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

[Matt Arsenault via llvm-commits]

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@@ -626,6 +626,89 @@ 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
+static bool isFSqrtDivToFMulLegal(Instruction *X,
+                                  const SmallVectorImpl<Instruction *> &R1,
+                                  const SmallVectorImpl<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 instructions in R1.
+  if (!all_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.
+  if (!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());
+      }))
+    return false;
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arsenm wrote:

any_of negated condition? 

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