[Hexagon] Handle decreasing of stack alignment in frame lowering

llvm-svn: 306124

1 files changed, 30 insertions, 0 deletions

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;