[mips] Add an IR transformation pass that optimizes calls to sqrt.

The pass emits a call to sqrt that has attribute "read-none". This call will be
converted to an ISD::FSQRT node during DAG construction, which will turn into
a mips native sqrt instruction.
 


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@183802 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Target/Mips/MipsOptimizeMathLibCalls.cpp

@@ -0,0 +1,175 @@
+//===---- MipsOptimizeMathLibCalls.cpp - Optimize math lib calls.      ----===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This pass does an IR transformation which enables the backend to emit native
+// math instructions.
+//
+//===----------------------------------------------------------------------===//
+
+#include "MipsTargetMachine.h"
+#include "llvm/IR/IRBuilder.h"
+#include "llvm/IR/Intrinsics.h"
+#include "llvm/Pass.h"
+#include "llvm/Support/CommandLine.h"
+#include "llvm/Target/TargetLibraryInfo.h"
+#include "llvm/Transforms/Utils/BasicBlockUtils.h"
+
+using namespace llvm;
+
+static cl::opt<bool> DisableOpt("disable-mips-math-optimization",
+                                cl::init(false),
+                                cl::desc("MIPS: Disable math lib call "
+                                         "optimization."), cl::Hidden);
+
+namespace {
+  class MipsOptimizeMathLibCalls : public FunctionPass {
+  public:
+    static char ID;
+
+    MipsOptimizeMathLibCalls(MipsTargetMachine &TM_) :
+      FunctionPass(ID), TM(TM_) {}
+
+    virtual const char *getPassName() const {
+      return "MIPS: Optimize calls to math library functions.";
+    }
+
+    virtual void getAnalysisUsage(AnalysisUsage &AU) const;
+
+    virtual bool runOnFunction(Function &F);
+
+  private:
+    /// Optimize calls to sqrt.
+    bool optimizeSQRT(CallInst *Call, Function *CalledFunc,
+                      BasicBlock &CurrBB,
+                      Function::iterator &BB);
+
+    const TargetMachine &TM;
+  };
+
+  char MipsOptimizeMathLibCalls::ID = 0;
+}
+
+FunctionPass *llvm::createMipsOptimizeMathLibCalls(MipsTargetMachine &TM) {
+  return new MipsOptimizeMathLibCalls(TM);
+}
+
+void MipsOptimizeMathLibCalls::getAnalysisUsage(AnalysisUsage &AU) const {
+  AU.addRequired<TargetLibraryInfo>();
+  FunctionPass::getAnalysisUsage(AU);
+}
+
+bool MipsOptimizeMathLibCalls::runOnFunction(Function &F) {
+  if (DisableOpt)
+    return false;
+
+  const MipsSubtarget &Subtarget = TM.getSubtarget<MipsSubtarget>();
+
+  if (Subtarget.inMips16Mode())
+    return false;
+
+  bool Changed = false;
+  Function::iterator CurrBB;
+  const TargetLibraryInfo *LibInfo = &getAnalysis<TargetLibraryInfo>();
+
+  for (Function::iterator BB = F.begin(), BE = F.end(); BB != BE;) {
+    CurrBB = BB++;
+
+    for (BasicBlock::iterator II = CurrBB->begin(), IE = CurrBB->end();
+         II != IE; ++II) {
+      CallInst *Call = dyn_cast<CallInst>(&*II);
+      Function *CalledFunc;
+
+      if (!Call || !(CalledFunc = Call->getCalledFunction()))
+        continue;
+
+      LibFunc::Func LibFunc;
+      Attribute A = CalledFunc->getAttributes()
+        .getAttribute(AttributeSet::FunctionIndex, "use-soft-float");
+
+      // Skip if function has "use-soft-float" attribute.
+      if ((A.isStringAttribute() && (A.getValueAsString() == "true")) ||
+          TM.Options.UseSoftFloat)
+        continue;
+
+      // Skip if function either has local linkage or is not a known library
+      // function.
+      if (CalledFunc->hasLocalLinkage() || !CalledFunc->hasName() ||
+          !LibInfo->getLibFunc(CalledFunc->getName(), LibFunc))
+        continue;
+
+      switch (LibFunc) {
+      case LibFunc::sqrtf:
+      case LibFunc::sqrt:
+        if (optimizeSQRT(Call, CalledFunc, *CurrBB, BB))
+          break;
+        continue;
+      default:
+        continue;
+      }
+
+      Changed = true;
+      break;
+    }
+  }
+
+  return Changed;
+}
+
+bool MipsOptimizeMathLibCalls::optimizeSQRT(CallInst *Call,
+                                            Function *CalledFunc,
+                                            BasicBlock &CurrBB,
+                                            Function::iterator &BB) {
+  // There is no need to change the IR, since backend will emit sqrt
+  // instruction if the call has already been marked read-only.
+  if (Call->onlyReadsMemory())
+    return false;
+
+  // Do the following transformation:
+  //
+  // (before)
+  // dst = sqrt(src)
+  //
+  // (after)
+  // v0 = sqrt_noreadmem(src) # native sqrt instruction.
+  // if (v0 is a NaN)
+  //   v1 = sqrt(src)         # library call.
+  // dst = phi(v0, v1)
+  //
+
+  // Move all instructions following Call to newly created block JoinBB.
+  // Create phi and replace all uses.
+  BasicBlock *JoinBB = llvm::SplitBlock(&CurrBB, Call->getNextNode(), this);
+  IRBuilder<> Builder(JoinBB, JoinBB->begin());
+  PHINode *Phi = Builder.CreatePHI(Call->getType(), 2);
+  Call->replaceAllUsesWith(Phi);
+
+  // Create basic block LibCallBB and insert a call to library function sqrt.
+  BasicBlock *LibCallBB = BasicBlock::Create(CurrBB.getContext(), "call.sqrt",
+                                             CurrBB.getParent(), JoinBB);
+  Builder.SetInsertPoint(LibCallBB);
+  Instruction *LibCall = Call->clone();
+  Builder.Insert(LibCall);
+  Builder.CreateBr(JoinBB);
+
+  // Add attribute "readnone" so that backend can use a native sqrt instruction
+  // for this call. Insert a FP compare instruction and a conditional branch
+  // at the end of CurrBB.
+  Call->addAttribute(AttributeSet::FunctionIndex, Attribute::ReadNone);
+  CurrBB.getTerminator()->eraseFromParent();
+  Builder.SetInsertPoint(&CurrBB);
+  Value *FCmp = Builder.CreateFCmpOEQ(Call, Call);
+  Builder.CreateCondBr(FCmp, JoinBB, LibCallBB);
+
+  // Add phi operands.
+  Phi->addIncoming(Call, &CurrBB);
+  Phi->addIncoming(LibCall, LibCallBB);
+
+  BB = JoinBB;
+  return true;
+}