AArch64: Fix emergency spillslot being out of reach for large callframes

Large callframes (calls with several hundreds or thousands or
parameters) could lead to situations in which the emergency spillslot is
out of range to be addressed relative to the stack pointer.
This commit forces the use of a frame pointer in the presence of large
callframes.

This commit does several things:
- Compute max callframe size at the end of instruction selection.
- Add mirFileLoaded target callback. Use it to compute the max callframe size
  after loading a .mir file when the size wasn't specified in the file.
- Let TargetFrameLowering::hasFP() return true if there exists a
  callframe > 255 bytes.
- Always place the emergency spillslot close to FP if we have a frame
  pointer.
- Note that `useFPForScavengingIndex()` would previously return false
  when a base pointer was available leading to the emergency spillslot
  getting allocated late (that's the whole effect of this callback).
  Which made no sense to me so I took this case out: Even though the
  emergency spillslot is technically not referenced by FP in this case
  we still want it allocated early.

Differential Revision: https://reviews.llvm.org/D40876

llvm-svn: 322200

1 files changed, 3 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/TargetSubtargetInfo.cpp b/llvm/lib/CodeGen/TargetSubtargetInfo.cpp
index 1a317cd865f..7aab869f025 100644
--- a/llvm/lib/CodeGen/TargetSubtargetInfo.cpp
+++ b/llvm/lib/CodeGen/TargetSubtargetInfo.cpp
@@ -111,3 +111,6 @@ std::string TargetSubtargetInfo::getSchedInfoStr(MCInst const &MCI) const {
       TSchedModel.computeInstrRThroughput(MCI.getOpcode());
   return createSchedInfoStr(Latency, RThroughput);
 }
+
+void TargetSubtargetInfo::mirFileLoaded(MachineFunction &MF) const {
+}