[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

3 files changed, 96 insertions, 2 deletions

diff --git a/llvm/lib/Target/WebAssembly/WebAssemblyISelDAGToDAG.cpp b/llvm/lib/Target/WebAssembly/WebAssemblyISelDAGToDAG.cpp
index 1efbb3b067b..26339eaef37 100644
--- a/llvm/lib/Target/WebAssembly/WebAssemblyISelDAGToDAG.cpp
+++ b/llvm/lib/Target/WebAssembly/WebAssemblyISelDAGToDAG.cpp
@@ -179,9 +179,17 @@ void WebAssemblyDAGToDAGISel::Select(SDNode *Node) {
       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);
     }
 
diff --git a/llvm/test/CodeGen/WebAssembly/tls-general-dynamic.ll b/llvm/test/CodeGen/WebAssembly/tls-general-dynamic.ll
new file mode 100644
index 00000000000..3f6d9d325c6
--- /dev/null
+++ b/llvm/test/CodeGen/WebAssembly/tls-general-dynamic.ll
@@ -0,0 +1,86 @@
+; RUN: not llc < %s -asm-verbose=false -disable-wasm-fallthrough-return-opt -wasm-disable-explicit-locals -mattr=+bulk-memory 2>&1 | FileCheck %s --check-prefix=ERROR
+; RUN: not llc < %s -asm-verbose=false -disable-wasm-fallthrough-return-opt -wasm-disable-explicit-locals -mattr=+bulk-memory -fast-isel 2>&1 | FileCheck %s --check-prefix=ERROR
+; RUN: llc < %s -asm-verbose=false -disable-wasm-fallthrough-return-opt -wasm-disable-explicit-locals -mattr=+bulk-memory --mtriple wasm32-unknown-emscripten | FileCheck %s --check-prefixes=CHECK,TLS
+; RUN: llc < %s -asm-verbose=false -disable-wasm-fallthrough-return-opt -wasm-disable-explicit-locals -mattr=+bulk-memory --mtriple wasm32-unknown-emscripten -fast-isel | FileCheck %s --check-prefixes=CHECK,TLS
+; RUN: llc < %s -asm-verbose=false -disable-wasm-fallthrough-return-opt -wasm-disable-explicit-locals -mattr=-bulk-memory | FileCheck %s --check-prefixes=CHECK,NO-TLS
+target datalayout = "e-m:e-p:32:32-i64:64-n32:64-S128"
+target triple = "wasm32-unknown-unknown"
+
+; ERROR: LLVM ERROR: only -ftls-model=local-exec is supported for now on non-Emscripten OSes: variable tls
+
+; CHECK-LABEL: address_of_tls:
+; CHECK-NEXT: .functype  address_of_tls () -> (i32)
+define i32 @address_of_tls() {
+  ; TLS-DAG: global.get __tls_base
+  ; TLS-DAG: i32.const tls
+  ; TLS-NEXT: i32.add
+  ; TLS-NEXT: return
+
+  ; NO-TLS-NEXT: i32.const tls
+  ; NO-TLS-NEXT: return
+  ret i32 ptrtoint(i32* @tls to i32)
+}
+
+; CHECK-LABEL: ptr_to_tls:
+; CHECK-NEXT: .functype ptr_to_tls () -> (i32)
+define i32* @ptr_to_tls() {
+  ; TLS-DAG: global.get __tls_base
+  ; TLS-DAG: i32.const tls
+  ; TLS-NEXT: i32.add
+  ; TLS-NEXT: return
+
+  ; NO-TLS-NEXT: i32.const tls
+  ; NO-TLS-NEXT: return
+  ret i32* @tls
+}
+
+; CHECK-LABEL: tls_load:
+; CHECK-NEXT: .functype tls_load () -> (i32)
+define i32 @tls_load() {
+  ; TLS-DAG: global.get __tls_base
+  ; TLS-DAG: i32.const tls
+  ; TLS-NEXT: i32.add
+  ; TLS-NEXT: i32.load 0
+  ; TLS-NEXT: return
+
+  ; NO-TLS-NEXT: i32.const 0
+  ; NO-TLS-NEXT: i32.load tls
+  ; NO-TLS-NEXT: return
+  %tmp = load i32, i32* @tls, align 4
+  ret i32 %tmp
+}
+
+; CHECK-LABEL: tls_store:
+; CHECK-NEXT: .functype tls_store (i32) -> ()
+define void @tls_store(i32 %x) {
+  ; TLS-DAG: global.get __tls_base
+  ; TLS-DAG: i32.const tls
+  ; TLS-NEXT: i32.add
+  ; TLS-NEXT: i32.store 0
+  ; TLS-NEXT: return
+
+  ; NO-TLS-NEXT: i32.const 0
+  ; NO-TLS-NEXT: i32.store tls
+  ; NO-TLS-NEXT: return
+  store i32 %x, i32* @tls, align 4
+  ret void
+}
+
+; CHECK-LABEL: tls_size:
+; CHECK-NEXT: .functype tls_size () -> (i32)
+define i32 @tls_size() {
+; CHECK-NEXT: global.get __tls_size
+; CHECK-NEXT: return
+  %1 = call i32 @llvm.wasm.tls.size.i32()
+  ret i32 %1
+}
+
+; CHECK: .type tls,@object
+; TLS-NEXT: .section .tbss.tls,"",@
+; NO-TLS-NEXT: .section .bss.tls,"",@
+; CHECK-NEXT: .p2align 2
+; CHECK-NEXT: tls:
+; CHECK-NEXT: .int32 0
+@tls = internal thread_local global i32 0
+
+declare i32 @llvm.wasm.tls.size.i32()
diff --git a/llvm/test/CodeGen/WebAssembly/tls.ll b/llvm/test/CodeGen/WebAssembly/tls-local-exec.ll
index 02979a28af9..02979a28af9 100644
--- a/llvm/test/CodeGen/WebAssembly/tls.ll
+++ b/llvm/test/CodeGen/WebAssembly/tls-local-exec.ll