[AVR] Reserve the Y register in all functions

llvm-svn: 302017

1 files changed, 12 insertions, 6 deletions

diff --git a/llvm/lib/Target/AVR/AVRRegisterInfo.cpp b/llvm/lib/Target/AVR/AVRRegisterInfo.cpp
index 5cc7eaf8add..2813e24d2ac 100644
--- a/llvm/lib/Target/AVR/AVRRegisterInfo.cpp
+++ b/llvm/lib/Target/AVR/AVRRegisterInfo.cpp
@@ -65,12 +65,18 @@ BitVector AVRRegisterInfo::getReservedRegs(const MachineFunction &MF) const {
   Reserved.set(AVR::SPH);
   Reserved.set(AVR::SP);
 
-  // Reserve the frame pointer registers r28 and r29 if the function requires one.
-  if (TFI->hasFP(MF)) {
-    Reserved.set(AVR::R28);
-    Reserved.set(AVR::R29);
-    Reserved.set(AVR::R29R28);
-  }
+  // We tenatively reserve the frame pointer register r29:r28 because the
+  // function may require one, but we cannot tell until register allocation
+  // is complete, which can be too late.
+  //
+  // Instead we just unconditionally reserve the Y register.
+  //
+  // TODO: Write a pass to enumerate functions which reserved the Y register
+  //       but didn't end up needing a frame pointer. In these, we can
+  //       convert one or two of the spills inside to use the Y register.
+  Reserved.set(AVR::R28);
+  Reserved.set(AVR::R29);
+  Reserved.set(AVR::R29R28);
 
   return Reserved;
 }