[WebAssembly] Compile all TLS on Emscripten as local-exec
Summary:
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
Reviewers: tlively, aheejin, sbc100
Reviewed By: aheejin
Subscribers: dschuff, jgravelle-google, hiraditya, sunfish, llvm-commits
Tags: #llvm
Differential Revision: https://reviews.llvm.org/D64776
llvm-svn: 366275
diff --git a/llvm/lib/Target/WebAssembly/WebAssemblyISelDAGToDAG.cpp b/llvm/lib/Target/WebAssembly/WebAssemblyISelDAGToDAG.cpp
index 1efbb3b..26339ea 100644
--- a/llvm/lib/Target/WebAssembly/WebAssemblyISelDAGToDAG.cpp
+++ b/llvm/lib/Target/WebAssembly/WebAssemblyISelDAGToDAG.cpp
@@ -179,9 +179,17 @@
report_fatal_error("cannot use thread-local storage without bulk memory",
false);
+ // Currently Emscripten does not support dynamic linking with threads.
+ // Therefore, if we have thread-local storage, only the local-exec model
+ // is possible.
+ // TODO: remove this and implement proper TLS models once Emscripten
+ // supports dynamic linking with threads.
if (GA->getGlobal()->getThreadLocalMode() !=
- GlobalValue::LocalExecTLSModel) {
- report_fatal_error("only -ftls-model=local-exec is supported for now",
+ GlobalValue::LocalExecTLSModel &&
+ !Subtarget->getTargetTriple().isOSEmscripten()) {
+ report_fatal_error("only -ftls-model=local-exec is supported for now on "
+ "non-Emscripten OSes: variable " +
+ GA->getGlobal()->getName(),
false);
}