Re-apply r112883:

"For ARM stack frames that utilize variable sized objects and have either
large local stack areas or require dynamic stack realignment, allocate a
base register via which to access the local frame. This allows efficient
access to frame indices not accessible via the FP (either due to being out
of range or due to dynamic realignment) or the SP (due to variable sized
object allocation). In particular, this greatly improves efficiency of access
to spill slots in Thumb functions which contain VLAs."

r112986 fixed a latent bug exposed by the above.




git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@112989 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 76 insertions, 23 deletions

diff --git a/lib/Target/ARM/ARMBaseRegisterInfo.cpp b/lib/Target/ARM/ARMBaseRegisterInfo.cpp
index 8c1b913..be33ed6 100644
--- a/lib/Target/ARM/ARMBaseRegisterInfo.cpp
+++ b/lib/Target/ARM/ARMBaseRegisterInfo.cpp
@@ -50,6 +50,10 @@ EnableLocalStackAlloc("enable-local-stack-alloc", cl::init(true), cl::Hidden,
 
 using namespace llvm;
 
+static cl::opt<bool>
+EnableBasePointer("arm-use-base-pointer", cl::Hidden, cl::init(true),
+          cl::desc("Enable use of a base pointer for complex stack frames"));
+
 unsigned ARMBaseRegisterInfo::getRegisterNumbering(unsigned RegEnum,
                                                    bool *isSPVFP) {
   if (isSPVFP)
@@ -146,7 +150,8 @@ ARMBaseRegisterInfo::ARMBaseRegisterInfo(const ARMBaseInstrInfo &tii,
                                          const ARMSubtarget &sti)
   : ARMGenRegisterInfo(ARM::ADJCALLSTACKDOWN, ARM::ADJCALLSTACKUP),
     TII(tii), STI(sti),
-    FramePtr((STI.isTargetDarwin() || STI.isThumb()) ? ARM::R7 : ARM::R11) {
+    FramePtr((STI.isTargetDarwin() || STI.isThumb()) ? ARM::R7 : ARM::R11),
+    BasePtr(ARM::R6) {
 }
 
 const unsigned*
@@ -182,6 +187,8 @@ getReservedRegs(const MachineFunction &MF) const {
   Reserved.set(ARM::FPSCR);
   if (hasFP(MF))
     Reserved.set(FramePtr);
+  if (hasBasePointer(MF))
+    Reserved.set(BasePtr);
   // Some targets reserve R9.
   if (STI.isR9Reserved())
     Reserved.set(ARM::R9);
@@ -195,6 +202,10 @@ bool ARMBaseRegisterInfo::isReservedReg(const MachineFunction &MF,
   case ARM::SP:
   case ARM::PC:
     return true;
+  case ARM::R6:
+    if (hasBasePointer(MF))
+      return true;
+    break;
   case ARM::R7:
   case ARM::R11:
     if (FramePtr == Reg && hasFP(MF))
@@ -625,35 +636,49 @@ bool ARMBaseRegisterInfo::hasFP(const MachineFunction &MF) const {
           MFI->isFrameAddressTaken());
 }
 
-bool ARMBaseRegisterInfo::canRealignStack(const MachineFunction &MF) const {
+bool ARMBaseRegisterInfo::hasBasePointer(const MachineFunction &MF) const {
   const MachineFrameInfo *MFI = MF.getFrameInfo();
   const ARMFunctionInfo *AFI = MF.getInfo<ARMFunctionInfo>();
-  return (RealignStack &&
-          !AFI->isThumb1OnlyFunction() &&
-          !MFI->hasVarSizedObjects());
+
+  if (!EnableBasePointer)
+    return false;
+
+  if (needsStackRealignment(MF) && MFI->hasVarSizedObjects())
+    return true;
+
+  // Thumb has trouble with negative offsets from the FP. Thumb2 has a limited
+  // negative range for ldr/str (255), and thumb1 is positive offsets only.
+  // It's going to be better to use the SP or Base Pointer instead. When there
+  // are variable sized objects, we can't reference off of the SP, so we
+  // reserve a Base Pointer.
+  if (AFI->isThumbFunction() && MFI->hasVarSizedObjects()) {
+    // Conservatively estimate whether the negative offset from the frame
+    // pointer will be sufficient to reach. If a function has a smallish
+    // frame, it's less likely to have lots of spills and callee saved
+    // space, so it's all more likely to be within range of the frame pointer.
+    // If it's wrong, the scavenger will still enable access to work, it just
+    // won't be optimal.
+    if (AFI->isThumb2Function() && MFI->getLocalFrameSize() < 128)
+      return false;
+    return true;
+  }
+
+  return false;
+}
+
+bool ARMBaseRegisterInfo::canRealignStack(const MachineFunction &MF) const {
+  const ARMFunctionInfo *AFI = MF.getInfo<ARMFunctionInfo>();
+  return (RealignStack && !AFI->isThumb1OnlyFunction());
 }
 
 bool ARMBaseRegisterInfo::
 needsStackRealignment(const MachineFunction &MF) const {
   const MachineFrameInfo *MFI = MF.getFrameInfo();
   const Function *F = MF.getFunction();
-  const ARMFunctionInfo *AFI = MF.getInfo<ARMFunctionInfo>();
   unsigned StackAlign = MF.getTarget().getFrameInfo()->getStackAlignment();
   bool requiresRealignment = ((MFI->getLocalFrameMaxAlign() > StackAlign) ||
                                F->hasFnAttr(Attribute::StackAlignment));
 
-  // FIXME: Currently we don't support stack realignment for functions with
-  //        variable-sized allocas.
-  // FIXME: It's more complicated than this...
-  if (0 && requiresRealignment && MFI->hasVarSizedObjects())
-    report_fatal_error(
-      "Stack realignment in presense of dynamic allocas is not supported");
-
-  // FIXME: This probably isn't the right place for this.
-  if (0 && requiresRealignment && AFI->isThumb1OnlyFunction())
-    report_fatal_error(
-      "Stack realignment in thumb1 functions is not supported");
-
   return requiresRealignment && canRealignStack(MF);
 }
 
@@ -776,6 +801,10 @@ ARMBaseRegisterInfo::processFunctionBeforeCalleeSavedScan(MachineFunction &MF,
   if (AFI->isThumb1OnlyFunction() && AFI->getVarArgsRegSaveSize() > 0)
     MF.getRegInfo().setPhysRegUsed(ARM::LR);
 
+  // Spill the BasePtr if it's used.
+  if (hasBasePointer(MF))
+    MF.getRegInfo().setPhysRegUsed(BasePtr);
+
   // Don't spill FP if the frame can be eliminated. This is determined
   // by scanning the callee-save registers to see if any is used.
   const unsigned *CSRegs = getCalleeSavedRegs();
@@ -1022,13 +1051,14 @@ ARMBaseRegisterInfo::ResolveFrameIndexReference(const MachineFunction &MF,
     return Offset - AFI->getDPRCalleeSavedAreaOffset();
 
   // When dynamically realigning the stack, use the frame pointer for
-  // parameters, and the stack pointer for locals.
+  // parameters, and the stack/base pointer for locals.
   if (needsStackRealignment(MF)) {
     assert (hasFP(MF) && "dynamic stack realignment without a FP!");
     if (isFixed) {
       FrameReg = getFrameRegister(MF);
       Offset = FPOffset;
-    }
+    } else if (MFI->hasVarSizedObjects())
+      FrameReg = BasePtr;
     return Offset;
   }
 
@@ -1036,9 +1066,13 @@ ARMBaseRegisterInfo::ResolveFrameIndexReference(const MachineFunction &MF,
   if (hasFP(MF) && AFI->hasStackFrame()) {
     // Use frame pointer to reference fixed objects. Use it for locals if
     // there are VLAs (and thus the SP isn't reliable as a base).
-    if (isFixed || MFI->hasVarSizedObjects()) {
+    if (isFixed || (MFI->hasVarSizedObjects() && !hasBasePointer(MF))) {
       FrameReg = getFrameRegister(MF);
       Offset = FPOffset;
+    } else if (MFI->hasVarSizedObjects()) {
+      assert(hasBasePointer(MF) && "missing base pointer!");
+      // Use the base register since we have it.
+      FrameReg = BasePtr;
     } else if (AFI->isThumb2Function()) {
       // In Thumb2 mode, the negative offset is very limited. Try to avoid
       // out of range references.
@@ -1052,6 +1086,9 @@ ARMBaseRegisterInfo::ResolveFrameIndexReference(const MachineFunction &MF,
       Offset = FPOffset;
     }
   }
+  // Use the base pointer if we have one.
+  if (hasBasePointer(MF))
+    FrameReg = BasePtr;
   return Offset;
 }
 
@@ -1089,7 +1126,8 @@ unsigned ARMBaseRegisterInfo::getRegisterPairEven(unsigned Reg,
   case ARM::R5:
     return ARM::R4;
   case ARM::R7:
-    return isReservedReg(MF, ARM::R7)  ? 0 : ARM::R6;
+    return (isReservedReg(MF, ARM::R7) || isReservedReg(MF, ARM::R6))
+      ? 0 : ARM::R6;
   case ARM::R9:
     return isReservedReg(MF, ARM::R9)  ? 0 :ARM::R8;
   case ARM::R11:
@@ -1178,7 +1216,8 @@ unsigned ARMBaseRegisterInfo::getRegisterPairOdd(unsigned Reg,
   case ARM::R4:
     return ARM::R5;
   case ARM::R6:
-    return isReservedReg(MF, ARM::R7)  ? 0 : ARM::R7;
+    return (isReservedReg(MF, ARM::R7) || isReservedReg(MF, ARM::R6))
+      ? 0 : ARM::R7;
   case ARM::R8:
     return isReservedReg(MF, ARM::R9)  ? 0 :ARM::R9;
   case ARM::R10:
@@ -1878,6 +1917,20 @@ emitPrologue(MachineFunction &MF) const {
     AFI->setShouldRestoreSPFromFP(true);
   }
 
+  // If we need a base pointer, set it up here. It's whatever the value
+  // of the stack pointer is at this point. Any variable size objects
+  // will be allocated after this, so we can still use the base pointer
+  // to reference locals.
+  if (hasBasePointer(MF)) {
+    if (isARM)
+      BuildMI(MBB, MBBI, dl, TII.get(ARM::MOVr), BasePtr)
+        .addReg(ARM::SP)
+        .addImm((unsigned)ARMCC::AL).addReg(0).addReg(0);
+    else
+      BuildMI(MBB, MBBI, dl, TII.get(ARM::tMOVgpr2gpr), BasePtr)
+        .addReg(ARM::SP);
+  }
+
   // If the frame has variable sized objects then the epilogue must restore
   // the sp from fp.
   if (!AFI->shouldRestoreSPFromFP() && MFI->hasVarSizedObjects())