[WebAssembly] Add pass to infer prototypes for prototype-less functions

[sbc100]

Summary

Without this change llvm will generate invalid wasm object files.

See https://bugs.llvm.org/show_bug.cgi?id=35385

[sunfish]

My biggest concern here is that this pass shouldn't affect prototype-possessing functions, and that appears to be satisfied :-).

[sbc100]

• fix typo
• cleanup

[dschuff]

Otherwise LGTM

lib/Target/WebAssembly/WebAssemblyAddMissingPrototypes.cpp

75

This code more-or-less functions as a kind of "assert" on clang's convention of using vararg-only functions for prototypeless C functions. If that changes, or if someone accidentally creates nonconforming IR (say, with a different frontend) then they'll end up here, so it's probably worth just putting a big comment here saying that's what we're doing. Fortunately the use of the attribute minimizes risk of accidentally having this IR, so I think the actual code is fine.

test/CodeGen/WebAssembly/add-prototypes.ll

12

can we add a test with a non-call use as well (e.g. storing a pointer to the function)?

[sbc100]

• add to test

[sbc100]

• add to test
• add comment

lib/Target/WebAssembly/CMakeLists.txt

 set(LLVM_TARGET_DEFINITIONS WebAssembly.td)
 
 tablegen(LLVM WebAssemblyGenAsmMatcher.inc -gen-asm-matcher)
 tablegen(LLVM WebAssemblyGenAsmWriter.inc -gen-asm-writer)
 tablegen(LLVM WebAssemblyGenDAGISel.inc -gen-dag-isel)
 tablegen(LLVM WebAssemblyGenDisassemblerTables.inc -gen-disassembler)
 tablegen(LLVM WebAssemblyGenFastISel.inc -gen-fast-isel)
 tablegen(LLVM WebAssemblyGenInstrInfo.inc -gen-instr-info)
 tablegen(LLVM WebAssemblyGenMCCodeEmitter.inc -gen-emitter)
 tablegen(LLVM WebAssemblyGenRegisterInfo.inc -gen-register-info)
 tablegen(LLVM WebAssemblyGenSubtargetInfo.inc -gen-subtarget)
 
 add_public_tablegen_target(WebAssemblyCommonTableGen)
 
 add_llvm_target(WebAssemblyCodeGen
+  WebAssemblyAddMissingPrototypes.cpp
   WebAssemblyArgumentMove.cpp
   WebAssemblyAsmPrinter.cpp
   WebAssemblyCallIndirectFixup.cpp
   WebAssemblyCFGStackify.cpp
   WebAssemblyCFGSort.cpp
   WebAssemblyLateEHPrepare.cpp
   WebAssemblyExceptionInfo.cpp
   WebAssemblyExplicitLocals.cpp

lib/Target/WebAssembly/WebAssembly.h

 
 class WebAssemblyTargetMachine;
 class ModulePass;
 class FunctionPass;
 
 // LLVM IR passes.
 ModulePass *createWebAssemblyLowerEmscriptenEHSjLj(bool DoEH, bool DoSjLj);
 ModulePass *createWebAssemblyLowerGlobalDtors();
+ModulePass *createWebAssemblyAddMissingPrototypes();
 ModulePass *createWebAssemblyFixFunctionBitcasts();
 FunctionPass *createWebAssemblyOptimizeReturned();
 
 // ISel and immediate followup passes.
 FunctionPass *createWebAssemblyISelDag(WebAssemblyTargetMachine &TM,
                                        CodeGenOpt::Level OptLevel);
 FunctionPass *createWebAssemblyArgumentMove();
 FunctionPass *createWebAssemblySetP2AlignOperands();

 FunctionPass *createWebAssemblyCFGSort();
 FunctionPass *createWebAssemblyCFGStackify();
 FunctionPass *createWebAssemblyLowerBrUnless();
 FunctionPass *createWebAssemblyRegNumbering();
 FunctionPass *createWebAssemblyPeephole();
 FunctionPass *createWebAssemblyCallIndirectFixup();
 
 // PassRegistry initialization declarations.
+void initializeWebAssemblyAddMissingPrototypesPass(PassRegistry &);
 void initializeWebAssemblyLowerEmscriptenEHSjLjPass(PassRegistry &);
 void initializeLowerGlobalDtorsPass(PassRegistry &);
 void initializeFixFunctionBitcastsPass(PassRegistry &);
 void initializeOptimizeReturnedPass(PassRegistry &);
 void initializeWebAssemblyArgumentMovePass(PassRegistry &);
 void initializeWebAssemblySetP2AlignOperandsPass(PassRegistry &);
 void initializeWebAssemblyReplacePhysRegsPass(PassRegistry &);
 void initializeWebAssemblyPrepareForLiveIntervalsPass(PassRegistry &);

lib/Target/WebAssembly/WebAssemblyAddMissingPrototypes.cpp

+//===-- WebAssemblyAddMissingPrototypes.cpp - Fix prototypeless functions -===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+///
+/// \file
+/// Add prototypes to prototypes-less functions.
+///
+/// WebAssembly has strict function prototype checking so we need functions
+/// declarations to match the call sites.  Clang treats prototype-less functions
+/// as varargs (foo(...)) which happens to work on existing platforms but
+/// doesn't under WebAssembly.  This pass will find all the call sites of each
+/// prototype-less function, ensure they agree, and then set the signature
+/// on the function declaration accordingly.
+///
+//===----------------------------------------------------------------------===//
+
+#include "WebAssembly.h"
+#include "llvm/IR/Constants.h"
+#include "llvm/IR/IRBuilder.h"
+#include "llvm/IR/Module.h"
+#include "llvm/IR/Operator.h"
+#include "llvm/Transforms/Utils/ModuleUtils.h"
+#include "llvm/Transforms/Utils/Local.h"
+#include "llvm/Pass.h"
+#include "llvm/Support/Debug.h"
+using namespace llvm;
+
+#define DEBUG_TYPE "wasm-add-missing-prototypes"
+
+namespace {
+class WebAssemblyAddMissingPrototypes final : public ModulePass {
+  StringRef getPassName() const override {
+    return "Add prototypes to prototypes-less functions";
+  }
+
+  void getAnalysisUsage(AnalysisUsage &AU) const override {
+    AU.setPreservesCFG();
+    ModulePass::getAnalysisUsage(AU);
+  }
+
+  bool runOnModule(Module &M) override;
+
+public:
+  static char ID;
+  WebAssemblyAddMissingPrototypes() : ModulePass(ID) {}
+};
+} // End anonymous namespace
+
+char WebAssemblyAddMissingPrototypes::ID = 0;
+INITIALIZE_PASS(WebAssemblyAddMissingPrototypes, DEBUG_TYPE,
+                "Add prototypes to prototypes-less functions", false, false)
+
+ModulePass *llvm::createWebAssemblyAddMissingPrototypes() {
+  return new WebAssemblyAddMissingPrototypes();
+}
+
+bool WebAssemblyAddMissingPrototypes::runOnModule(Module &M) {
+  LLVM_DEBUG(dbgs() << "runnning AddMissingPrototypes\n");
+
+  std::vector<std::pair<Function*, Function*>> Replacements;
+
+  // Find all the prototype-less function declarations
+  for (Function &F : M) {
+    if (!F.isDeclaration() || !F.hasFnAttribute("no-prototype"))
+      continue;
+
+    LLVM_DEBUG(dbgs() << "Found no-prototype function: " << F.getName() << "\n");
+
+    if (!F.isVarArg())
+      report_fatal_error(
+          "Functions with 'no-prototype' attribute must take varargs: " +
+          F.getName());
+    if (F.getFunctionType()->getNumParams() != 0)
+      report_fatal_error(
+          "Functions with 'no-prototype' attribute should not have params: " +
+          F.getName());
+
+
+    // Create a function prototype based on the first call site (first bitcast)
+    // that we find.
+    FunctionType *NewType = nullptr;
+    Function* NewF = nullptr;
+    for (Use &U : F.uses()) {
+      LLVM_DEBUG(dbgs() << "prototype-less use: " << F.getName() << "\n");
+      if (BitCastOperator *BC = dyn_cast<BitCastOperator>(U.getUser())) {
+        FunctionType *DestType =
+            cast<FunctionType>(BC->getDestTy()->getPointerElementType());
+
+        // Create a new function with the correct type
+        NewType = DestType;
+        NewF = Function::Create(NewType, F.getLinkage(), F.getName());
+        NewF->setAttributes(F.getAttributes());
+        NewF->removeFnAttr("no-prototype");
+        break;
+      }
+    }
+
+    if (!NewType) {
+      LLVM_DEBUG(
+          dbgs() << "could not derive a function prototype from usage: " +
+                        F.getName() + "\n");
+      continue;
+    }
+
+    for (Use &U : F.uses()) {
+      if (BitCastOperator *BC = dyn_cast<BitCastOperator>(U.getUser())) {
+        FunctionType *DestType =
+            cast<FunctionType>(BC->getDestTy()->getPointerElementType());
+        if (NewType != DestType) {
+          report_fatal_error(
+              "Prototypeless function used with conflicting signatures: " +
+              F.getName());
+        }
+        BC->replaceAllUsesWith(NewF);
+        Replacements.emplace_back(&F, NewF);
+      } else if (isa<StoreInst>(U.getUser())) {
+        LLVM_DEBUG(dbgs() << "Use in instruction:\n"; U.getUser()->dump());
+      } else if (isa<SelectInst>(U.getUser())) {
+        LLVM_DEBUG(dbgs() << "Use in instruction:\n"; U.getUser()->dump());
+      } else {
+        dbgs() << *U.getUser()->getType() << "\n";
+        U.getUser()->dump();
+        report_fatal_error(
+            "unhandled use of prototypeless function: " + F.getName() + "\n");
+      }
+    }
+  }
+
+  // Finally replace the old function declarations with the new ones
+  for (auto &Pair : Replacements) {
+    Function* Old = Pair.first;
+    Function* New = Pair.second;
+    Old->eraseFromParent();
+    M.getFunctionList().push_back(New);
+  }
+
+  return !Replacements.empty();
+}

lib/Target/WebAssembly/WebAssemblyTargetMachine.cpp

extern "C" void LLVMInitializeWebAssemblyTarget() {
   // Register the target.
   RegisterTargetMachine<WebAssemblyTargetMachine> X(
       getTheWebAssemblyTarget32());
   RegisterTargetMachine<WebAssemblyTargetMachine> Y(
       getTheWebAssemblyTarget64());
 
   // Register backend passes
   auto &PR = *PassRegistry::getPassRegistry();
+  initializeWebAssemblyAddMissingPrototypesPass(PR);
   initializeWebAssemblyLowerEmscriptenEHSjLjPass(PR);
   initializeLowerGlobalDtorsPass(PR);
   initializeFixFunctionBitcastsPass(PR);
   initializeOptimizeReturnedPass(PR);
   initializeWebAssemblyArgumentMovePass(PR);
   initializeWebAssemblySetP2AlignOperandsPass(PR);
   initializeWebAssemblyReplacePhysRegsPass(PR);
   initializeWebAssemblyPrepareForLiveIntervalsPass(PR);
if (TM->Options.ThreadModel == ThreadModel::Single) {
     addPass(createLowerAtomicPass());
     addPass(new StripThreadLocal());
   } else {
     // Expand some atomic operations. WebAssemblyTargetLowering has hooks which
     // control specifically what gets lowered.
     addPass(createAtomicExpandPass());
   }
 
+  // Add signatures to prototype-less function declarations
+  addPass(createWebAssemblyAddMissingPrototypes());
+
   // Lower .llvm.global_dtors into .llvm_global_ctors with __cxa_atexit calls.
   addPass(createWebAssemblyLowerGlobalDtors());
 
   // Fix function bitcasts, as WebAssembly requires caller and callee signatures
   // to match.
   addPass(createWebAssemblyFixFunctionBitcasts());
 
   // Optimize "returned" function attributes.

test/CodeGen/WebAssembly/add-prototypes.ll

+; RUN: opt -S -wasm-add-missing-prototypes %s | FileCheck %s
+
+target datalayout = "e-m:e-p:32:32-i64:64-n32:64-S128"
+target triple = "wasm32-unknown-unknown"
+
+@foo_addr = global i64 (i32)* bitcast (i64 (...)* @foo to i64 (i32)*), align 8
+
+define void @bar(i32 %a) {
+entry:
+  %call = call i64 bitcast (i64 (...)* @foo to i64 (i32)*)(i32 42)
+  ret void
+}
+
+declare i64 @foo(...) #1
+
+attributes #1 = { "no-prototype" }
+
+; CHECK: %call = call i64 @foo(i32 42)
+; CHECK: declare i64 @foo(i32)
+; CHECK-NOT: attributes {{.*}} = { {{.*}}"no-prototype"{{.*}} }