Making use of VFP / NEON floating point multiply-accumulate / subtraction is

difficult on current ARM implementations for a few reasons.
1. Even though a single vmla has latency that is one cycle shorter than a pair
   of vmul + vadd, a RAW hazard during the first (4? on Cortex-a8) can cause
   additional pipeline stall. So it's frequently better to single codegen
   vmul + vadd.
2. A vmla folowed by a vmul, vmadd, or vsub causes the second fp instruction to
   stall for 4 cycles. We need to schedule them apart.
3. A vmla followed vmla is a special case. Obvious issuing back to back RAW
   vmla + vmla is very bad. But this isn't ideal either:
     vmul
     vadd
     vmla
   Instead, we want to expand the second vmla:
     vmla
     vmul
     vadd
   Even with the 4 cycle vmul stall, the second sequence is still 2 cycles
   faster.

Up to now, isel simply avoid codegen'ing fp vmla / vmls. This works well enough
but it isn't the optimial solution. This patch attempts to make it possible to
use vmla / vmls in cases where it is profitable.

A. Add missing isel predicates which cause vmla to be codegen'ed.
B. Make sure the fmul in (fadd (fmul)) has a single use. We don't want to
   compute a fmul and a fmla.
C. Add additional isel checks for vmla, avoid cases where vmla is feeding into
   fp instructions (except for the #3 exceptional case).
D. Add ARM hazard recognizer to model the vmla / vmls hazards.
E. Add a special pre-regalloc case to expand vmla / vmls when it's likely the
   vmla / vmls will trigger one of the special hazards.

Work in progress, only A+B are enabled.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@120960 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 53 insertions, 0 deletions

diff --git a/lib/Target/ARM/ARMHazardRecognizer.h b/lib/Target/ARM/ARMHazardRecognizer.h
new file mode 100644
index 0000000000..d1919d8cf4
--- /dev/null
+++ b/lib/Target/ARM/ARMHazardRecognizer.h
@@ -0,0 +1,53 @@
+//===-- ARMHazardRecognizer.h - ARM Hazard Recognizers ----------*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file defines hazard recognizers for scheduling ARM functions.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef ARMHAZARDRECOGNIZER_H
+#define ARMHAZARDRECOGNIZER_H
+
+#include "llvm/CodeGen/PostRAHazardRecognizer.h"
+
+namespace llvm {
+
+class ARMBaseInstrInfo;
+class ARMBaseRegisterInfo;
+class ARMSubtarget;
+class MachineInstr;
+
+class ARMHazardRecognizer : public PostRAHazardRecognizer {
+  const ARMBaseInstrInfo &TII;
+  const ARMBaseRegisterInfo &TRI;
+  const ARMSubtarget &STI;
+
+  MachineInstr *LastMI;
+  unsigned Stalls;
+  unsigned ITBlockSize;  // No. of MIs in current IT block yet to be scheduled.
+  MachineInstr *ITBlockMIs[4];
+
+public:
+  ARMHazardRecognizer(const InstrItineraryData *ItinData,
+                      const ARMBaseInstrInfo &tii,
+                      const ARMBaseRegisterInfo &tri,
+                      const ARMSubtarget &sti) :
+    PostRAHazardRecognizer(ItinData), TII(tii), TRI(tri), STI(sti),
+    LastMI(0), ITBlockSize(0) {}
+
+  virtual HazardType getHazardType(SUnit *SU);
+  virtual void Reset();
+  virtual void EmitInstruction(SUnit *SU);
+  virtual void AdvanceCycle();
+};
+
+
+} // end namespace llvm
+
+#endif // ARMHAZARDRECOGNIZER_H