1 files changed, 15 insertions, 0 deletions

diff --git a/llvm/lib/Target/AVR/AVRRegisterInfo.cpp b/llvm/lib/Target/AVR/AVRRegisterInfo.cpp
index 8dc31fe066b..0aae34d7dfd 100644
--- a/llvm/lib/Target/AVR/AVRRegisterInfo.cpp
+++ b/llvm/lib/Target/AVR/AVRRegisterInfo.cpp
@@ -16,6 +16,7 @@
 #include "llvm/CodeGen/MachineFrameInfo.h"
 #include "llvm/CodeGen/MachineFunction.h"
 #include "llvm/CodeGen/MachineInstrBuilder.h"
+#include "llvm/CodeGen/MachineRegisterInfo.h"
 #include "llvm/IR/Function.h"
 #include "llvm/CodeGen/TargetFrameLowering.h"
 
@@ -272,4 +273,18 @@ void AVRRegisterInfo::splitReg(unsigned Reg,
     HiReg = getSubReg(Reg, AVR::sub_hi);
 }
 
+bool AVRRegisterInfo::shouldCoalesce(MachineInstr *MI,
+                                     const TargetRegisterClass *SrcRC,
+                                     unsigned SubReg,
+                                     const TargetRegisterClass *DstRC,
+                                     unsigned DstSubReg,
+                                     const TargetRegisterClass *NewRC,
+                                     LiveIntervals &LIS) const {
+  if(this->getRegClass(AVR::PTRDISPREGSRegClassID)->hasSubClassEq(NewRC)) {
+    return false;
+  }
+
+  return TargetRegisterInfo::shouldCoalesce(MI, SrcRC, SubReg, DstRC, DstSubReg, NewRC, LIS);
+}
+
 } // end of namespace llvm