[PATCH] D64776: [WebAssembly] Compile all TLS on Emscripten as local-exec

[Guanzhong Chen via Phabricator via llvm-commits]

quantum created this revision.
quantum added reviewers: tlively, aheejin, sbc100.
Herald added subscribers: llvm-commits, sunfish, hiraditya, jgravelle-google, dschuff.
Herald added a project: LLVM.

Currently, on Emscripten, dynamic linking is not supported with threads.
This means that if thread-local storage is used, it must be used in a
statically-linked executable. Hence, local-exec is the only possible model.

This diff compiles all TLS variables to use local-exec on Emscripten as a
temporary measure until dynamic linking is supported with threads.

The goal for this is to allow C++ types with constructors to be thread-local.

Currently, when `clang` compiles a `thread_local` variable with a constructor,
it generates `__tls_guard` variable:

  @__tls_guard = internal thread_local global i8 0, align 1

As no TLS model is specified, this is treated as general-dynamic, which we do
not support (and cannot support without implementing dynamic linking support
with threads in Emscripten). As a result, any C++ constructor in `thread_local`
variables would not compile.

By compiling all `thread_local` as local-exec, `__tls_guard` will compile and
we can support C++ constructors with TLS without implementing dynamic linking
with threads.

Depends on D64537 <https://reviews.llvm.org/D64537>


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D64776

Files:
  llvm/lib/Target/WebAssembly/WebAssemblyISelDAGToDAG.cpp


Index: llvm/lib/Target/WebAssembly/WebAssemblyISelDAGToDAG.cpp
===================================================================
--- llvm/lib/Target/WebAssembly/WebAssemblyISelDAGToDAG.cpp
+++ llvm/lib/Target/WebAssembly/WebAssemblyISelDAGToDAG.cpp
@@ -179,8 +179,12 @@
       report_fatal_error("cannot use thread-local storage without bulk memory",
                          false);
 
+    // XXX Emscripten: currently Emscripten does not support dynamic linking
+    // with threads. Therefore, if we have thread-local storage, only the
+    // local-exec model is possible.
     if (GA->getGlobal()->getThreadLocalMode() !=
-        GlobalValue::LocalExecTLSModel) {
+            GlobalValue::LocalExecTLSModel &&
+        !Subtarget->getTargetTriple().isOSEmscripten()) {
       report_fatal_error("only -ftls-model=local-exec is supported for now",
                          false);
     }


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