[ARM] Generate consistent frame records for Thumb2

There is not an official documented ABI for frame pointers in Thumb2,
but we should try to emit something which is useful.

We use r7 as the frame pointer for Thumb code, which currently means
that if a function needs to save a high register (r8-r11), it will get
pushed to the stack between the frame pointer (r7) and link register
(r14). This means that while a stack unwinder can follow the chain of
frame pointers up the stack, it cannot know the offset to lr, so does
not know which functions correspond to the stack frames.

To fix this, we need to push the callee-saved registers in two batches,
with the first push saving the low registers, fp and lr, and the second
push saving the high registers. This is already implemented, but
previously only used for iOS. This patch turns it on for all Thumb2
targets when frame pointers are required by the ABI, and the frame
pointer is r7 (Windows uses r11, so this isn't a problem there). If
frame pointer elimination is enabled we still emit a single push/pop
even if we need a frame pointer for other reasons, to avoid increasing
code size.

We must also ensure that lr is pushed to the stack when using a frame
pointer, so that we end up with a complete frame record. Situations that
could cause this were rare, because we already push lr in most
situations so that we can return using the pop instruction.

Differential Revision: https://reviews.llvm.org/D23516

llvm-svn: 279506

1 files changed, 29 insertions, 22 deletions

diff --git a/llvm/lib/Target/ARM/ARMFrameLowering.cpp b/llvm/lib/Target/ARM/ARMFrameLowering.cpp
index 4c8646c..61be18c 100644
--- a/llvm/lib/Target/ARM/ARMFrameLowering.cpp
+++ b/llvm/lib/Target/ARM/ARMFrameLowering.cpp
@@ -57,16 +57,14 @@ bool ARMFrameLowering::noFramePointerElim(const MachineFunction &MF) const {
 /// or if frame pointer elimination is disabled.
 bool ARMFrameLowering::hasFP(const MachineFunction &MF) const {
   const TargetRegisterInfo *RegInfo = MF.getSubtarget().getRegisterInfo();
+  const MachineFrameInfo &MFI = MF.getFrameInfo();
 
-  // iOS requires FP not to be clobbered for backtracing purpose.
-  if (STI.isTargetIOS() || STI.isTargetWatchOS())
+  // ABI-required frame pointer.
+  if (MF.getTarget().Options.DisableFramePointerElim(MF))
     return true;
 
-  const MachineFrameInfo &MFI = MF.getFrameInfo();
-  // Always eliminate non-leaf frame pointers.
-  return ((MF.getTarget().Options.DisableFramePointerElim(MF) &&
-           MFI.hasCalls()) ||
-          RegInfo->needsStackRealignment(MF) ||
+  // Frame pointer required for use within this function.
+  return (RegInfo->needsStackRealignment(MF) ||
           MFI.hasVarSizedObjects() ||
           MFI.isFrameAddressTaken());
 }
@@ -352,7 +350,7 @@ void ARMFrameLowering::emitPrologue(MachineFunction &MF,
     case ARM::R10:
     case ARM::R11:
     case ARM::R12:
-      if (STI.splitFramePushPop()) {
+      if (STI.splitFramePushPop(MF)) {
         GPRCS2Size += 4;
         break;
       }
@@ -557,7 +555,7 @@ void ARMFrameLowering::emitPrologue(MachineFunction &MF,
       case ARM::R10:
       case ARM::R11:
       case ARM::R12:
-        if (STI.splitFramePushPop())
+        if (STI.splitFramePushPop(MF))
           break;
         LLVM_FALLTHROUGH;
       case ARM::R0:
@@ -590,7 +588,7 @@ void ARMFrameLowering::emitPrologue(MachineFunction &MF,
       case ARM::R10:
       case ARM::R11:
       case ARM::R12:
-        if (STI.splitFramePushPop()) {
+        if (STI.splitFramePushPop(MF)) {
           unsigned DwarfReg =  MRI->getDwarfRegNum(Reg, true);
           unsigned Offset = MFI.getObjectOffset(FI);
           unsigned CFIIndex = MMI.addFrameInst(
@@ -902,7 +900,7 @@ void ARMFrameLowering::emitPushInst(MachineBasicBlock &MBB,
     unsigned LastReg = 0;
     for (; i != 0; --i) {
       unsigned Reg = CSI[i-1].getReg();
-      if (!(Func)(Reg, STI.splitFramePushPop())) continue;
+      if (!(Func)(Reg, STI.splitFramePushPop(MF))) continue;
 
       // D-registers in the aligned area DPRCS2 are NOT spilled here.
       if (Reg >= ARM::D8 && Reg < ARM::D8 + NumAlignedDPRCS2Regs)
@@ -983,7 +981,7 @@ void ARMFrameLowering::emitPopInst(MachineBasicBlock &MBB,
     bool DeleteRet = false;
     for (; i != 0; --i) {
       unsigned Reg = CSI[i-1].getReg();
-      if (!(Func)(Reg, STI.splitFramePushPop())) continue;
+      if (!(Func)(Reg, STI.splitFramePushPop(MF))) continue;
 
       // The aligned reloads from area DPRCS2 are not inserted here.
       if (Reg >= ARM::D8 && Reg < ARM::D8 + NumAlignedDPRCS2Regs)
@@ -1547,7 +1545,7 @@ void ARMFrameLowering::determineCalleeSaves(MachineFunction &MF,
     if (Spilled) {
       NumGPRSpills++;
 
-      if (!STI.splitFramePushPop()) {
+      if (!STI.splitFramePushPop(MF)) {
         if (Reg == ARM::LR)
           LRSpilled = true;
         CS1Spilled = true;
@@ -1569,7 +1567,7 @@ void ARMFrameLowering::determineCalleeSaves(MachineFunction &MF,
         break;
       }
     } else {
-      if (!STI.splitFramePushPop()) {
+      if (!STI.splitFramePushPop(MF)) {
         UnspilledCS1GPRs.push_back(Reg);
         continue;
       }
@@ -1634,6 +1632,23 @@ void ARMFrameLowering::determineCalleeSaves(MachineFunction &MF,
   if (BigStack || !CanEliminateFrame || RegInfo->cannotEliminateFrame(MF)) {
     AFI->setHasStackFrame(true);
 
+    if (hasFP(MF)) {
+      SavedRegs.set(FramePtr);
+      // If the frame pointer is required by the ABI, also spill LR so that we
+      // emit a complete frame record.
+      if (MF.getTarget().Options.DisableFramePointerElim(MF) && !LRSpilled) {
+        SavedRegs.set(ARM::LR);
+        LRSpilled = true;
+        NumGPRSpills++;
+      }
+      auto FPPos = find(UnspilledCS1GPRs, FramePtr);
+      if (FPPos != UnspilledCS1GPRs.end())
+        UnspilledCS1GPRs.erase(FPPos);
+      NumGPRSpills++;
+      if (FramePtr == ARM::R7)
+        CS1Spilled = true;
+    }
+
     // If LR is not spilled, but at least one of R4, R5, R6, and R7 is spilled.
     // Spill LR as well so we can fold BX_RET to the registers restore (LDM).
     if (!LRSpilled && CS1Spilled) {
@@ -1648,14 +1663,6 @@ void ARMFrameLowering::determineCalleeSaves(MachineFunction &MF,
       ExtraCSSpill = true;
     }
 
-    if (hasFP(MF)) {
-      SavedRegs.set(FramePtr);
-      auto FPPos = find(UnspilledCS1GPRs, FramePtr);
-      if (FPPos != UnspilledCS1GPRs.end())
-        UnspilledCS1GPRs.erase(FPPos);
-      NumGPRSpills++;
-    }
-
     // If stack and double are 8-byte aligned and we are spilling an odd number
     // of GPRs, spill one extra callee save GPR so we won't have to pad between
     // the integer and double callee save areas.