[GlobalISel] Add a fallback path to SDISel.

When global-isel fails on a MachineFunction MF, MF will be cleaned up
and given to SDISel.
Thanks to this fallback, we can already perform correctness test even if
we support only a small portion of the functions in a test.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@279891 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/CodeGen/Passes.h

@@ -53,6 +53,8 @@ namespace llvm {
   /// using the MIR serialization format.
   MachineFunctionPass *createPrintMIRPass(raw_ostream &OS);
 
+  MachineFunctionPass *createResetMachineFunctionPass();
+
   /// createCodeGenPreparePass - Transform the code to expose more pattern
   /// matching during instruction selection.
   FunctionPass *createCodeGenPreparePass(const TargetMachine *TM = nullptr);

include/llvm/InitializePasses.h

@@ -292,6 +292,7 @@ void initializeRegionPrinterPass(PassRegistry&);
 void initializeRegionViewerPass(PassRegistry&);
 void initializeRegisterCoalescerPass(PassRegistry&);
 void initializeRenameIndependentSubregsPass(PassRegistry&);
+void initializeResetMachineFunctionPass(PassRegistry &);
 void initializeReversePostOrderFunctionAttrsLegacyPassPass(PassRegistry&);
 void initializeRewriteStatepointsForGCPass(PassRegistry&);
 void initializeRewriteSymbolsLegacyPassPass(PassRegistry&);

lib/CodeGen/CMakeLists.txt

@@ -105,6 +105,7 @@ add_llvm_library(LLVMCodeGen
   RegisterUsageInfo.cpp
   RegUsageInfoCollector.cpp
   RegUsageInfoPropagate.cpp
+  ResetMachineFunctionPass.cpp
   SafeStack.cpp
   SafeStackColoring.cpp
   SafeStackLayout.cpp

lib/CodeGen/LLVMTargetMachine.cpp

@@ -168,6 +168,15 @@ addPassesToGenerateCode(LLVMTargetMachine *TM, PassManagerBase &PM,
     if (PassConfig->addGlobalInstructionSelect())
       return nullptr;
 
+    // Pass to reset the MachineFunction if the ISel failed.
+    PM.add(createResetMachineFunctionPass());
+
+    // Provide a fallback path when we do not want to abort on
+    // not-yet-supported input.
+    if (LLVM_UNLIKELY(!PassConfig->isGlobalISelAbortEnabled()) &&
+        PassConfig->addInstSelector())
+      return nullptr;
+
   } else if (PassConfig->addInstSelector())
     return nullptr;
 

lib/CodeGen/ResetMachineFunctionPass.cpp

@@ -0,0 +1,53 @@
+//===-- ResetMachineFunctionPass.cpp - Machine Loop Invariant Code Motion Pass ---------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/CodeGen/Passes.h"
+#include "llvm/CodeGen/MachineFunction.h"
+#include "llvm/CodeGen/MachineFunctionPass.h"
+#include "llvm/Support/Debug.h"
+using namespace llvm;
+
+#define DEBUG_TYPE "reset-machine-function"
+
+namespace {
+  class ResetMachineFunction : public MachineFunctionPass {
+  public:
+    static char ID; // Pass identification, replacement for typeid
+    ResetMachineFunction() :
+      MachineFunctionPass(ID) {
+    }
+
+    const char *getPassName() const override {
+      return "ResetMachineFunction";
+    }
+
+    bool runOnMachineFunction(MachineFunction &MF) override {
+      if (MF.getProperties().hasProperty(
+              MachineFunctionProperties::Property::FailedISel)) {
+        DEBUG(dbgs() << "Reseting: " << MF.getName() << '\n');
+        MF.reset();
+        return true;
+      }
+      return false;
+    }
+
+  };
+} // end anonymous namespace
+
+char ResetMachineFunction::ID = 0;
+INITIALIZE_PASS(ResetMachineFunction, DEBUG_TYPE,
+                "reset machine function if ISel failed", false, false)
+
+MachineFunctionPass *
+llvm::createResetMachineFunctionPass() {
+  return new ResetMachineFunction();
+}

test/CodeGen/AArch64/GlobalISel/arm64-fallback.ll

@@ -0,0 +1,19 @@
+; RUN: not llc -O0 -global-isel -verify-machineinstrs %s -o - 2>&1 | FileCheck %s --check-prefix=ERROR
+; RUN: llc -O0 -global-isel -global-isel-abort=false -verify-machineinstrs %s -o - 2>&1 | FileCheck %s --check-prefix=FALLBACK
+; This file checks that the fallback path to selection dag works.
+; The test is fragile in the sense that it must be updated to expose
+; something that fails with global-isel.
+; When we cannot produce a test case anymore, that means we can remove
+; the fallback path.
+
+target datalayout = "e-m:o-i64:64-i128:128-n32:64-S128"
+target triple = "aarch64-apple-ios"
+
+; ERROR: Unable to lower arguments
+; FALLBACK: ldr q0,
+; FALLBACK-NEXT: bl ___fixunstfti
+define i128 @ABIi128(i128 %arg1) {
+  %farg1 =       bitcast i128 %arg1 to fp128 
+  %res = fptoui fp128 %farg1 to i128
+  ret i128 %res
+}