commit 7d6aa1f7ae73206460bc36c97d5b48bdbf7c2cd8
author Alex Bradbury <asb@lowrisc.org> Thu Jan 18 11:34:02 2018 +0000
committer Alex Bradbury <asb@lowrisc.org> Thu Jan 18 11:34:02 2018 +0000
tree 19a95f1c99019c338f9250c313ec36b0722de084
parent c806d39182c1aa6c647d044d19d58aad9f5c95df

[RISCV] Implement frame pointer elimination

llvm-svn: 322839

llvm/lib/Target/RISCV/RISCVFrameLowering.cpp

diff --git a/llvm/lib/Target/RISCV/RISCVFrameLowering.cpp b/llvm/lib/Target/RISCV/RISCVFrameLowering.cpp
index 32ed896..33703f5 100644
--- a/llvm/lib/Target/RISCV/RISCVFrameLowering.cpp
+++ b/llvm/lib/Target/RISCV/RISCVFrameLowering.cpp
@@ -22,7 +22,14 @@
 
 using namespace llvm;
 
-bool RISCVFrameLowering::hasFP(const MachineFunction &MF) const { return true; }
+bool RISCVFrameLowering::hasFP(const MachineFunction &MF) const {
+  const TargetRegisterInfo *RegInfo = MF.getSubtarget().getRegisterInfo();
+
+  const MachineFrameInfo &MFI = MF.getFrameInfo();
+  return MF.getTarget().Options.DisableFramePointerElim(MF) ||
+         RegInfo->needsStackRealignment(MF) || MFI.hasVarSizedObjects() ||
+         MFI.isFrameAddressTaken();
+}
 
 // Determines the size of the frame and maximum call frame size.
 void RISCVFrameLowering::determineFrameLayout(MachineFunction &MF) const {
@@ -103,11 +110,6 @@
                                       MachineBasicBlock &MBB) const {
   assert(&MF.front() == &MBB && "Shrink-wrapping not yet supported");
 
-  if (!hasFP(MF)) {
-    report_fatal_error(
-        "emitPrologue doesn't support framepointer-less functions");
-  }
-
   MachineFrameInfo &MFI = MF.getFrameInfo();
   auto *RVFI = MF.getInfo<RISCVMachineFunctionInfo>();
   MachineBasicBlock::iterator MBBI = MBB.begin();
@@ -143,17 +145,13 @@
   std::advance(MBBI, CSI.size());
 
   // Generate new FP.
-  adjustReg(MBB, MBBI, DL, FPReg, SPReg, StackSize - RVFI->getVarArgsSaveSize(),
-            MachineInstr::FrameSetup);
+  if (hasFP(MF))
+    adjustReg(MBB, MBBI, DL, FPReg, SPReg,
+              StackSize - RVFI->getVarArgsSaveSize(), MachineInstr::FrameSetup);
 }
 
 void RISCVFrameLowering::emitEpilogue(MachineFunction &MF,
                                       MachineBasicBlock &MBB) const {
-  if (!hasFP(MF)) {
-    report_fatal_error(
-        "emitEpilogue doesn't support framepointer-less functions");
-  }
-
   MachineBasicBlock::iterator MBBI = MBB.getLastNonDebugInstr();
   const RISCVRegisterInfo *RI = STI.getRegisterInfo();
   MachineFrameInfo &MFI = MF.getFrameInfo();
@@ -174,6 +172,7 @@
   // necessary if the stack pointer was modified, meaning the stack size is
   // unknown.
   if (RI->needsStackRealignment(MF) || MFI.hasVarSizedObjects()) {
+    assert(hasFP(MF) && "frame pointer should not have been eliminated");
     adjustReg(MBB, LastFrameDestroy, DL, SPReg, FPReg,
               -StackSize + RVFI->getVarArgsSaveSize(),
               MachineInstr::FrameDestroy);
@@ -210,8 +209,10 @@
     Offset += MF.getFrameInfo().getStackSize();
   } else {
     FrameReg = RI->getFrameRegister(MF);
-    assert(hasFP(MF) && "Offset calculation incorrect if !hasFP");
-    Offset += RVFI->getVarArgsSaveSize();
+    if (hasFP(MF))
+      Offset += RVFI->getVarArgsSaveSize();
+    else
+      Offset += MF.getFrameInfo().getStackSize();
   }
   return Offset;
 }
@@ -220,10 +221,12 @@
                                               BitVector &SavedRegs,
                                               RegScavenger *RS) const {
   TargetFrameLowering::determineCalleeSaves(MF, SavedRegs, RS);
-  // TODO: Once frame pointer elimination is implemented, don't
-  // unconditionally spill the frame pointer and return address.
-  SavedRegs.set(RISCV::X1);
-  SavedRegs.set(RISCV::X8);
+  // Unconditionally spill RA and FP only if the function uses a frame
+  // pointer.
+  if (hasFP(MF)) {
+    SavedRegs.set(RISCV::X1);
+    SavedRegs.set(RISCV::X8);
+  }
 }
 
 void RISCVFrameLowering::processFunctionBeforeFrameFinalized(