[llvm] AMDGPU: Libcall expand fast pow/powr/pown/rootn for float case (PR #180553)

[Steffen Larsen via llvm-commits]

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@@ -1170,6 +1198,260 @@ bool AMDGPULibCalls::fold_rootn(FPMathOperator *FPOp, IRBuilder<> &B,
   return false;
 }
 
+// is_integer(y) => trunc(y) == y
+static Value *emitIsInteger(IRBuilder<> &B, Value *Y) {
+  Value *TruncY = B.CreateUnaryIntrinsic(Intrinsic::trunc, Y);
+  return B.CreateFCmpOEQ(TruncY, Y);
+}
+
+static Value *emitIsEvenInteger(IRBuilder<> &B, Value *Y) {
+  // Even integers are still integers after division by 2.
+  auto *HalfY = B.CreateFMul(Y, ConstantFP::get(Y->getType(), 0.5));
+  return emitIsInteger(B, HalfY);
+}
+
+// is_odd_integer(y) => is_integer(y) && !is_even_integer(y)
+static Value *emitIsOddInteger(IRBuilder<> &B, Value *Y) {
+  Value *IsIntY = emitIsInteger(B, Y);
+  Value *IsEvenY = emitIsEvenInteger(B, Y);
+  Value *NotEvenY = B.CreateNot(IsEvenY);
+  return B.CreateAnd(IsIntY, NotEvenY);
+}
----------------
steffenlarsen wrote:

I couldn't let it go, so I came up with something that should work purely using a bitcast, integer arithmetics and logical operations: https://alive2.llvm.org/ce/z/qZ8E8-

At that point, I'm not sure whether there is any gain above an fmul anymore, but it's always fun to speculate!

https://github.com/llvm/llvm-project/pull/180553