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, 0 insertions, 53 deletions

diff --git a/lib/Target/ARM/Thumb2HazardRecognizer.cpp b/lib/Target/ARM/Thumb2HazardRecognizer.cpp
deleted file mode 100644
index 172908da22..0000000000
--- a/lib/Target/ARM/Thumb2HazardRecognizer.cpp
+++ /dev/null
@@ -1,53 +0,0 @@
-//===-- Thumb2HazardRecognizer.cpp - Thumb2 postra hazard recognizer ------===//
-//
-//                     The LLVM Compiler Infrastructure
-//
-// This file is distributed under the University of Illinois Open Source
-// License. See LICENSE.TXT for details.
-//
-//===----------------------------------------------------------------------===//
-
-#include "ARM.h"
-#include "Thumb2HazardRecognizer.h"
-#include "llvm/CodeGen/MachineInstr.h"
-#include "llvm/CodeGen/ScheduleDAG.h"
-using namespace llvm;
-
-ScheduleHazardRecognizer::HazardType
-Thumb2HazardRecognizer::getHazardType(SUnit *SU) {
-  if (ITBlockSize) {
-    MachineInstr *MI = SU->getInstr();
-    if (!MI->isDebugValue() && MI != ITBlockMIs[ITBlockSize-1])
-      return Hazard;
-  }
-
-  return PostRAHazardRecognizer::getHazardType(SU);
-}
-
-void Thumb2HazardRecognizer::Reset() {
-  ITBlockSize = 0;
-  PostRAHazardRecognizer::Reset();
-}
-
-void Thumb2HazardRecognizer::EmitInstruction(SUnit *SU) {
-  MachineInstr *MI = SU->getInstr();
-  unsigned Opcode = MI->getOpcode();
-  if (ITBlockSize) {
-    --ITBlockSize;
-  } else if (Opcode == ARM::t2IT) {
-    unsigned Mask = MI->getOperand(1).getImm();
-    unsigned NumTZ = CountTrailingZeros_32(Mask);
-    assert(NumTZ <= 3 && "Invalid IT mask!");
-    ITBlockSize = 4 - NumTZ;
-    MachineBasicBlock::iterator I = MI;
-    for (unsigned i = 0; i < ITBlockSize; ++i) {
-      // Advance to the next instruction, skipping any dbg_value instructions.
-      do {
-        ++I;
-      } while (I->isDebugValue());
-      ITBlockMIs[ITBlockSize-1-i] = &*I;
-    }
-  }
-
-  PostRAHazardRecognizer::EmitInstruction(SU);
-}