[WebAssembly] MC: Fix for data aliases with offsets (getelementptr)

[sbc100]

Summary

For some reason we hadn't seen such cases in the wild which makes
me think that clang and rustc don't generate these. In the bug which
reproduces it only occurs with LTO so my guess is that some LTO pass
is creating this alias + gep.

See: https://github.com/emscripten-core/emscripten/issues/8731

Of the 3 new tests added here the first 2 already passed, and
test/MC/WebAssembly/offset.s would previously crash.

[aheejin]

llvm/test/CodeGen/WebAssembly/aliases.ll

3

Can we also check if llvm-mc can correctly parse the result of this test? That will possibly break the current codebase.

llvm/test/MC/WebAssembly/offset.s

13

Does this handle something like

.set sym_c, 16

too? The code assumes there always exists a base symbol.

[sbc100]

llvm/test/CodeGen/WebAssembly/aliases.ll

3

None of the other tests in this directory to do that. In general I see this directory as testing CodeGen only. The ability for MC to parse this stuff is tested separately in llvm/test/MC/WebAssembly/. See the two new tests I added there.

llvm/test/MC/WebAssembly/offset.s

13

Yes, you are right it looks like we don't support absolute addresses like this yet. If its OK with you I'll leave that as a followup change? We might need a different symbol flag for that because currently all our data symbols need to be relative to a segment,.

[aheejin]

llvm/test/MC/WebAssembly/offset.s

13

Yeah sounds good.

[sbc100]

• rebase

llvm/lib/MC/WasmObjectWriter.cpp

private:
   void writeImportSection(ArrayRef<wasm::WasmImport> Imports, uint32_t DataSize,
                           uint32_t NumElements);
   void writeFunctionSection(ArrayRef<WasmFunction> Functions);
   void writeExportSection(ArrayRef<wasm::WasmExport> Exports);
   void writeElemSection(ArrayRef<uint32_t> TableElems);
   void writeDataCountSection();
   void writeCodeSection(const MCAssembler &Asm, const MCAsmLayout &Layout,
                         ArrayRef<WasmFunction> Functions);
-  void writeDataSection();
+  void writeDataSection(const MCAsmLayout &Layout);
   void writeEventSection(ArrayRef<wasm::WasmEventType> Events);
   void writeGlobalSection(ArrayRef<wasm::WasmGlobal> Globals);
   void writeRelocSection(uint32_t SectionIndex, StringRef Name,
                          std::vector<WasmRelocationEntry> &Relocations);
   void writeLinkingMetaDataSection(
       ArrayRef<wasm::WasmSymbolInfo> SymbolInfos,
       ArrayRef<std::pair<uint16_t, uint32_t>> InitFuncs,
       const std::map<StringRef, std::vector<WasmComdatEntry>> &Comdats);
   void writeCustomSection(WasmCustomSection &CustomSection,
                           const MCAssembler &Asm, const MCAsmLayout &Layout);
   void writeCustomRelocSections();
   void
   updateCustomSectionRelocations(const SmallVector<WasmFunction, 4> &Functions,
                                  const MCAsmLayout &Layout);
 
-  uint64_t getProvisionalValue(const WasmRelocationEntry &RelEntry);
+  uint64_t getProvisionalValue(const WasmRelocationEntry &RelEntry,
+                               const MCAsmLayout &Layout);
   void applyRelocations(ArrayRef<WasmRelocationEntry> Relocations,
-                        uint64_t ContentsOffset);
+                        uint64_t ContentsOffset, const MCAsmLayout &Layout);
 
   uint32_t getRelocationIndexValue(const WasmRelocationEntry &RelEntry);
   uint32_t getFunctionType(const MCSymbolWasm &Symbol);
   uint32_t getEventType(const MCSymbolWasm &Symbol);
   void registerFunctionType(const MCSymbolWasm &Symbol);
   void registerEventType(const MCSymbolWasm &Symbol);
 };
 
void WasmObjectWriter::recordRelocation(MCAssembler &Asm,
   // The .init_array isn't translated as data, so don't do relocations in it.
   if (FixupSection.getName().startswith(".init_array")) {
     SymA->setUsedInInitArray();
     return;
   }
 
   if (SymA->isVariable()) {
     const MCExpr *Expr = SymA->getVariableValue();
-    const auto *Inner = cast<MCSymbolRefExpr>(Expr);
+    if (const auto *Inner = dyn_cast<MCSymbolRefExpr>(Expr))
       if (Inner->getKind() == MCSymbolRefExpr::VK_WEAKREF)
         llvm_unreachable("weakref used in reloc not yet implemented");
   }
 
   // Put any constant offset in an addend. Offsets can be negative, and
   // LLVM expects wrapping, in contrast to wasm's immediates which can't
   // be negative and don't wrap.
   FixedValue = 0;
 
   unsigned Type = TargetObjectWriter->getRelocType(Target, Fixup);
if (FixupSection.isWasmData()) {
     CodeRelocations.push_back(Rec);
   } else if (FixupSection.getKind().isMetadata()) {
     CustomSectionsRelocations[&FixupSection].push_back(Rec);
   } else {
     llvm_unreachable("unexpected section type");
   }
 }
 
-static const MCSymbolWasm *resolveSymbol(const MCSymbolWasm &Symbol) {
-  const MCSymbolWasm* Ret = &Symbol;
-  while (Ret->isVariable()) {
-    const MCExpr *Expr = Ret->getVariableValue();
-    auto *Inner = cast<MCSymbolRefExpr>(Expr);
-    Ret = cast<MCSymbolWasm>(&Inner->getSymbol());
-  }
-  return Ret;
-}
-
 // Compute a value to write into the code at the location covered
 // by RelEntry. This value isn't used by the static linker; it just serves
 // to make the object format more readable and more likely to be directly
 // useable.
 uint64_t
-WasmObjectWriter::getProvisionalValue(const WasmRelocationEntry &RelEntry) {
+WasmObjectWriter::getProvisionalValue(const WasmRelocationEntry &RelEntry,
+                                      const MCAsmLayout &Layout) {
   if ((RelEntry.Type == wasm::R_WASM_GLOBAL_INDEX_LEB ||
        RelEntry.Type == wasm::R_WASM_GLOBAL_INDEX_I32) &&
       !RelEntry.Symbol->isGlobal()) {
     assert(GOTIndices.count(RelEntry.Symbol) > 0 && "symbol not found in GOT index space");
     return GOTIndices[RelEntry.Symbol];
   }
 
   switch (RelEntry.Type) {
   case wasm::R_WASM_TABLE_INDEX_REL_SLEB:
   case wasm::R_WASM_TABLE_INDEX_SLEB:
   case wasm::R_WASM_TABLE_INDEX_I32: {
     // Provisional value is table address of the resolved symbol itself
-    const MCSymbolWasm *Sym = resolveSymbol(*RelEntry.Symbol);
-    assert(Sym->isFunction());
+    const MCSymbolWasm *Base =
+        cast<MCSymbolWasm>(Layout.getBaseSymbol(*RelEntry.Symbol));
+    assert(Base->isFunction());
     if (RelEntry.Type == wasm::R_WASM_TABLE_INDEX_REL_SLEB)
-      return TableIndices[Sym] - InitialTableOffset;
+      return TableIndices[Base] - InitialTableOffset;
     else
-      return TableIndices[Sym];
+      return TableIndices[Base];
   }
   case wasm::R_WASM_TYPE_INDEX_LEB:
     // Provisional value is same as the index
     return getRelocationIndexValue(RelEntry);
   case wasm::R_WASM_FUNCTION_INDEX_LEB:
   case wasm::R_WASM_GLOBAL_INDEX_LEB:
   case wasm::R_WASM_GLOBAL_INDEX_I32:
   case wasm::R_WASM_EVENT_INDEX_LEB:
WasmObjectWriter::getProvisionalValue(const WasmRelocationEntry &RelEntry,
   case wasm::R_WASM_MEMORY_ADDR_LEB64:
   case wasm::R_WASM_MEMORY_ADDR_SLEB:
   case wasm::R_WASM_MEMORY_ADDR_SLEB64:
   case wasm::R_WASM_MEMORY_ADDR_REL_SLEB:
   case wasm::R_WASM_MEMORY_ADDR_REL_SLEB64:
   case wasm::R_WASM_MEMORY_ADDR_I32:
   case wasm::R_WASM_MEMORY_ADDR_I64: {
     // Provisional value is address of the global
-    const MCSymbolWasm *Sym = resolveSymbol(*RelEntry.Symbol);
+    const MCSymbolWasm *Base =
+        cast<MCSymbolWasm>(Layout.getBaseSymbol(*RelEntry.Symbol));
     // For undefined symbols, use zero
-    if (!Sym->isDefined())
+    if (!Base->isDefined())
       return 0;
-    const wasm::WasmDataReference &Ref = DataLocations[Sym];
+    const wasm::WasmDataReference &Ref = DataLocations[Base];
     const WasmDataSegment &Segment = DataSegments[Ref.Segment];
     // Ignore overflow. LLVM allows address arithmetic to silently wrap.
     return Segment.Offset + Ref.Offset + RelEntry.Addend;
   }
   default:
     llvm_unreachable("invalid relocation type");
   }
 }
WasmObjectWriter::getRelocationIndexValue(const WasmRelocationEntry &RelEntry) {
   }
 
   return RelEntry.Symbol->getIndex();
 }
 
 // Apply the portions of the relocation records that we can handle ourselves
 // directly.
 void WasmObjectWriter::applyRelocations(
-    ArrayRef<WasmRelocationEntry> Relocations, uint64_t ContentsOffset) {
+    ArrayRef<WasmRelocationEntry> Relocations, uint64_t ContentsOffset,
+    const MCAsmLayout &Layout) {
   auto &Stream = static_cast<raw_pwrite_stream &>(W.OS);
   for (const WasmRelocationEntry &RelEntry : Relocations) {
     uint64_t Offset = ContentsOffset +
                       RelEntry.FixupSection->getSectionOffset() +
                       RelEntry.Offset;
 
     LLVM_DEBUG(dbgs() << "applyRelocation: " << RelEntry << "\n");
-    auto Value = getProvisionalValue(RelEntry);
+    auto Value = getProvisionalValue(RelEntry, Layout);
 
     switch (RelEntry.Type) {
     case wasm::R_WASM_FUNCTION_INDEX_LEB:
     case wasm::R_WASM_TYPE_INDEX_LEB:
     case wasm::R_WASM_GLOBAL_INDEX_LEB:
     case wasm::R_WASM_MEMORY_ADDR_LEB:
     case wasm::R_WASM_EVENT_INDEX_LEB:
       writePatchableLEB<5>(Stream, Value, Offset);
if (!Func.Sym->getSize()->evaluateAsAbsolute(Size, Layout))
       report_fatal_error(".size expression must be evaluatable");
 
     encodeULEB128(Size, W.OS);
     FuncSection.setSectionOffset(W.OS.tell() - Section.ContentsOffset);
     Asm.writeSectionData(W.OS, &FuncSection, Layout);
   }
 
   // Apply fixups.
-  applyRelocations(CodeRelocations, Section.ContentsOffset);
+  applyRelocations(CodeRelocations, Section.ContentsOffset, Layout);
 
   endSection(Section);
 }
 
-void WasmObjectWriter::writeDataSection() {
+void WasmObjectWriter::writeDataSection(const MCAsmLayout &Layout) {
   if (DataSegments.empty())
     return;
 
   SectionBookkeeping Section;
   startSection(Section, wasm::WASM_SEC_DATA);
   DataSectionIndex = Section.Index;
 
   encodeULEB128(DataSegments.size(), W.OS); // count
 
   for (const WasmDataSegment &Segment : DataSegments) {
     encodeULEB128(Segment.InitFlags, W.OS); // flags
     if (Segment.InitFlags & wasm::WASM_SEGMENT_HAS_MEMINDEX)
       encodeULEB128(0, W.OS); // memory index
     if ((Segment.InitFlags & wasm::WASM_SEGMENT_IS_PASSIVE) == 0) {
       W.OS << char(wasm::WASM_OPCODE_I32_CONST);
       encodeSLEB128(Segment.Offset, W.OS); // offset
       W.OS << char(wasm::WASM_OPCODE_END);
     }
     encodeULEB128(Segment.Data.size(), W.OS); // size
     Segment.Section->setSectionOffset(W.OS.tell() - Section.ContentsOffset);
     W.OS << Segment.Data; // data
   }
 
   // Apply fixups.
-  applyRelocations(DataRelocations, Section.ContentsOffset);
+  applyRelocations(DataRelocations, Section.ContentsOffset, Layout);
 
   endSection(Section);
 }
 
 void WasmObjectWriter::writeRelocSection(
     uint32_t SectionIndex, StringRef Name,
     std::vector<WasmRelocationEntry> &Relocs) {
   // See: https://github.com/WebAssembly/tool-conventions/blob/master/Linking.md
void WasmObjectWriter::writeCustomSection(WasmCustomSection &CustomSection,
 
   CustomSection.OutputContentsOffset = Section.ContentsOffset;
   CustomSection.OutputIndex = Section.Index;
 
   endSection(Section);
 
   // Apply fixups.
   auto &Relocations = CustomSectionsRelocations[CustomSection.Section];
-  applyRelocations(Relocations, CustomSection.OutputContentsOffset);
+  applyRelocations(Relocations, CustomSection.OutputContentsOffset, Layout);
 }
 
 uint32_t WasmObjectWriter::getFunctionType(const MCSymbolWasm &Symbol) {
   assert(Symbol.isFunction());
   assert(TypeIndices.count(&Symbol));
   return TypeIndices[&Symbol];
 }
 
 uint32_t WasmObjectWriter::getEventType(const MCSymbolWasm &Symbol) {
   assert(Symbol.isEvent());
   assert(TypeIndices.count(&Symbol));
   return TypeIndices[&Symbol];
 }
 
 void WasmObjectWriter::registerFunctionType(const MCSymbolWasm &Symbol) {
   assert(Symbol.isFunction());
 
   WasmSignature S;
-  const MCSymbolWasm *ResolvedSym = resolveSymbol(Symbol);
-  if (auto *Sig = ResolvedSym->getSignature()) {
+
+  if (auto *Sig = Symbol.getSignature()) {
     S.Returns = Sig->Returns;
     S.Params = Sig->Params;
   }
 
   auto Pair = SignatureIndices.insert(std::make_pair(S, Signatures.size()));
   if (Pair.second)
     Signatures.push_back(S);
   TypeIndices[&Symbol] = Pair.first->second;
uint64_t WasmObjectWriter::writeObject(MCAssembler &Asm,
   // Populate SignatureIndices, and Imports and WasmIndices for undefined
   // symbols.  This must be done before populating WasmIndices for defined
   // symbols.
   for (const MCSymbol &S : Asm.symbols()) {
     const auto &WS = static_cast<const MCSymbolWasm &>(S);
 
     // Register types for all functions, including those with private linkage
     // (because wasm always needs a type signature).
-    if (WS.isFunction())
-      registerFunctionType(WS);
+    if (WS.isFunction()) {
+      const MCSymbolWasm *Base = cast<MCSymbolWasm>(Layout.getBaseSymbol(S));
+      registerFunctionType(*Base);
+    }
 
     if (WS.isEvent())
       registerEventType(WS);
 
     if (WS.isTemporary())
       continue;
 
     // If the symbol is not defined in this translation unit, import it.
uint64_t WasmObjectWriter::writeObject(MCAssembler &Asm,
   // target of the alias before the alias itself and the symbols are not
   // necessarily ordered in this way.
   for (const MCSymbol &S : Asm.symbols()) {
     if (!S.isVariable())
       continue;
 
     assert(S.isDefined());
 
+    const MCSymbolWasm *Base = cast<MCSymbolWasm>(Layout.getBaseSymbol(S));
+
     // Find the target symbol of this weak alias and export that index
     const auto &WS = static_cast<const MCSymbolWasm &>(S);
-    const MCSymbolWasm *ResolvedSym = resolveSymbol(WS);
-    LLVM_DEBUG(dbgs() << WS.getName() << ": weak alias of '" << *ResolvedSym
-                      << "'\n");
-
-    if (ResolvedSym->isFunction()) {
-      assert(WasmIndices.count(ResolvedSym) > 0);
-      uint32_t WasmIndex = WasmIndices.find(ResolvedSym)->second;
+    LLVM_DEBUG(dbgs() << WS.getName() << ": weak alias of '" << *Base << "'\n");
+
+    if (Base->isFunction()) {
+      assert(WasmIndices.count(Base) > 0);
+      uint32_t WasmIndex = WasmIndices.find(Base)->second;
       assert(WasmIndices.count(&WS) == 0);
       WasmIndices[&WS] = WasmIndex;
       LLVM_DEBUG(dbgs() << "  -> index:" << WasmIndex << "\n");
-    } else if (ResolvedSym->isData()) {
-      assert(DataLocations.count(ResolvedSym) > 0);
-      const wasm::WasmDataReference &Ref =
-          DataLocations.find(ResolvedSym)->second;
+    } else if (Base->isData()) {
+      auto &DataSection = static_cast<MCSectionWasm &>(WS.getSection());
+      uint64_t Offset = Layout.getSymbolOffset(S);
+      int64_t Size = 0;
+      // For data symbol alias we use the size of the base symbol as the
+      // size of the alias.  When an offset from the base is involved this
+      // can result in a offset + size goes past the end of the data section
+      // which out object format doesn't support.  So we must clamp it.
+      if (!Base->getSize()->evaluateAsAbsolute(Size, Layout))
+        report_fatal_error(".size expression must be evaluatable");
+      const WasmDataSegment &Segment =
+          DataSegments[DataSection.getSegmentIndex()];
+      Size =
+          std::min(static_cast<uint64_t>(Size), Segment.Data.size() - Offset);
+      wasm::WasmDataReference Ref = wasm::WasmDataReference{
+          DataSection.getSegmentIndex(),
+          static_cast<uint32_t>(Layout.getSymbolOffset(S)),
+          static_cast<uint32_t>(Size)};
       DataLocations[&WS] = Ref;
       LLVM_DEBUG(dbgs() << "  -> index:" << Ref.Segment << "\n");
     } else {
       report_fatal_error("don't yet support global/event aliases");
     }
   }
 
   // Finally, populate the symbol table itself, in its "natural" order.
auto HandleReloc = [&](const WasmRelocationEntry &Rel) {
       // Functions referenced by a relocation need to put in the table.  This is
       // purely to make the object file's provisional values readable, and is
       // ignored by the linker, which re-calculates the relocations itself.
       if (Rel.Type != wasm::R_WASM_TABLE_INDEX_I32 &&
           Rel.Type != wasm::R_WASM_TABLE_INDEX_SLEB &&
           Rel.Type != wasm::R_WASM_TABLE_INDEX_REL_SLEB)
         return;
       assert(Rel.Symbol->isFunction());
-      const MCSymbolWasm &WS = *resolveSymbol(*Rel.Symbol);
-      uint32_t FunctionIndex = WasmIndices.find(&WS)->second;
+      const MCSymbolWasm *Base =
+          cast<MCSymbolWasm>(Layout.getBaseSymbol(*Rel.Symbol));
+      uint32_t FunctionIndex = WasmIndices.find(Base)->second;
       uint32_t TableIndex = TableElems.size() + InitialTableOffset;
-      if (TableIndices.try_emplace(&WS, TableIndex).second) {
-        LLVM_DEBUG(dbgs() << "  -> adding " << WS.getName()
+      if (TableIndices.try_emplace(Base, TableIndex).second) {
+        LLVM_DEBUG(dbgs() << "  -> adding " << Base->getName()
                           << " to table: " << TableIndex << "\n");
         TableElems.push_back(FunctionIndex);
-        registerFunctionType(WS);
+        registerFunctionType(*Base);
       }
     };
 
     for (const WasmRelocationEntry &RelEntry : CodeRelocations)
       HandleReloc(RelEntry);
     for (const WasmRelocationEntry &RelEntry : DataRelocations)
       HandleReloc(RelEntry);
   }
uint64_t WasmObjectWriter::writeObject(MCAssembler &Asm,
   // Skip the "table" section; we import the table instead.
   // Skip the "memory" section; we import the memory instead.
   writeEventSection(Events);
   writeGlobalSection(Globals);
   writeExportSection(Exports);
   writeElemSection(TableElems);
   writeDataCountSection();
   writeCodeSection(Asm, Layout, Functions);
-  writeDataSection();
+  writeDataSection(Layout);
   for (auto &CustomSection : CustomSections)
     writeCustomSection(CustomSection, Asm, Layout);
   writeLinkingMetaDataSection(SymbolInfos, InitFuncs, Comdats);
   writeRelocSection(CodeSectionIndex, "CODE", CodeRelocations);
   writeRelocSection(DataSectionIndex, "DATA", DataRelocations);
   writeCustomRelocSections();
   if (ProducersSection)
     writeCustomSection(*ProducersSection, Asm, Layout);

llvm/test/CodeGen/WebAssembly/aliases.ll

+; Based llvm/test/CodeGen/X86/aliases.ll
+; RUN: llc < %s -mtriple=wasm32-unknown-uknown -asm-verbose=false | FileCheck %s
+
+@bar = global i32 42
+
+; CHECK-DAG: .globl foo1
+; CHECK-DAG: .set foo1, bar
+@foo1 = alias i32, i32* @bar
+
+; CHECK-DAG: .globl foo2
+; CHECK-DAG: .set foo2, bar
+@foo2 = alias i32, i32* @bar
+
+%FunTy = type i32()
+
+define i32 @foo_f() {
+  ret i32 0
+}
+
+; CHECK-DAG: .weak  bar_f
+; CHECK-DAG: .type  bar_f,@function
+; CHECK-DAG: .set bar_f, foo_f
+@bar_f = weak alias %FunTy, %FunTy* @foo_f
+
+; CHECK-DAG: .weak  bar_l
+; CHECK-DAG: .set bar_l, bar
+@bar_l = linkonce_odr alias i32, i32* @bar
+
+; CHECK-DAG: .set bar_i, bar
+@bar_i = internal alias i32, i32* @bar
+
+; CHECK-DAG: .globl A
+@A = alias i64, bitcast (i32* @bar to i64*)
+
+; CHECK-DAG: .globl bar_h
+; CHECK-DAG: .hidden  bar_h
+; CHECK-DAG: .set bar_h, bar
+@bar_h = hidden alias i32, i32* @bar
+
+; CHECK-DAG: .globl bar_p
+; CHECK-DAG: .protected bar_p
+; CHECK-DAG: .set bar_p, bar
+@bar_p = protected alias i32, i32* @bar
+
+; CHECK-DAG: .set test2, bar+4
+@test2 = alias i32, getelementptr(i32, i32* @bar, i32 1)
+
+; CHECK-DAG: .set test3, 42
+@test3 = alias i32, inttoptr(i32 42 to i32*)
+
+; CHECK-DAG: .set test4, bar
+@test4 = alias i32, inttoptr(i64 ptrtoint (i32* @bar to i64) to i32*)
+
+; CHECK-DAG: .set test5, test2-bar
+@test5 = alias i32, inttoptr(i32 sub (i32 ptrtoint (i32* @test2 to i32),
+                                 i32 ptrtoint (i32* @bar to i32)) to i32*)
+
+; CHECK-DAG: .globl test
+define i32 @test() {
+entry:
+   %tmp = load i32, i32* @foo1
+   %tmp1 = load i32, i32* @foo2
+   %tmp0 = load i32, i32* @bar_i
+   %tmp2 = call i32 @foo_f()
+   %tmp3 = add i32 %tmp, %tmp2
+   %tmp4 = call i32 @bar_f()
+   %tmp5 = add i32 %tmp3, %tmp4
+   %tmp6 = add i32 %tmp1, %tmp5
+   %tmp7 = add i32 %tmp6, %tmp0
+   ret i32 %tmp7
+}

llvm/test/MC/WebAssembly/alias.s

+# RUN: llvm-mc -triple=wasm32-unknown-unknown -filetype=obj %s | llvm-objdump -t - | FileCheck %s
+
+.section    .data,"",@
+foo:
+  .int32 1
+  .size foo, 4
+sym_a:
+  .int32 2
+  .size sym_a, 4
+
+.set sym_b, sym_a
+
+# CHECK: 00000000 l     O DATA foo
+# CHECK: 00000004 l     O DATA sym_a
+# CHECK: 00000004 l     O DATA sym_b

llvm/test/MC/WebAssembly/offset.s

+# RUN: llvm-mc -triple=wasm32-unknown-unknown -filetype=obj %s | llvm-objdump -t - | FileCheck %s
+
+.section    .data,"",@
+foo:
+  .int32 0
+  .size foo, 4
+sym_a:
+  .int32 1
+  .int32 2
+  .size sym_a, 8
+
+.set sym_b, sym_a + 4
+
+# CHECK: 00000000 l     O DATA foo
+# CHECK: 00000004 l     O DATA sym_a
+# CHECK: 00000008 l     O DATA sym_b