Prepare to make r0 an allocatable register on PPC

Currently the PPC r0 register is unconditionally reserved. There are two reasons
for this:

 1. r0 is treated specially (as the constant 0) by certain instructions, and so
    cannot be used with those instructions as a regular register.

 2. r0 is used as a temporary register in the CR-register spilling process
    (where, under some circumstances, we require two GPRs).

This change addresses the first reason by introducing a restricted register
class (without r0) for use by those instructions that treat r0 specially. These
register classes have a new pseudo-register, ZERO, which represents the r0-as-0
use. This has the side benefit of making the existing target code simpler (and
easier to understand), and will make it clear to the register allocator that
uses of r0 as 0 don't conflict will real uses of the r0 register.

Once the CR spilling code is improved, we'll be able to allocate r0.

Adding these extra register classes, for some reason unclear to me, causes
requests to the target to copy 32-bit registers to 64-bit registers. The
resulting code seems correct (and causes no test-suite failures), and the new
test case covers this new kind of asymmetric copy.

As r0 is still reserved, no functionality change intended.

llvm-svn: 177423

1 files changed, 12 insertions, 0 deletions

diff --git a/llvm/lib/Target/PowerPC/PPCRegisterInfo.cpp b/llvm/lib/Target/PowerPC/PPCRegisterInfo.cpp
index 74261a3a504..bd7d285431b 100644
--- a/llvm/lib/Target/PowerPC/PPCRegisterInfo.cpp
+++ b/llvm/lib/Target/PowerPC/PPCRegisterInfo.cpp
@@ -76,6 +76,12 @@ PPCRegisterInfo::PPCRegisterInfo(const PPCSubtarget &ST,
 const TargetRegisterClass *
 PPCRegisterInfo::getPointerRegClass(const MachineFunction &MF, unsigned Kind)
                                                                        const {
+  if (Kind == 1) {
+    if (Subtarget.isPPC64())
+      return &PPC::G8RC_NOX0RegClass;
+    return &PPC::GPRC_NOR0RegClass;
+  }
+
   if (Subtarget.isPPC64())
     return &PPC::G8RCRegClass;
   return &PPC::GPRCRegClass;
@@ -104,6 +110,10 @@ BitVector PPCRegisterInfo::getReservedRegs(const MachineFunction &MF) const {
   const PPCFrameLowering *PPCFI =
     static_cast<const PPCFrameLowering*>(MF.getTarget().getFrameLowering());
 
+  // The ZERO register is not really a register, but the representation of r0
+  // when used in instructions that treat r0 as the constant 0.
+  Reserved.set(PPC::ZERO);
+
   Reserved.set(PPC::R0);
   Reserved.set(PPC::R1);
   Reserved.set(PPC::LR);
@@ -148,6 +158,8 @@ PPCRegisterInfo::getRegPressureLimit(const TargetRegisterClass *RC,
   switch (RC->getID()) {
   default:
     return 0;
+  case PPC::G8RC_NOX0RegClassID:
+  case PPC::GPRC_NOR0RegClassID: 
   case PPC::G8RCRegClassID:
   case PPC::GPRCRegClassID: {
     unsigned FP = TFI->hasFP(MF) ? 1 : 0;