Factor out LiveIntervalAnalysis' code to determine whether an instruction

is trivially rematerializable and integrate it into
TargetInstrInfo::isTriviallyReMaterializable. This way, all places that
need to know whether an instruction is rematerializable will get the
same answer.

This enables the useful parts of the aggressive-remat option by
default -- using AliasAnalysis to determine whether a memory location
is invariant, and removes the questionable parts -- rematting operations
with virtual register inputs that may not be live everywhere.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@83687 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/TargetInstrInfoImpl.cpp

@@ -13,11 +13,14 @@
 //===----------------------------------------------------------------------===//
 
 #include "llvm/Target/TargetInstrInfo.h"
+#include "llvm/Target/TargetMachine.h"
+#include "llvm/Target/TargetRegisterInfo.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/CodeGen/MachineFrameInfo.h"
 #include "llvm/CodeGen/MachineInstr.h"
 #include "llvm/CodeGen/MachineInstrBuilder.h"
 #include "llvm/CodeGen/MachineMemOperand.h"
+#include "llvm/CodeGen/MachineRegisterInfo.h"
 #include "llvm/CodeGen/PseudoSourceValue.h"
 #include "llvm/Support/ErrorHandling.h"
 #include "llvm/Support/raw_ostream.h"
@@ -238,3 +241,88 @@ TargetInstrInfo::foldMemoryOperand(MachineFunction &MF,
 
   return NewMI;
 }
+
+bool
+TargetInstrInfo::isReallyTriviallyReMaterializableGeneric(const MachineInstr *
+                                                            MI,
+                                                          AliasAnalysis *
+                                                            AA) const {
+  const MachineFunction &MF = *MI->getParent()->getParent();
+  const MachineRegisterInfo &MRI = MF.getRegInfo();
+  const TargetMachine &TM = MF.getTarget();
+  const TargetInstrInfo &TII = *TM.getInstrInfo();
+  const TargetRegisterInfo &TRI = *TM.getRegisterInfo();
+
+  // A load from a fixed stack slot can be rematerialized. This may be
+  // redundant with subsequent checks, but it's target-independent,
+  // simple, and a common case.
+  int FrameIdx = 0;
+  if (TII.isLoadFromStackSlot(MI, FrameIdx) &&
+      MF.getFrameInfo()->isImmutableObjectIndex(FrameIdx))
+    return true;
+
+  const TargetInstrDesc &TID = MI->getDesc();
+
+  // Avoid instructions obviously unsafe for remat.
+  if (TID.hasUnmodeledSideEffects() || TID.isNotDuplicable() ||
+      TID.mayStore())
+    return false;
+
+  // Avoid instructions which load from potentially varying memory.
+  if (TID.mayLoad() && !MI->isInvariantLoad(AA))
+    return false;
+
+  // If any of the registers accessed are non-constant, conservatively assume
+  // the instruction is not rematerializable.
+  for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
+    const MachineOperand &MO = MI->getOperand(i);
+    if (!MO.isReg()) continue;
+    unsigned Reg = MO.getReg();
+    if (Reg == 0)
+      continue;
+
+    // Check for a well-behaved physical register.
+    if (TargetRegisterInfo::isPhysicalRegister(Reg)) {
+      if (MO.isUse()) {
+        // If the physreg has no defs anywhere, it's just an ambient register
+        // and we can freely move its uses. Alternatively, if it's allocatable,
+        // it could get allocated to something with a def during allocation.
+        if (!MRI.def_empty(Reg))
+          return false;
+        BitVector AllocatableRegs = TRI.getAllocatableSet(MF, 0);
+        if (AllocatableRegs.test(Reg))
+          return false;
+        // Check for a def among the register's aliases too.
+        for (const unsigned *Alias = TRI.getAliasSet(Reg); *Alias; ++Alias) {
+          unsigned AliasReg = *Alias;
+          if (!MRI.def_empty(AliasReg))
+            return false;
+          if (AllocatableRegs.test(AliasReg))
+            return false;
+        }
+      } else {
+        // A physreg def. We can't remat it.
+        return false;
+      }
+      continue;
+    }
+
+    // Only allow one virtual-register def, and that in the first operand.
+    if (MO.isDef() != (i == 0))
+      return false;
+
+    // For the def, it should be the only def of that register.
+    if (MO.isDef() && (next(MRI.def_begin(Reg)) != MRI.def_end() ||
+                       MRI.isLiveIn(Reg)))
+      return false;
+
+    // Don't allow any virtual-register uses. Rematting an instruction with
+    // virtual register uses would length the live ranges of the uses, which
+    // is not necessarily a good idea, certainly not "trivial".
+    if (MO.isUse())
+      return false;
+  }
+
+  // Everything checked out.
+  return true;
+}