MachineFrameInfo: Simplify pristine register calculation.

About pristine regsiters:
Pristine registers "hold a value that is useless to the current
function, but that must be preserved - they are callee saved registers
that have not been saved." This concept saves compile time as it frees
the prologue/epilogue inserter from adding every such register to every
basic blocks live-in list.

However the current code in getPristineRegs is formulated in a
complicated way: Inside the function prologue and epilogue all callee
saves are considered pristine, while in the rest of the code only the
non-saved ones are considered pristine.  This requires logic to
differentiate between prologue/epilogue and the rest and in the presence
of shrink-wrapping this even becomes complicated/expensive.  It's also
unnecessary because the prologue epilogue inserters already mark
callee-save registers that are saved/restores properly in the respective
blocks in the prologue/epilogue (see updateLiveness() in
PrologueEpilogueInserter.cpp). So only declaring non-saved/restored
callee saved registers as pristine just works.

Differential Revision: http://reviews.llvm.org/D10101

llvm-svn: 238524

1 files changed, 4 insertions, 46 deletions

diff --git a/llvm/lib/CodeGen/MachineFunction.cpp b/llvm/lib/CodeGen/MachineFunction.cpp
index 8ec63f823e1..09662b6e48d 100644
--- a/llvm/lib/CodeGen/MachineFunction.cpp
+++ b/llvm/lib/CodeGen/MachineFunction.cpp
@@ -584,12 +584,8 @@ int MachineFrameInfo::CreateFixedSpillStackObject(uint64_t Size,
   return -++NumFixedObjects;
 }
 
-BitVector
-MachineFrameInfo::getPristineRegs(const MachineBasicBlock *MBB) const {
-  assert(MBB && "MBB must be valid");
-  const MachineFunction *MF = MBB->getParent();
-  assert(MF && "MBB must be part of a MachineFunction");
-  const TargetRegisterInfo *TRI = MF->getSubtarget().getRegisterInfo();
+BitVector MachineFrameInfo::getPristineRegs(const MachineFunction &MF) const {
+  const TargetRegisterInfo *TRI = MF.getSubtarget().getRegisterInfo();
   BitVector BV(TRI->getNumRegs());
 
   // Before CSI is calculated, no registers are considered pristine. They can be
@@ -597,14 +593,10 @@ MachineFrameInfo::getPristineRegs(const MachineBasicBlock *MBB) const {
   if (!isCalleeSavedInfoValid())
     return BV;
 
-  for (const MCPhysReg *CSR = TRI->getCalleeSavedRegs(MF); CSR && *CSR; ++CSR)
+  for (const MCPhysReg *CSR = TRI->getCalleeSavedRegs(&MF); CSR && *CSR; ++CSR)
     BV.set(*CSR);
 
-  // Each MBB before the save point has all CSRs pristine.
-  if (isBeforeSavePoint(*MF, *MBB))
-    return BV;
-
-  // On other MBBs the saved CSRs are not pristine.
+  // Saved CSRs are not pristine.
   const std::vector<CalleeSavedInfo> &CSI = getCalleeSavedInfo();
   for (std::vector<CalleeSavedInfo>::const_iterator I = CSI.begin(),
          E = CSI.end(); I != E; ++I)
@@ -613,40 +605,6 @@ MachineFrameInfo::getPristineRegs(const MachineBasicBlock *MBB) const {
   return BV;
 }
 
-// Note: We could use some sort of caching mecanism, but we lack the ability
-// to know when the cache is invalid, i.e., the CFG changed.
-// Assuming we have that, we can simply compute all the set of MBBs
-// that are before the save point.
-bool MachineFrameInfo::isBeforeSavePoint(const MachineFunction &MF,
-                                         const MachineBasicBlock &MBB) const {
-  // Early exit if shrink-wrapping did not kick.
-  if (!Save)
-    return &MBB == &MF.front();
-
-  // Starting from MBB, check if there is a path leading to Save that do
-  // not cross Restore.
-  SmallPtrSet<const MachineBasicBlock *, 8> Visited;
-  SmallVector<const MachineBasicBlock *, 8> WorkList;
-  WorkList.push_back(&MBB);
-  Visited.insert(&MBB);
-  do {
-    const MachineBasicBlock *CurBB = WorkList.pop_back_val();
-    // By construction, the region that is after the save point is
-    // dominated by the Save and post-dominated by the Restore.
-    // If we do not reach Restore and still reach Save, this
-    // means MBB is before Save.
-    if (CurBB == Save)
-      return true;
-    if (CurBB == Restore)
-      continue;
-    // Enqueue all the successors not already visited.
-    for (MachineBasicBlock *SuccBB : CurBB->successors())
-      if (Visited.insert(SuccBB).second)
-        WorkList.push_back(SuccBB);
-  } while (!WorkList.empty());
-  return false;
-}
-
 unsigned MachineFrameInfo::estimateStackSize(const MachineFunction &MF) const {
   const TargetFrameLowering *TFI = MF.getSubtarget().getFrameLowering();
   const TargetRegisterInfo *RegInfo = MF.getSubtarget().getRegisterInfo();