[WebAssembly] call_indirect causes indirect function table import

For wasm-ld table linking work to proceed, object files should indicate
if they use an indirect function table.  In the future this will be done
by the usual symbols and relocations mechanism, but until that support
lands in the linker, the presence of an `__indirect_function_table` in
the object file's import section shows that the object file needs an
indirect function table.

Prior to https://reviews.llvm.org/D91637, this condition was met by all
object files residualizing an `__indirect_function_table` import.

Since https://reviews.llvm.org/D91637, the intention has been that only
those object files needing an indirect function table would have the
`__indirect_function_table` import.  However, we missed the case of
object files which use the table via `call_indirect` but which
themselves do not declare any indirect functions.

This changeset makes it so that when we lower a call to `call_indirect`,
that we ensure that a `__indirect_function_table` symbol is present and
that it will be propagated to the linker.

A followup patch will revise this mechanism to make an explicit link
between `call_indirect` and its associated indirect function table; see
https://reviews.llvm.org/D90948.

Differential Revision: https://reviews.llvm.org/D92840

llvm/lib/MC/WasmObjectWriter.cpp

@@ -405,6 +405,13 @@ void WasmObjectWriter::writeHeader(const MCAssembler &Asm) {
 
 void WasmObjectWriter::executePostLayoutBinding(MCAssembler &Asm,
                                                 const MCAsmLayout &Layout) {
+  // As a stopgap measure until call_indirect instructions start explicitly
+  // referencing the indirect function table via TABLE_NUMBER relocs, ensure
+  // that the indirect function table import makes it to the output if anything
+  // in the compilation unit has caused it to be present.
+  if (auto *Sym = Asm.getContext().lookupSymbol("__indirect_function_table"))
+    Asm.registerSymbol(*Sym);
+
   // Build a map of sections to the function that defines them, for use
   // in recordRelocation.
   for (const MCSymbol &S : Asm.symbols()) {

llvm/lib/Target/WebAssembly/AsmParser/WebAssemblyAsmParser.cpp

@@ -160,6 +160,24 @@ struct WebAssemblyOperand : public MCParsedAsmOperand {
   }
 };
 
+static MCSymbolWasm *GetOrCreateFunctionTableSymbol(MCContext &Ctx,
+                                                    const StringRef &Name) {
+  // FIXME: Duplicates functionality from
+  // MC/WasmObjectWriter::recordRelocation, as well as WebAssemblyCodegen's
+  // WebAssembly:getOrCreateFunctionTableSymbol.
+  MCSymbolWasm *Sym = cast_or_null<MCSymbolWasm>(Ctx.lookupSymbol(Name));
+  if (Sym) {
+    if (!Sym->isFunctionTable())
+      Ctx.reportError(SMLoc(), "symbol is not a wasm funcref table");
+  } else {
+    Sym = cast<MCSymbolWasm>(Ctx.getOrCreateSymbol(Name));
+    Sym->setFunctionTable();
+    // The default function table is synthesized by the linker.
+    Sym->setUndefined();
+  }
+  return Sym;
+}
+
 class WebAssemblyAsmParser final : public MCTargetAsmParser {
   MCAsmParser &Parser;
   MCAsmLexer &Lexer;
@@ -531,6 +549,15 @@ class WebAssemblyAsmParser final : public MCTargetAsmParser {
         return true;
     } else if (Name == "call_indirect" || Name == "return_call_indirect") {
       ExpectFuncType = true;
+      // Ensure that the object file has a __indirect_function_table import, as
+      // we call_indirect against it.
+      auto &Ctx = getStreamer().getContext();
+      MCSymbolWasm *Sym =
+          GetOrCreateFunctionTableSymbol(Ctx, "__indirect_function_table");
+      // Until call_indirect emits TABLE_NUMBER relocs against this symbol, mark
+      // it as NO_STRIP so as to ensure that the indirect function table makes
+      // it to linked output.
+      Sym->setNoStrip();
     } else if (Name == "ref.null") {
       ExpectHeapType = true;
     }

llvm/lib/Target/WebAssembly/WebAssemblyFastISel.cpp

@@ -20,12 +20,14 @@
 #include "WebAssemblyMachineFunctionInfo.h"
 #include "WebAssemblySubtarget.h"
 #include "WebAssemblyTargetMachine.h"
+#include "WebAssemblyUtilities.h"
 #include "llvm/Analysis/BranchProbabilityInfo.h"
 #include "llvm/CodeGen/FastISel.h"
 #include "llvm/CodeGen/FunctionLoweringInfo.h"
 #include "llvm/CodeGen/MachineConstantPool.h"
 #include "llvm/CodeGen/MachineFrameInfo.h"
 #include "llvm/CodeGen/MachineInstrBuilder.h"
+#include "llvm/CodeGen/MachineModuleInfo.h"
 #include "llvm/CodeGen/MachineRegisterInfo.h"
 #include "llvm/IR/DataLayout.h"
 #include "llvm/IR/DerivedTypes.h"
@@ -878,6 +880,15 @@ bool WebAssemblyFastISel::selectCall(const Instruction *I) {
     // Add placeholders for the type index and immediate flags
     MIB.addImm(0);
     MIB.addImm(0);
+
+    // Ensure that the object file has a __indirect_function_table import, as we
+    // call_indirect against it.
+    MCSymbolWasm *Sym = WebAssembly::getOrCreateFunctionTableSymbol(
+        MF->getMMI().getContext(), "__indirect_function_table");
+    // Until call_indirect emits TABLE_NUMBER relocs against this symbol, mark
+    // it as NO_STRIP so as to ensure that the indirect function table makes it
+    // to linked output.
+    Sym->setNoStrip();
   }
 
   for (unsigned ArgReg : Args)

llvm/lib/Target/WebAssembly/WebAssemblyISelLowering.cpp

@@ -16,6 +16,7 @@
 #include "WebAssemblyMachineFunctionInfo.h"
 #include "WebAssemblySubtarget.h"
 #include "WebAssemblyTargetMachine.h"
+#include "WebAssemblyUtilities.h"
 #include "llvm/CodeGen/Analysis.h"
 #include "llvm/CodeGen/CallingConvLower.h"
 #include "llvm/CodeGen/MachineInstrBuilder.h"
@@ -477,6 +478,15 @@ static MachineBasicBlock *LowerCallResults(MachineInstr &CallResults,
   if (IsIndirect) {
     MIB.addImm(0);
     MIB.addImm(0);
+
+    // Ensure that the object file has a __indirect_function_table import, as we
+    // call_indirect against it.
+    MCSymbolWasm *Sym = WebAssembly::getOrCreateFunctionTableSymbol(
+        MF.getContext(), "__indirect_function_table");
+    // Until call_indirect emits TABLE_NUMBER relocs against this symbol, mark
+    // it as NO_STRIP so as to ensure that the indirect function table makes it
+    // to linked output.
+    Sym->setNoStrip();
   }
 
   for (auto Use : CallParams.uses())

llvm/lib/Target/WebAssembly/WebAssemblyUtilities.cpp

@@ -15,6 +15,7 @@
 #include "WebAssemblyMachineFunctionInfo.h"
 #include "llvm/CodeGen/MachineInstr.h"
 #include "llvm/CodeGen/MachineLoopInfo.h"
+#include "llvm/MC/MCContext.h"
 using namespace llvm;
 
 const char *const WebAssembly::ClangCallTerminateFn = "__clang_call_terminate";
@@ -96,3 +97,21 @@ const MachineOperand &WebAssembly::getCalleeOp(const MachineInstr &MI) {
     llvm_unreachable("Not a call instruction");
   }
 }
+
+MCSymbolWasm *
+WebAssembly::getOrCreateFunctionTableSymbol(MCContext &Ctx,
+                                            const StringRef &Name) {
+  // FIXME: Duplicates functionality from
+  // MC/WasmObjectWriter::recordRelocation.
+  MCSymbolWasm *Sym = cast_or_null<MCSymbolWasm>(Ctx.lookupSymbol(Name));
+  if (Sym) {
+    if (!Sym->isFunctionTable())
+      Ctx.reportError(SMLoc(), "symbol is not a wasm funcref table");
+  } else {
+    Sym = cast<MCSymbolWasm>(Ctx.getOrCreateSymbol(Name));
+    Sym->setFunctionTable();
+    // The default function table is synthesized by the linker.
+    Sym->setUndefined();
+  }
+  return Sym;
+}

llvm/lib/Target/WebAssembly/WebAssemblyUtilities.h

@@ -19,6 +19,9 @@ namespace llvm {
 
 class MachineInstr;
 class MachineOperand;
+class MCContext;
+class MCSymbolWasm;
+class StringRef;
 class WebAssemblyFunctionInfo;
 
 namespace WebAssembly {
@@ -37,6 +40,11 @@ extern const char *const PersonalityWrapperFn;
 /// instruction.
 const MachineOperand &getCalleeOp(const MachineInstr &MI);
 
+/// Returns the operand number of a callee, assuming the argument is a call
+/// instruction.
+MCSymbolWasm *getOrCreateFunctionTableSymbol(MCContext &Ctx,
+                                             const StringRef &Name);
+
 } // end namespace WebAssembly
 
 } // end namespace llvm

llvm/test/CodeGen/WebAssembly/call-indirect.ll

@@ -0,0 +1,28 @@
+; RUN: llc < %s -asm-verbose=false -O2 | FileCheck --check-prefix=CHECK %s
+; RUN: llc < %s -asm-verbose=false -O2 --filetype=obj | obj2yaml | FileCheck --check-prefix=OBJ %s
+
+; Test that compilation units with call_indirect but without any
+; function pointer declarations still get a table.
+
+target datalayout = "e-m:e-p:32:32-i64:64-n32:64-S128"
+target triple = "wasm32-unknown-unknown"
+
+; CHECK-LABEL: call_indirect_void:
+; CHECK-NEXT: .functype call_indirect_void (i32) -> ()
+; CHECK-NEXT: local.get 0
+; CHECK-NEXT: call_indirect () -> ()
+; CHECK-NEXT: end_function
+define void @call_indirect_void(void ()* %callee) {
+  call void %callee()
+  ret void
+}
+
+; OBJ:    Imports:
+; OBJ-NEXT:      - Module:          env
+; OBJ-NEXT:        Field:           __linear_memory
+; OBJ-NEXT:        Kind:            MEMORY
+; OBJ-NEXT:        Memory:
+; OBJ-NEXT:          Initial:         0x0
+; OBJ-NEXT:      - Module:          env
+; OBJ-NEXT:        Field:           __indirect_function_table
+; OBJ-NEXT:        Kind:            TABLE

llvm/test/MC/WebAssembly/type-index.s

@@ -38,6 +38,14 @@ test0:
 # BIN-NEXT:         Kind:            MEMORY
 # BIN-NEXT:         Memory:
 # BIN-NEXT:           Initial:         0x0
+# BIN-NEXT:       - Module:          env
+# BIN-NEXT:         Field:           __indirect_function_table
+# BIN-NEXT:         Kind:            TABLE
+# BIN-NEXT:         Table:
+# BIN-NEXT:           Index:           0
+# BIN-NEXT:           ElemType:        FUNCREF
+# BIN-NEXT:           Limits:
+# BIN-NEXT:             Initial:         0x0
 # BIN-NEXT:   - Type:            FUNCTION
 # BIN-NEXT:     FunctionTypes:   [ 0 ]
 # BIN-NEXT:   - Type:            CODE