[BACKEND] Explicitly allow specialization of FMA in llvm (#407)

src/codegen/llvm/intrin_rule_llvm.cc

@@ -67,6 +67,9 @@ inline void DispatchLLVMIntrin(const TVMArgs& targs, TVMRetValue* rv) {
 TVM_REGISTER_GLOBAL("tvm.intrin.rule.llvm.exp")
 .set_body(DispatchLLVMPureIntrin<::llvm::Intrinsic::exp>);
 
+TVM_REGISTER_GLOBAL("tvm.intrin.rule.llvm.fma")
+.set_body(DispatchLLVMPureIntrin<::llvm::Intrinsic::fmuladd>);
+
 TVM_REGISTER_GLOBAL("tvm.intrin.rule.llvm.log")
 .set_body(DispatchLLVMPureIntrin<::llvm::Intrinsic::log>);
 

src/pass/lower_intrin.cc

@@ -16,11 +16,12 @@ namespace ir {
 class IntrinInjecter : public IRMutator {
  public:
   explicit IntrinInjecter(std::string target) {
-    patterns_.push_back("tvm.intrin.rule." + target + ".");
-    if (!strncmp(target.c_str(), "llvm", 4) && target != "llvm") {
-      patterns_.push_back("tvm.intrin.rule.llvm.");
-    }
+    std::istringstream is(target);
+    std::string starget;
+    is >> starget;
+    patterns_.push_back("tvm.intrin.rule." + starget + ".");
     patterns_.push_back("tvm.intrin.rule.default.");
+    fma_ = runtime::Registry::Get(patterns_[0] + "fma");
   }
 
   Expr Mutate_(const Call* op, const Expr& e) final {
@@ -32,6 +33,22 @@ class IntrinInjecter : public IRMutator {
     return IRMutator::Mutate_(op, e);
   }
 
+  Expr Mutate_(const Add* op, const Expr& e) final {
+    if (fma_ == nullptr || !op->type.is_float()) {
+      return IRMutator::Mutate_(op, e);
+    }
+    if (const Mul* mb = op->b.as<Mul>()) {
+      Expr r = (*fma_)(Call::make(
+          op->type, "fma", {mb->a, mb->b, op->a}, Call::PureIntrinsic));
+      if (r.defined()) return r;
+    } else if (const Mul* ma = op->a.as<Mul>()) {
+      Expr r = (*fma_)(Call::make(
+          op->type, "fma", {ma->a, ma->b, op->b}, Call::PureIntrinsic));
+      if (r.defined()) return r;
+    }
+    return IRMutator::Mutate_(op, e);
+  }
+
  private:
   Expr ApplyPattern(const std::string& name, const Expr& e) {
     for (size_t i = 0; i < patterns_.size(); ++i) {
@@ -54,6 +71,7 @@ class IntrinInjecter : public IRMutator {
   }
   // patterns
   std::vector<std::string> patterns_;
+  const PackedFunc* fma_{nullptr};
 };
 
 LoweredFunc