[WebAssembly] MC: Ensure that FUNCTION_OFFSET relocations are always against function symbols.

The getAtom() method wasn't doing what we needed in all cases. We want
the symbols for the function which defines that section. We can compute
this easily enough and we know that we have at most one function in each
section.

Once this lands I will revert rL331412 which is no longer needed.

Fixes PR37409

Differential Revision: https://reviews.llvm.org/D46970

llvm-svn: 332517

1 files changed, 17 insertions, 1 deletions

diff --git a/llvm/lib/MC/WasmObjectWriter.cpp b/llvm/lib/MC/WasmObjectWriter.cpp
index 9c4371d48ba..c9733328a7b 100644
--- a/llvm/lib/MC/WasmObjectWriter.cpp
+++ b/llvm/lib/MC/WasmObjectWriter.cpp
@@ -225,6 +225,9 @@ class WasmObjectWriter : public MCObjectWriter {
   DenseMap<const MCSectionWasm *, std::vector<WasmRelocationEntry>>
       CustomSectionsRelocations;
 
+  // Map from section to fintining function.
+  DenseMap<const MCSection *, const MCSymbol *> SectionFunctions;
+
   DenseMap<WasmFunctionType, int32_t, WasmFunctionTypeDenseMapInfo>
       FunctionTypeIndices;
   SmallVector<WasmFunctionType, 4> FunctionTypes;
@@ -265,6 +268,7 @@ private:
     FunctionTypes.clear();
     Globals.clear();
     DataSegments.clear();
+    SectionFunctions.clear();
     MCObjectWriter::reset();
     NumFunctionImports = 0;
     NumGlobalImports = 0;
@@ -383,6 +387,18 @@ void WasmObjectWriter::writeHeader(const MCAssembler &Asm) {
 
 void WasmObjectWriter::executePostLayoutBinding(MCAssembler &Asm,
                                                 const MCAsmLayout &Layout) {
+  // Build a map of sections to the function that defines them, for use
+  // in recordRelocation.
+  for (const MCSymbol &S : Asm.symbols()) {
+    const auto &WS = static_cast<const MCSymbolWasm &>(S);
+    if (WS.isDefined() && WS.isFunction() && !WS.isVariable()) {
+      const auto &Sec = static_cast<const MCSectionWasm &>(S.getSection());
+      auto Pair = SectionFunctions.insert(std::make_pair(&Sec, &S));
+      if (!Pair.second)
+        report_fatal_error("section already has a defining function: " +
+                           Sec.getSectionName());
+    }
+  }
 }
 
 void WasmObjectWriter::recordRelocation(MCAssembler &Asm,
@@ -475,7 +491,7 @@ void WasmObjectWriter::recordRelocation(MCAssembler &Asm,
     const MCSymbol *SectionSymbol = nullptr;
     const MCSection &SecA = SymA->getSection();
     if (SecA.getKind().isText())
-      SectionSymbol = SecA.begin()->getAtom();
+      SectionSymbol = SectionFunctions.find(&SecA)->second;
     else
       SectionSymbol = SecA.getBeginSymbol();
     if (!SectionSymbol)