[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

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 @@ class WasmObjectWriter : public MCObjectWriter {
     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)

llvm/test/MC/WebAssembly/debug-info.ll

@@ -204,6 +204,13 @@
 ; CHECK-NEXT:  }
 ; CHECK-NEXT:]
 
+; generated from the following C code using: clang --target=wasm32 -g -O0 -S -emit-llvm test.c
+; extern int myextern;
+; void f2(void) { return; }
+;
+; int* foo = &myextern;
+; void (*ptr2)(void) = f2;
+
 target triple = "wasm32-unknown-unknown"
 
 source_filename = "test.c"
@@ -226,7 +233,7 @@ attributes #0 = { noinline nounwind optnone "correctly-rounded-divide-sqrt-fp-ma
 
 !0 = !DIGlobalVariableExpression(var: !1, expr: !DIExpression())
 !1 = distinct !DIGlobalVariable(name: "foo", scope: !2, file: !3, line: 4, type: !11, isLocal: false, isDefinition: true)
-!2 = distinct !DICompileUnit(language: DW_LANG_C99, file: !3, producer: "clang version 6.0.0 (trunk 315924) (llvm/trunk 315960)", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug, enums: !4, globals: !5)
+!2 = distinct !DICompileUnit(language: DW_LANG_C99, file: !3, producer: "clang version 7.0.0 (trunk 332303) (llvm/trunk 332406)", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug, enums: !4, globals: !5)
 !3 = !DIFile(filename: "test.c", directory: "/usr/local/google/home/sbc/dev/wasm/simple")
 !4 = !{}
 !5 = !{!0, !6}
@@ -240,6 +247,6 @@ attributes #0 = { noinline nounwind optnone "correctly-rounded-divide-sqrt-fp-ma
 !13 = !{i32 2, !"Dwarf Version", i32 4}
 !14 = !{i32 2, !"Debug Info Version", i32 3}
 !15 = !{i32 1, !"wchar_size", i32 4}
-!16 = !{!"clang version 6.0.0 (trunk 315924) (llvm/trunk 315960)"}
+!16 = !{!"clang version 7.0.0 (trunk 332303) (llvm/trunk 332406)"}
 !17 = distinct !DISubprogram(name: "f2", scope: !3, file: !3, line: 2, type: !9, isLocal: false, isDefinition: true, scopeLine: 2, flags: DIFlagPrototyped, isOptimized: false, unit: !2, retainedNodes: !4)
-!18 = !DILocation(line: 2, column: 16, scope: !17)
+!18 = !DILocation(line: 2, column: 17, scope: !17)