[Hexagon] Handle decreasing of stack alignment in frame lowering

llvm-svn: 306124

4 files changed, 51 insertions, 2 deletions

diff --git a/llvm/lib/Target/Hexagon/HexagonFrameLowering.cpp b/llvm/lib/Target/Hexagon/HexagonFrameLowering.cpp
index f0b7f587379..601a5d303cd 100644
--- a/llvm/lib/Target/Hexagon/HexagonFrameLowering.cpp
+++ b/llvm/lib/Target/Hexagon/HexagonFrameLowering.cpp
@@ -552,7 +552,7 @@ void HexagonFrameLowering::insertPrologueInBlock(MachineBasicBlock &MBB,
   auto &HRI = *HST.getRegisterInfo();
   DebugLoc dl;
 
-  unsigned MaxAlign = std::max(MFI.getMaxAlignment(), getStackAlignment());
+  unsigned MaxAlign = std::max(getMaxStackAlignment(MF), getStackAlignment());
 
   // Calculate the total stack frame size.
   // Get the number of bytes to allocate from the FrameInfo.
@@ -2354,11 +2354,26 @@ void HexagonFrameLowering::expandAlloca(MachineInstr *AI,
   }
 }
 
+unsigned
+HexagonFrameLowering::getMaxStackAlignment(const MachineFunction &MF) const {
+  const MachineFrameInfo &MFI = MF.getFrameInfo();
+  // MFI's MaxAlignment can only grow, but we can actually reduce it
+  // for vector spills.
+  unsigned MaxAlign = 0;
+  for (int i = 0, e = MFI.getObjectIndexEnd(); i != e; ++i) {
+    if (MFI.isDeadObjectIndex(i))
+      continue;
+    unsigned Align = MFI.getObjectAlignment(i);
+    MaxAlign = std::max(MaxAlign, Align);
+  }
+  return MaxAlign;
+}
+
 bool HexagonFrameLowering::needsAligna(const MachineFunction &MF) const {
   const MachineFrameInfo &MFI = MF.getFrameInfo();
   if (!MFI.hasVarSizedObjects())
     return false;
-  unsigned MaxA = MFI.getMaxAlignment();
+  unsigned MaxA = getMaxStackAlignment(MF);
   if (MaxA <= getStackAlignment())
     return false;
   return true;
diff --git a/llvm/lib/Target/Hexagon/HexagonFrameLowering.h b/llvm/lib/Target/Hexagon/HexagonFrameLowering.h
index 529a61d4a5b..29bafaee78c 100644
--- a/llvm/lib/Target/Hexagon/HexagonFrameLowering.h
+++ b/llvm/lib/Target/Hexagon/HexagonFrameLowering.h
@@ -82,6 +82,8 @@ public:
       const TargetRegisterInfo *TRI, std::vector<CalleeSavedInfo> &CSI)
       const override;
 
+  unsigned getMaxStackAlignment(const MachineFunction &MF) const;
+
   bool needsAligna(const MachineFunction &MF) const;
   const MachineInstr *getAlignaInstr(const MachineFunction &MF) const;
 
diff --git a/llvm/lib/Target/Hexagon/HexagonRegisterInfo.cpp b/llvm/lib/Target/Hexagon/HexagonRegisterInfo.cpp
index 1fc157900ed..b44177b0a35 100644
--- a/llvm/lib/Target/Hexagon/HexagonRegisterInfo.cpp
+++ b/llvm/lib/Target/Hexagon/HexagonRegisterInfo.cpp
@@ -283,6 +283,36 @@ bool HexagonRegisterInfo::useFPForScavengingIndex(const MachineFunction &MF)
   return MF.getSubtarget<HexagonSubtarget>().getFrameLowering()->hasFP(MF);
 }
 
+// The stack alignment on Hexagon can actually decrease in some cases,
+// specifically in some subset of cases when a variable-sized stack object
+// is present.
+// The issue is two-fold:
+// First of all, if there is a variable-sized object and the stack needs
+// extra alignment (due to pre-existing local objects), then a special
+// register will be reserved up front, acting as the aligned stack pointer
+// (call it AP). This register is only guaranteed to be live for accessing
+// these pre-existing local objects (the ones with the higher alignment).
+// Now, if the register allocator introduces vector register spills, their
+// spill slots will initially have an alignment equal to the register size,
+// which is higher than the normal stack alignment. Ideally, they should be
+// loaded/stored using AP, but AP may not be available at all required
+// places. To avoid this issue, the vector spill slots will have their
+// alignment lowered to 8, and they will be loaded/stored using unaligned
+// instructions.
+//
+// The lowering of the stack alignment may happen if the stack had a
+// variable-sized object, but otherwise retained its default alignment
+// up until register allocation. If the register allocator introduces
+// a vector spill, it will cause the max stack alignment to grow
+// (inside MachineFrameInfo). When the alignment of the spills is reset
+// back to the default stack alignment, MFI's max stack alignment will
+// not reflect that (since it cannot be lowered). Relying on that during
+// frame lowering will cause an unnecessary stack realignment.
+bool HexagonRegisterInfo::needsStackRealignment(const MachineFunction &MF)
+      const {
+  auto &HFI = *MF.getSubtarget<HexagonSubtarget>().getFrameLowering();
+  return HFI.getMaxStackAlignment(MF) > HFI.getStackAlignment();
+}
 
 unsigned HexagonRegisterInfo::getFirstCallerSavedNonParamReg() const {
   return Hexagon::R6;
diff --git a/llvm/lib/Target/Hexagon/HexagonRegisterInfo.h b/llvm/lib/Target/Hexagon/HexagonRegisterInfo.h
index 5f65fad2cc0..568bdd0b806 100644
--- a/llvm/lib/Target/Hexagon/HexagonRegisterInfo.h
+++ b/llvm/lib/Target/Hexagon/HexagonRegisterInfo.h
@@ -55,6 +55,8 @@ public:
     return true;
   }
 
+  bool needsStackRealignment(const MachineFunction &MF) const;
+
   /// Returns true if the frame pointer is valid.
   bool useFPForScavengingIndex(const MachineFunction &MF) const override;