[Triton-MLIR] Enable libdevice for ptx backend when has external functions. (#848)

At the phase from ptx to cubin, check whether llvm::Module has external
functions. if has, link with libdevice at:
https://github.com/openai/triton/blob/triton-mlir/python/triton/language/libdevice.10.bc

lib/Target/LLVMIR/LLVMIRTranslation.cpp

@@ -151,7 +151,7 @@ translateTritonGPUToLLVMIR(llvm::LLVMContext *llvmContext,
     return nullptr;
   }
 
-  std::map<std::string, std::string> extern_libs;
+  std::map<std::string, std::string> externLibs;
   SmallVector<LLVM::LLVMFuncOp> funcs;
   module.walk([&](LLVM::LLVMFuncOp func) {
     if (func.isExternal())
@@ -166,7 +166,7 @@ translateTritonGPUToLLVMIR(llvm::LLVMContext *llvmContext,
           func.getOperation()->getAttr("libpath").dyn_cast<StringAttr>();
       if (name) {
         std::string lib_name = name.str();
-        extern_libs[lib_name] = path.str();
+        externLibs[lib_name] = path.str();
       }
     }
   }
@@ -176,7 +176,7 @@ translateTritonGPUToLLVMIR(llvm::LLVMContext *llvmContext,
                     ->getAttr("triton_gpu.externs")
                     .dyn_cast<DictionaryAttr>();
     for (auto &attr : dict) {
-      extern_libs[attr.getName().strref().trim().str()] =
+      externLibs[attr.getName().strref().trim().str()] =
           attr.getValue().dyn_cast<StringAttr>().strref().trim().str();
     }
   }
@@ -188,20 +188,9 @@ translateTritonGPUToLLVMIR(llvm::LLVMContext *llvmContext,
   }
 
   llvm::SMDiagnostic err;
-  for (auto &lib : extern_libs) {
-    auto ext_mod = llvm::parseIRFile(lib.second, err, *llvmContext);
-    if (!ext_mod) {
-      llvm::errs() << "Failed to load extern lib " << lib.first;
+  for (auto &lib : externLibs) {
+    if (linkExternLib(*llvmir, lib.second))
       return nullptr;
-    }
-    ext_mod->setTargetTriple(llvmir->getTargetTriple());
-    ext_mod->setDataLayout(llvmir->getDataLayout());
-
-    if (llvm::Linker::linkModules(*llvmir, std::move(ext_mod),
-                                  llvm::Linker::Flags::LinkOnlyNeeded)) {
-      llvm::errs() << "Failed to link extern lib " << lib.first;
-      return nullptr;
-    }
   }
 
   return llvmir;
@@ -227,5 +216,27 @@ void addExternalLibs(mlir::ModuleOp &module,
   return;
 }
 
+bool linkExternLib(llvm::Module &module, llvm::StringRef path) {
+  llvm::SMDiagnostic err;
+  auto &ctx = module.getContext();
+
+  auto extMod = llvm::parseIRFile(path, err, ctx);
+  if (!extMod) {
+    llvm::errs() << "Failed to load " << path;
+    return true;
+  }
+
+  extMod->setTargetTriple(module.getTargetTriple());
+  extMod->setDataLayout(module.getDataLayout());
+
+  if (llvm::Linker::linkModules(module, std::move(extMod),
+                                llvm::Linker::Flags::LinkOnlyNeeded)) {
+    llvm::errs() << "Failed to link " << path;
+    return true;
+  }
+
+  return false;
+}
+
 } // namespace triton
 } // namespace mlir

lib/Target/PTX/PTXTranslation.cpp

@@ -29,6 +29,7 @@
 #include "llvm/Target/TargetOptions.h"
 #include "llvm/Transforms/Scalar.h"
 #include "llvm/Transforms/Utils/Cloning.h"
+#include <filesystem>
 #include <regex>
 
 namespace triton {
@@ -61,6 +62,43 @@ static bool find_and_replace(std::string &str, const std::string &begin,
 }
 
 static std::string llir_to_ptx(llvm::Module *module, int capability, int ptx) {
+  bool hasExternal = false;
+  for (auto &func : *module) {
+    if (func.hasExternalLinkage()) {
+      hasExternal = true;
+      break;
+    }
+  }
+
+  if (hasExternal) {
+    namespace fs = std::filesystem;
+    // [triton root dir]/python/triton/language/libdevice.10.bc
+    static const fs::path libdevice = fs::path(__FILE__)
+                                          .parent_path()
+                                          .parent_path()
+                                          .parent_path()
+                                          .parent_path() /
+                                      "python" / "triton" / "language" /
+                                      "libdevice.10.bc";
+    if (mlir::triton::linkExternLib(*module, libdevice.string()))
+      llvm::errs() << "link failed for: " << libdevice.string();
+  }
+
+  // please check https://llvm.org/docs/NVPTXUsage.html#reflection-parameters
+  // this will enable fast math path in libdevice
+  // for example, when enable nvvm-reflect-ftz, sqrt.approx.f32 will change to
+  // sqrt.approx.ftz.f32
+  {
+    auto &ctx = module->getContext();
+    llvm::Type *I32 = llvm::Type::getInt32Ty(ctx);
+    llvm::Metadata *mdFour =
+        llvm::ConstantAsMetadata::get(llvm::ConstantInt::getSigned(I32, 4));
+    llvm::Metadata *mdName = llvm::MDString::get(ctx, "nvvm-reflect-ftz");
+    llvm::Metadata *mdOne =
+        llvm::ConstantAsMetadata::get(llvm::ConstantInt::getSigned(I32, 1));
+    llvm::MDNode *reflect = llvm::MDNode::get(ctx, {mdFour, mdName, mdOne});
+    module->addModuleFlag(reflect);
+  }
   // LLVM version in use may not officially support target hardware
   int max_nvvm_cc = 75;
   // int max_nvvm_ptx = 74;