[InstCombine] Transform high latency, dependent FSQRT/FDIV into FMUL

The proposed patch, in general, tries to transform the below code sequence:
x = 1.0 / sqrt (a);
r1 = x * x;  // same as 1.0 / a
r2 = a / sqrt(a); // same as sqrt (a)

TO

(If x, r1 and r2 are all used further in the code)
tmp1 = 1.0 / a
tmp2 = sqrt (a)
tmp3 = tmp1 * tmp2
x = tmp3
r1 = tmp1
r2 = tmp2

The transform tries to make high latency sqrt and div operations independent and also saves on one multiplication.

The patch was tested with SPEC17 suite with cpu=neoverse-v2.
The performance uplift achieved was:
544.nab_r   ~4%

No other regressions were observed. Also, no compile time differences were observed with the patch.

Closes #54652

Author: sushgokh

llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp

@@ -13,6 +13,7 @@
 
 #include "InstCombineInternal.h"
 #include "llvm/ADT/APInt.h"
+#include "llvm/ADT/SmallPtrSet.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/Analysis/InstructionSimplify.h"
 #include "llvm/Analysis/ValueTracking.h"
@@ -666,6 +667,94 @@ Instruction *InstCombinerImpl::foldPowiReassoc(BinaryOperator &I) {
   return nullptr;
 }
 
+// If we have the following pattern,
+// X = 1.0/sqrt(a)
+// R1 = X * X
+// R2 = a/sqrt(a)
+// then this method collects all the instructions that match R1 and R2.
+static bool getFSqrtDivOptPattern(Instruction *Div,
+                                  SmallPtrSetImpl<Instruction *> &R1,
+                                  SmallPtrSetImpl<Instruction *> &R2) {
+  Value *A;
+  if (match(Div, m_FDiv(m_FPOne(), m_Sqrt(m_Value(A)))) ||
+      match(Div, m_FDiv(m_SpecificFP(-1.0), m_Sqrt(m_Value(A))))) {
+    for (User *U : Div->users()) {
+      Instruction *I = cast<Instruction>(U);
+      if (match(I, m_FMul(m_Specific(Div), m_Specific(Div))))
+        R1.insert(I);
+    }
+
+    CallInst *CI = cast<CallInst>(Div->getOperand(1));
+    for (User *U : CI->users()) {
+      Instruction *I = cast<Instruction>(U);
+      if (match(I, m_FDiv(m_Specific(A), m_Sqrt(m_Specific(A)))))
+        R2.insert(I);
+    }
+  }
+  return !R1.empty() && !R2.empty();
+}
+
+// Check legality for transforming
+// x = 1.0/sqrt(a)
+// r1 = x * x;
+// r2 = a/sqrt(a);
+//
+// TO
+//
+// r1 = 1/a
+// r2 = sqrt(a)
+// x = r1 * r2
+// This transform works only when 'a' is known positive.
+static bool isFSqrtDivToFMulLegal(Instruction *X,
+                                  SmallPtrSetImpl<Instruction *> &R1,
+                                  SmallPtrSetImpl<Instruction *> &R2) {
+  // Check if the required pattern for the transformation exists.
+  if (!getFSqrtDivOptPattern(X, R1, R2))
+    return false;
+
+  BasicBlock *BBx = X->getParent();
+  BasicBlock *BBr1 = (*R1.begin())->getParent();
+  BasicBlock *BBr2 = (*R2.begin())->getParent();
+
+  CallInst *FSqrt = cast<CallInst>(X->getOperand(1));
+  if (!FSqrt->hasAllowReassoc() || !FSqrt->hasNoNaNs() ||
+      !FSqrt->hasNoSignedZeros() || !FSqrt->hasNoInfs())
+    return false;
+
+  // We change x = 1/sqrt(a) to x = sqrt(a) * 1/a . This change isn't allowed
+  // by recip fp as it is strictly meant to transform ops of type a/b to
+  // a * 1/b. So, this can be considered as algebraic rewrite and reassoc flag
+  // has been used(rather abused)in the past for algebraic rewrites.
+  if (!X->hasAllowReassoc() || !X->hasAllowReciprocal() || !X->hasNoInfs())
+    return false;
+
+  // Check the constraints on X, R1 and R2 combined.
+  // fdiv instruction and one of the multiplications must reside in the same
+  // block. If not, the optimized code may execute more ops than before and
+  // this may hamper the performance.
+  if (BBx != BBr1 && BBx != BBr2)
+    return false;
+
+  // Check the constraints on instructions in R1.
+  if (any_of(R1, [BBr1](Instruction *I) {
+        // When you have multiple instructions residing in R1 and R2
+        // respectively, it's difficult to generate combinations of (R1,R2) and
+        // then check if we have the required pattern. So, for now, just be
+        // conservative.
+        return (I->getParent() != BBr1 || !I->hasAllowReassoc());
+      }))
+    return false;
+
+  // Check the constraints on instructions in R2.
+  return all_of(R2, [BBr2](Instruction *I) {
+    // When you have multiple instructions residing in R1 and R2
+    // respectively, it's difficult to generate combination of (R1,R2) and
+    // then check if we have the required pattern. So, for now, just be
+    // conservative.
+    return (I->getParent() == BBr2 && I->hasAllowReassoc());
+  });
+}
+
 Instruction *InstCombinerImpl::foldFMulReassoc(BinaryOperator &I) {
   Value *Op0 = I.getOperand(0);
   Value *Op1 = I.getOperand(1);
@@ -1917,6 +2006,75 @@ static Instruction *foldFDivSqrtDivisor(BinaryOperator &I,
   return BinaryOperator::CreateFMulFMF(Op0, NewSqrt, &I);
 }
 
+// Change
+// X = 1/sqrt(a)
+// R1 = X * X
+// R2 = a * X
+//
+// TO
+//
+// FDiv = 1/a
+// FSqrt = sqrt(a)
+// FMul = FDiv * FSqrt
+// Replace Uses Of R1 With FDiv
+// Replace Uses Of R2 With FSqrt
+// Replace Uses Of X With FMul
+static Instruction *
+convertFSqrtDivIntoFMul(CallInst *CI, Instruction *X,
+                        const SmallPtrSetImpl<Instruction *> &R1,
+                        const SmallPtrSetImpl<Instruction *> &R2,
+                        InstCombiner::BuilderTy &B, InstCombinerImpl *IC) {
+
+  B.SetInsertPoint(X);
+
+  // Have an instruction that is representative of all of instructions in R1 and
+  // get the most common fpmath metadata and fast-math flags on it.
+  Value *SqrtOp = CI->getArgOperand(0);
+  auto *FDiv = cast<Instruction>(
+      B.CreateFDiv(ConstantFP::get(X->getType(), 1.0), SqrtOp));
+  auto *R1FPMathMDNode = (*R1.begin())->getMetadata(LLVMContext::MD_fpmath);
+  FastMathFlags R1FMF = (*R1.begin())->getFastMathFlags(); // Common FMF
+  for (Instruction *I : R1) {
+    R1FPMathMDNode = MDNode::getMostGenericFPMath(
+        R1FPMathMDNode, I->getMetadata(LLVMContext::MD_fpmath));
+    R1FMF &= I->getFastMathFlags();
+    IC->replaceInstUsesWith(*I, FDiv);
+    IC->eraseInstFromFunction(*I);
+  }
+  FDiv->setMetadata(LLVMContext::MD_fpmath, R1FPMathMDNode);
+  FDiv->copyFastMathFlags(R1FMF);
+
+  // Have a single sqrt call instruction that is representative of all of
+  // instructions in R2 and get the most common fpmath metadata and fast-math
+  // flags on it.
+  auto *FSqrt = cast<CallInst>(CI->clone());
+  FSqrt->insertBefore(CI);
+  auto *R2FPMathMDNode = (*R2.begin())->getMetadata(LLVMContext::MD_fpmath);
+  FastMathFlags R2FMF = (*R2.begin())->getFastMathFlags(); // Common FMF
+  for (Instruction *I : R2) {
+    R2FPMathMDNode = MDNode::getMostGenericFPMath(
+        R2FPMathMDNode, I->getMetadata(LLVMContext::MD_fpmath));
+    R2FMF &= I->getFastMathFlags();
+    IC->replaceInstUsesWith(*I, FSqrt);
+    IC->eraseInstFromFunction(*I);
+  }
+  FSqrt->setMetadata(LLVMContext::MD_fpmath, R2FPMathMDNode);
+  FSqrt->copyFastMathFlags(R2FMF);
+
+  Instruction *FMul;
+  // If X = -1/sqrt(a) initially,then FMul = -(FDiv * FSqrt)
+  if (match(X, m_FDiv(m_SpecificFP(-1.0), m_Specific(CI)))) {
+    Value *Mul = B.CreateFMul(FDiv, FSqrt);
+    FMul = cast<Instruction>(B.CreateFNeg(Mul));
+  } else
+    FMul = cast<Instruction>(B.CreateFMul(FDiv, FSqrt));
+  FMul->copyMetadata(*X);
+  FMul->copyFastMathFlags(FastMathFlags::intersectRewrite(R1FMF, R2FMF) |
+                          FastMathFlags::unionValue(R1FMF, R2FMF));
+  IC->replaceInstUsesWith(*X, FMul);
+  return IC->eraseInstFromFunction(*X);
+}
+
 Instruction *InstCombinerImpl::visitFDiv(BinaryOperator &I) {
   Module *M = I.getModule();
 
@@ -1941,6 +2099,24 @@ Instruction *InstCombinerImpl::visitFDiv(BinaryOperator &I) {
     return R;
 
   Value *Op0 = I.getOperand(0), *Op1 = I.getOperand(1);
+
+  // Convert
+  // x = 1.0/sqrt(a)
+  // r1 = x * x;
+  // r2 = a/sqrt(a);
+  //
+  // TO
+  //
+  // r1 = 1/a
+  // r2 = sqrt(a)
+  // x = r1 * r2
+  SmallPtrSet<Instruction *, 2> R1, R2;
+  if (isFSqrtDivToFMulLegal(&I, R1, R2)) {
+    CallInst *CI = cast<CallInst>(I.getOperand(1));
+    if (Instruction *D = convertFSqrtDivIntoFMul(CI, &I, R1, R2, Builder, this))
+      return D;
+  }
+
   if (isa<Constant>(Op0))
     if (SelectInst *SI = dyn_cast<SelectInst>(Op1))
       if (Instruction *R = FoldOpIntoSelect(I, SI))