[PPC][AIX] Set needsFP to true when base pointer is used in prologue/epilogue

When the base pointer r30 is used to hold the stack pointer, r30 is spilled
in the prologue. On AIX registers are saved from highest to lowest, so r31
also needs to be saved. Setting needsFP to true on AIX when the base pointer
is used allows r31 to also be saved and restored.

Author: Zaara Syeda

llvm/lib/Target/PowerPC/PPCFrameLowering.cpp

@@ -376,6 +376,9 @@ bool PPCFrameLowering::needsFP(const MachineFunction &MF) const {
   if (MF.getFunction().hasFnAttribute(Attribute::Naked))
     return false;
 
+  if (Subtarget.isAIXABI() && Subtarget.getRegisterInfo()->hasBasePointer(MF))
+    return true;
+
   return MF.getTarget().Options.DisableFramePointerElim(MF) ||
          MFI.hasVarSizedObjects() || MFI.hasStackMap() || MFI.hasPatchPoint() ||
          MF.exposesReturnsTwice() ||

llvm/test/CodeGen/PowerPC/aix-base-pointer.ll

@@ -6,8 +6,9 @@
 
 ; Use an overaligned buffer to force base-pointer usage. Test verifies:
 ; - base pointer register (r30) is saved/defined/restored.
+; - frame pointer register (r31) is saved/defined/restored.
 ; - stack frame is allocated with correct alignment.
-; - Address of %AlignedBuffer is calculated based off offset from the stack
+; - Address of %AlignedBuffer is calculated based off offset from the frame
 ;   pointer.
 
 define float @caller(float %f) {
@@ -19,23 +20,29 @@ define float @caller(float %f) {
 declare void @callee(ptr)
 
 ; 32BIT-LABEL: .caller:
+; 32BIT:         stw 31, -12(1)
 ; 32BIT:         stw 30, -16(1)
 ; 32BIT:         mr 30, 1
 ; 32BIT:         clrlwi  0, 1, 27
 ; 32BIT:         subfic 0, 0, -224
 ; 32BIT:         stwux 1, 1, 0
-; 32BIT:         addi 3, 1, 64
+; 32BIT:         mr 31, 1
+; 32BIT:         addi 3, 31, 64
 ; 32BIT:         bl .callee
 ; 32BIT:         mr 1, 30
+; 32BIT:         lwz 31, -12(1)
 ; 32BIT:         lwz 30, -16(1)
 
 ; 64BIT-LABEL: .caller:
+; 64BIT:         std 31, -16(1)
 ; 64BIT:         std 30, -24(1)
 ; 64BIT:         mr 30, 1
 ; 64BIT:         clrldi  0, 1, 59
 ; 64BIT:         subfic 0, 0, -288
 ; 64BIT:         stdux 1, 1, 0
-; 64BIT:         addi 3, 1, 128
+; 64BIT:         mr 31, 1
+; 64BIT:         addi 3, 31, 128
 ; 64BIT:         bl .callee
 ; 64BIT:         mr 1, 30
+; 64BIT:         ld 31, -16(1)
 ; 64BIT:         ld 30, -24(1)