Avoid write-after-write issue hazards for Cortex-A9.

Add a avoidWriteAfterWrite() target hook to identify register classes that
suffer from write-after-write hazards. For those register classes, try to avoid
writing the same register in two consecutive instructions.

This is currently disabled by default.  We should not spill to avoid hazards!
The command line flag -avoid-waw-hazard can be used to enable waw avoidance.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@129772 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Target/TargetRegisterInfo.h

@@ -624,6 +624,14 @@ public:
     return 0;
   }
 
+  /// avoidWriteAfterWrite - Return true if the register allocator should avoid
+  /// writing a register from RC in two consecutive instructions.
+  /// This can avoid pipeline stalls on certain architectures.
+  /// It does cause increased register pressure, though.
+  virtual bool avoidWriteAfterWrite(const TargetRegisterClass *RC) const {
+    return false;
+  }
+
   /// UpdateRegAllocHint - A callback to allow target a chance to update
   /// register allocation hints when a register is "changed" (e.g. coalesced)
   /// to another register. e.g. On ARM, some virtual registers should target

lib/CodeGen/RegAllocLinearScan.cpp

@@ -67,6 +67,11 @@ TrivCoalesceEnds("trivial-coalesce-ends",
                   cl::desc("Attempt trivial coalescing of interval ends"),
                   cl::init(false), cl::Hidden);
 
+static cl::opt<bool>
+AvoidWAWHazard("avoid-waw-hazard",
+               cl::desc("Avoid write-write hazards for some register classes"),
+               cl::init(false), cl::Hidden);
+
 static RegisterRegAlloc
 linearscanRegAlloc("linearscan", "linear scan register allocator",
                    createLinearScanRegisterAllocator);
@@ -110,6 +115,7 @@ namespace {
       if (NumRecentlyUsedRegs > 0)
         RecentRegs.resize(NumRecentlyUsedRegs, 0);
       RecentNext = RecentRegs.begin();
+      avoidWAW_ = 0;
     }
 
     typedef std::pair<LiveInterval*, LiveInterval::iterator> IntervalPtr;
@@ -180,6 +186,9 @@ namespace {
     SmallVector<unsigned, 4> RecentRegs;
     SmallVector<unsigned, 4>::iterator RecentNext;
 
+    // Last write-after-write register written.
+    unsigned avoidWAW_;
+
     // Record that we just picked this register.
     void recordRecentlyUsed(unsigned reg) {
       assert(reg != 0 && "Recently used register is NOREG!");
@@ -227,8 +236,8 @@ namespace {
 
     // Determine if we skip this register due to its being recently used.
     bool isRecentlyUsed(unsigned reg) const {
-      return std::find(RecentRegs.begin(), RecentRegs.end(), reg) !=
-             RecentRegs.end();
+      return reg == avoidWAW_ ||
+       std::find(RecentRegs.begin(), RecentRegs.end(), reg) != RecentRegs.end();
     }
 
   private:
@@ -1116,6 +1125,12 @@ void RALinScan::assignRegOrStackSlotAtInterval(LiveInterval* cur) {
     active_.push_back(std::make_pair(cur, cur->begin()));
     handled_.push_back(cur);
 
+    // Remember physReg for avoiding a write-after-write hazard in the next
+    // instruction.
+    if (AvoidWAWHazard &&
+        tri_->avoidWriteAfterWrite(mri_->getRegClass(cur->reg)))
+      avoidWAW_ = physReg;
+
     // "Upgrade" the physical register since it has been allocated.
     UpgradeRegister(physReg);
     if (LiveInterval *NextReloadLI = hasNextReloadInterval(cur)) {
@@ -1446,7 +1461,7 @@ unsigned RALinScan::getFreePhysReg(LiveInterval* cur,
     if (reservedRegs_.test(Reg))
       continue;
     // Skip recently allocated registers.
-    if (isRegAvail(Reg) && !isRecentlyUsed(Reg)) {
+    if (isRegAvail(Reg) && (!SkipDGRegs || !isRecentlyUsed(Reg))) {
       FreeReg = Reg;
       if (FreeReg < inactiveCounts.size())
         FreeRegInactiveCount = inactiveCounts[FreeReg];
@@ -1477,7 +1492,8 @@ unsigned RALinScan::getFreePhysReg(LiveInterval* cur,
     if (reservedRegs_.test(Reg))
       continue;
     if (isRegAvail(Reg) && Reg < inactiveCounts.size() &&
-        FreeRegInactiveCount < inactiveCounts[Reg] && !isRecentlyUsed(Reg)) {
+        FreeRegInactiveCount < inactiveCounts[Reg] &&
+        (!SkipDGRegs || !isRecentlyUsed(Reg))) {
       FreeReg = Reg;
       FreeRegInactiveCount = inactiveCounts[Reg];
       if (FreeRegInactiveCount == MaxInactiveCount)
@@ -1528,12 +1544,10 @@ unsigned RALinScan::getFreePhysReg(LiveInterval *cur) {
       return Preference;
   }
 
-  if (!DowngradedRegs.empty()) {
-    unsigned FreeReg = getFreePhysReg(cur, RC, MaxInactiveCount, inactiveCounts,
-                                      true);
-    if (FreeReg)
-      return FreeReg;
-  }
+  unsigned FreeReg = getFreePhysReg(cur, RC, MaxInactiveCount, inactiveCounts,
+                                    true);
+  if (FreeReg)
+    return FreeReg;
   return getFreePhysReg(cur, RC, MaxInactiveCount, inactiveCounts, false);
 }
 

lib/Target/ARM/ARMBaseRegisterInfo.cpp

@@ -554,6 +554,29 @@ ARMBaseRegisterInfo::UpdateRegAllocHint(unsigned Reg, unsigned NewReg,
   }
 }
 
+bool
+ARMBaseRegisterInfo::avoidWriteAfterWrite(const TargetRegisterClass *RC) const {
+  // CortexA9 has a Write-after-write hazard for NEON registers.
+  if (!STI.isCortexA9())
+    return false;
+
+  switch (RC->getID()) {
+  case ARM::DPRRegClassID:
+  case ARM::DPR_8RegClassID:
+  case ARM::DPR_VFP2RegClassID:
+  case ARM::QPRRegClassID:
+  case ARM::QPR_8RegClassID:
+  case ARM::QPR_VFP2RegClassID:
+  case ARM::SPRRegClassID:
+  case ARM::SPR_8RegClassID:
+    // Avoid reusing S, D, and Q registers.
+    // Don't increase register pressure for QQ and QQQQ.
+    return true;
+  default:
+    return false;
+  }
+}
+
 bool ARMBaseRegisterInfo::hasBasePointer(const MachineFunction &MF) const {
   const MachineFrameInfo *MFI = MF.getFrameInfo();
   const ARMFunctionInfo *AFI = MF.getInfo<ARMFunctionInfo>();

lib/Target/ARM/ARMBaseRegisterInfo.h

@@ -142,6 +142,8 @@ public:
   void UpdateRegAllocHint(unsigned Reg, unsigned NewReg,
                           MachineFunction &MF) const;
 
+  virtual bool avoidWriteAfterWrite(const TargetRegisterClass *RC) const;
+
   bool hasBasePointer(const MachineFunction &MF) const;
 
   bool canRealignStack(const MachineFunction &MF) const;

test/CodeGen/ARM/fabss.ll

@@ -24,4 +24,4 @@ declare float @fabsf(float)
 ; CORTEXA8: test:
 ; CORTEXA8: 	vabs.f32	d1, d1
 ; CORTEXA9: test:
-; CORTEXA9: 	vabs.f32	s1, s1
+; CORTEXA9: 	vabs.f32	s{{.}}, s{{.}}

test/CodeGen/ARM/fadds.ll

@@ -20,4 +20,4 @@ entry:
 ; CORTEXA8: test:
 ; CORTEXA8: 	vadd.f32	d0, d1, d0
 ; CORTEXA9: test:
-; CORTEXA9: 	vadd.f32	s0, s1, s0
+; CORTEXA9: 	vadd.f32	s{{.}}, s{{.}}, s{{.}}

test/CodeGen/ARM/fdivs.ll

@@ -20,4 +20,4 @@ entry:
 ; CORTEXA8: test:
 ; CORTEXA8: 	vdiv.f32	s0, s1, s0
 ; CORTEXA9: test:
-; CORTEXA9: 	vdiv.f32	s0, s1, s0
+; CORTEXA9: 	vdiv.f32	s{{.}}, s{{.}}, s{{.}}

test/CodeGen/ARM/fmuls.ll

@@ -20,4 +20,4 @@ entry:
 ; CORTEXA8: test:
 ; CORTEXA8: 	vmul.f32	d0, d1, d0
 ; CORTEXA9: test:
-; CORTEXA9: 	vmul.f32	s0, s1, s0
+; CORTEXA9: 	vmul.f32	s{{.}}, s{{.}}, s{{.}}

test/CodeGen/ARM/fp_convert.ll

@@ -5,7 +5,7 @@
 
 define i32 @test1(float %a, float %b) {
 ; VFP2: test1:
-; VFP2: vcvt.s32.f32 s0, s0
+; VFP2: vcvt.s32.f32 s{{.}}, s{{.}}
 ; NEON: test1:
 ; NEON: vcvt.s32.f32 d0, d0
 entry:
@@ -16,7 +16,7 @@ entry:
 
 define i32 @test2(float %a, float %b) {
 ; VFP2: test2:
-; VFP2: vcvt.u32.f32 s0, s0
+; VFP2: vcvt.u32.f32 s{{.}}, s{{.}}
 ; NEON: test2:
 ; NEON: vcvt.u32.f32 d0, d0
 entry:
@@ -27,7 +27,7 @@ entry:
 
 define float @test3(i32 %a, i32 %b) {
 ; VFP2: test3:
-; VFP2: vcvt.f32.u32 s0, s0
+; VFP2: vcvt.f32.u32 s{{.}}, s{{.}}
 ; NEON: test3:
 ; NEON: vcvt.f32.u32 d0, d0
 entry:
@@ -38,7 +38,7 @@ entry:
 
 define float @test4(i32 %a, i32 %b) {
 ; VFP2: test4:
-; VFP2: vcvt.f32.s32 s0, s0
+; VFP2: vcvt.f32.s32 s{{.}}, s{{.}}
 ; NEON: test4:
 ; NEON: vcvt.f32.s32 d0, d0
 entry: