[ARM] Fix not saving FP when required to in frame-pointer=non-leaf. (#163699)

When the stars align to conspire against stack alignment, when we have
frame-pointer=non-leaf we can incorrectly skip preserving fp/r7 in the
prolog.

The fix here first makes sure we're using the right frame pointer
register in the context of preserving the incoming FP, and then make sure that we
save the FP when re-alignment is known to be necessary.

rdar://162462271

Author: aemerson

llvm/lib/Target/ARM/ARMFrameLowering.cpp

@@ -2536,7 +2536,7 @@ void ARMFrameLowering::determineCalleeSaves(MachineFunction &MF,
   MachineRegisterInfo &MRI = MF.getRegInfo();
   const TargetRegisterInfo *TRI = MF.getSubtarget().getRegisterInfo();
   (void)TRI;  // Silence unused warning in non-assert builds.
-  Register FramePtr = RegInfo->getFrameRegister(MF);
+  Register FramePtr = STI.getFramePointerReg();
   ARMSubtarget::PushPopSplitVariation PushPopSplit =
       STI.getPushPopSplitVariation(MF);
 
@@ -2783,7 +2783,11 @@ void ARMFrameLowering::determineCalleeSaves(MachineFunction &MF,
       !CanEliminateFrame || RegInfo->cannotEliminateFrame(MF)) {
     AFI->setHasStackFrame(true);
 
-    if (HasFP) {
+    // Save the FP if:
+    // 1. We currently need it (HasFP), OR
+    // 2. We might need it later due to stack realignment from aligned DPRCS2
+    //    saves (which will make hasFP() become true in emitPrologue).
+    if (HasFP || (isFPReserved(MF) && AFI->getNumAlignedDPRCS2Regs() > 0)) {
       SavedRegs.set(FramePtr);
       // If the frame pointer is required by the ABI, also spill LR so that we
       // emit a complete frame record.

llvm/test/CodeGen/ARM/save-fp-with-non-leaf.ll

@@ -0,0 +1,37 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 5
+; RUN: llc %s -o - | FileCheck %s --check-prefix=CHECK
+target datalayout = "e-m:o-p:32:32-Fi8-f64:32:64-v64:32:64-v128:32:128-a:0:32-n32-S32"
+target triple = "thumbv7-apple-darwin"
+
+; This test checks that even with NEON register induced stack re-alignment, and
+; with the frame-pointer=non-leaf option, that we still save fp aka r7 in the
+; prolog as required.
+
+define fastcc i32 @test_save_fp() #0 {
+; CHECK-LABEL: test_save_fp:
+; CHECK:       @ %bb.0:
+; CHECK-NEXT:    push {r4, r7, lr}
+; CHECK-NEXT:    add r7, sp, #4
+; CHECK-NEXT:    sub.w r4, sp, #64
+; CHECK-NEXT:    bfc r4, #0, #4
+; CHECK-NEXT:    mov sp, r4
+; CHECK-NEXT:    vst1.64 {d8, d9, d10, d11}, [r4:128]!
+; CHECK-NEXT:    movs r0, #0
+; CHECK-NEXT:    vst1.64 {d12, d13, d14, d15}, [r4:128]
+; CHECK-NEXT:    mov r4, sp
+; CHECK-NEXT:    @ InlineAsm Start
+; CHECK-NEXT:    vld1.16 {d0, d1, d2, d3}, [r0]
+; CHECK-NEXT:    vld1.16 {d4, d5, d6, d7}, [r0]
+; CHECK-NEXT:    vabdl.s16 q4, d0, d4
+; CHECK-EMPTY:
+; CHECK-NEXT:    @ InlineAsm End
+; CHECK-NEXT:    vld1.64 {d8, d9, d10, d11}, [r4:128]!
+; CHECK-NEXT:    vld1.64 {d12, d13, d14, d15}, [r4:128]
+; CHECK-NEXT:    subs r4, r7, #4
+; CHECK-NEXT:    mov sp, r4
+; CHECK-NEXT:    pop {r4, r7, pc}
+  tail call void asm sideeffect "vld1.i16 {q0,q1}, [$0]\0Avld1.i16 {q2,q3}, [$1]\0Avabdl.s16 q4, d0, d4\0A", "r,r,r,~{q0},~{q1},~{q2},~{q3},~{q4},~{q5},~{q6},~{q7},~{memory}"(ptr null, ptr null, ptr null)
+  ret i32 0
+}
+
+attributes #0 = { "frame-pointer"="non-leaf" }