[llvm] [InstCombine] Transform high latency, dependent FSQRT/FDIV into FMUL (PR #87474)

[Sushant Gokhale via llvm-commits]

================
@@ -666,6 +666,92 @@ Instruction *InstCombinerImpl::foldPowiReassoc(BinaryOperator &I) {
   return nullptr;
 }
 
+// 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, ArrayRef<Instruction *> R1,
+                                  ArrayRef<Instruction *> R2) {
+  BasicBlock *BBx = X->getParent();
+  BasicBlock *BBr1 = R1[0]->getParent();
+  BasicBlock *BBr2 = R2[0]->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 !any_of(R2, [BBr2](Instruction *I) {
----------------
sushgokh wrote:

After simplifying, this can be changed to all_of (as in current revision)

https://github.com/llvm/llvm-project/pull/87474