Reland "[WebAssembly][Object]Use file offset as function symbol address for linked files (#76198)"

WebAssembly doesn't have a single virtual memory space the way other object
formats or architectures do, so "addresses" mean different things depending
on the context.
Function symbol addresses in object files are offsets from the start of the code
section. This is good for linking and relocation. However when dealing with
linked binaries, offsets from the start of the file/module are more often
used (e.g. for stack traces in browsers), and are more useful for use
cases like binary size attribution. This PR changes Object to use
the file offset instead of the section offset for function symbols, but
only for linked (non-DSO) files.

This is a reland of fc5f51cf with a fix for the MSan failure (it was not caused
by this change, but it was revealed by the new tests).

1 files changed, 13 insertions, 4 deletions

diff --git a/llvm/lib/Object/WasmObjectFile.cpp b/llvm/lib/Object/WasmObjectFile.cpp
index 40665d6..94cd969 100644
--- a/llvm/lib/Object/WasmObjectFile.cpp
+++ b/llvm/lib/Object/WasmObjectFile.cpp
@@ -1351,6 +1351,7 @@ Error WasmObjectFile::parseExportSection(ReadContext &Ctx) {
       break;
     case wasm::WASM_EXTERNAL_TABLE:
       Info.Kind = wasm::WASM_SYMBOL_TYPE_TABLE;
+      Info.ElementIndex = Ex.Index;
       break;
     default:
       return make_error<GenericBinaryError>("unexpected export kind",
@@ -1667,10 +1668,18 @@ Expected<StringRef> WasmObjectFile::getSymbolName(DataRefImpl Symb) const {
 Expected<uint64_t> WasmObjectFile::getSymbolAddress(DataRefImpl Symb) const {
   auto &Sym = getWasmSymbol(Symb);
   if (Sym.Info.Kind == wasm::WASM_SYMBOL_TYPE_FUNCTION &&
-      isDefinedFunctionIndex(Sym.Info.ElementIndex))
-    return getDefinedFunction(Sym.Info.ElementIndex).CodeSectionOffset;
-  else
-    return getSymbolValue(Symb);
+      isDefinedFunctionIndex(Sym.Info.ElementIndex)) {
+    // For object files, use the section offset. The linker relies on this.
+    // For linked files, use the file offset. This behavior matches the way
+    // browsers print stack traces and is useful for binary size analysis.
+    // (see https://webassembly.github.io/spec/web-api/index.html#conventions)
+    uint32_t Adjustment = isRelocatableObject() || isSharedObject()
+                              ? 0
+                              : Sections[CodeSection].Offset;
+    return getDefinedFunction(Sym.Info.ElementIndex).CodeSectionOffset +
+           Adjustment;
+  }
+  return getSymbolValue(Symb);
 }
 
 uint64_t WasmObjectFile::getWasmSymbolValue(const WasmSymbol &Sym) const {