[WebAssembly] Initial implementation of PIC code generation

This change implements lowering of references global symbols in PIC
mode.

This change implements lowering of global references in PIC mode using a
new @GOT reference type. @GOT references can be used with function or
data symbol names combined with the get_global instruction. In this case
the linker will insert the wasm global that stores the address of the
symbol (either in memory for data symbols or in the wasm table for
function symbols).

For now I'm continuing to use the R_WASM_GLOBAL_INDEX_LEB relocation
type for this type of reference which means that this relocation type
can refer to either a global or a function or data symbol. We could
choose to introduce specific relocation types for GOT entries in the
future.  See the current dynamic linking proposal:

https://github.com/WebAssembly/tool-conventions/blob/master/DynamicLinking.md

Differential Revision: https://reviews.llvm.org/D54647

llvm-svn: 357022

1 files changed, 5 insertions, 1 deletions

diff --git a/llvm/lib/Object/WasmObjectFile.cpp b/llvm/lib/Object/WasmObjectFile.cpp
index 14b17bcfa5e..7a2769149ad 100644
--- a/llvm/lib/Object/WasmObjectFile.cpp
+++ b/llvm/lib/Object/WasmObjectFile.cpp
@@ -777,7 +777,11 @@ Error WasmObjectFile::parseRelocSection(StringRef Name, ReadContext &Ctx) {
                                               object_error::parse_failed);
       break;
     case wasm::R_WASM_GLOBAL_INDEX_LEB:
-      if (!isValidGlobalSymbol(Reloc.Index))
+      // R_WASM_GLOBAL_INDEX_LEB are can be used against function and data
+      // symbols to refer to thier GOT enties.
+      if (!isValidGlobalSymbol(Reloc.Index) &&
+          !isValidDataSymbol(Reloc.Index) &&
+          !isValidFunctionSymbol(Reloc.Index))
         return make_error<GenericBinaryError>("Bad relocation global index",
                                               object_error::parse_failed);
       break;