From: Guanzhong Chen <gzchen@google.com>
Date: Tue, 16 Jul 2019 22:00:45 +0000 (+0000)
Subject: [WebAssembly] Implement thread-local storage (local-exec model)
X-Git-Url: https://granicus.if.org/sourcecode?a=commitdiff_plain;h=c0300437e347c1c5f86cc19b671b331a42ee6377;p=clang

[WebAssembly] Implement thread-local storage (local-exec model)

Summary:
Thread local variables are placed inside a `.tdata` segment. Their symbols are
offsets from the start of the segment. The address of a thread local variable
is computed as `__tls_base` + the offset from the start of the segment.

`.tdata` segment is a passive segment and `memory.init` is used once per thread
to initialize the thread local storage.

`__tls_base` is a wasm global. Since each thread has its own wasm instance,
it is effectively thread local. Currently, `__tls_base` must be initialized
at thread startup, and so cannot be used with dynamic libraries.

`__tls_base` is to be initialized with a new linker-synthesized function,
`__wasm_init_tls`, which takes as an argument a block of memory to use as the
storage for thread locals. It then initializes the block of memory and sets
`__tls_base`. As `__wasm_init_tls` will handle the memory initialization,
the memory does not have to be zeroed.

To help allocating memory for thread-local storage, a new compiler intrinsic
is introduced: `__builtin_wasm_tls_size()`. This instrinsic function returns
the size of the thread-local storage for the current function.

The expected usage is to run something like the following upon thread startup:

    __wasm_init_tls(malloc(__builtin_wasm_tls_size()));

Reviewers: tlively, aheejin, kripken, sbc100

Subscribers: dschuff, jgravelle-google, hiraditya, sunfish, jfb, cfe-commits, llvm-commits

Tags: #clang, #llvm

Differential Revision: https://reviews.llvm.org/D64537

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@366272 91177308-0d34-0410-b5e6-96231b3b80d8
---

diff --git a/include/clang/Basic/BuiltinsWebAssembly.def b/include/clang/Basic/BuiltinsWebAssembly.def
index 57ebb27ab4..63177f016a 100644
--- a/include/clang/Basic/BuiltinsWebAssembly.def
+++ b/include/clang/Basic/BuiltinsWebAssembly.def
@@ -29,6 +29,9 @@ BUILTIN(__builtin_wasm_memory_grow, "zIiz", "n")
 TARGET_BUILTIN(__builtin_wasm_memory_init, "vIUiIUiv*UiUi", "", "bulk-memory")
 TARGET_BUILTIN(__builtin_wasm_data_drop, "vIUi", "", "bulk-memory")
 
+// Thread-local storage
+TARGET_BUILTIN(__builtin_wasm_tls_size, "z", "nc", "bulk-memory")
+
 // Floating point min/max
 BUILTIN(__builtin_wasm_min_f32, "fff", "nc")
 BUILTIN(__builtin_wasm_max_f32, "fff", "nc")
diff --git a/lib/CodeGen/CGBuiltin.cpp b/lib/CodeGen/CGBuiltin.cpp
index acaa81ae8a..1658be5a88 100644
--- a/lib/CodeGen/CGBuiltin.cpp
+++ b/lib/CodeGen/CGBuiltin.cpp
@@ -13913,6 +13913,11 @@ Value *CodeGenFunction::EmitWebAssemblyBuiltinExpr(unsigned BuiltinID,
     Function *Callee = CGM.getIntrinsic(Intrinsic::wasm_data_drop);
     return Builder.CreateCall(Callee, {Arg});
   }
+  case WebAssembly::BI__builtin_wasm_tls_size: {
+    llvm::Type *ResultType = ConvertType(E->getType());
+    Function *Callee = CGM.getIntrinsic(Intrinsic::wasm_tls_size, ResultType);
+    return Builder.CreateCall(Callee);
+  }
   case WebAssembly::BI__builtin_wasm_throw: {
     Value *Tag = EmitScalarExpr(E->getArg(0));
     Value *Obj = EmitScalarExpr(E->getArg(1));
diff --git a/test/CodeGen/builtins-wasm.c b/test/CodeGen/builtins-wasm.c
index 4784d6ff86..8a17fb3964 100644
--- a/test/CodeGen/builtins-wasm.c
+++ b/test/CodeGen/builtins-wasm.c
@@ -38,6 +38,12 @@ void data_drop() {
   // WEBASSEMBLY64: call void @llvm.wasm.data.drop(i32 3)
 }
 
+__SIZE_TYPE__ tls_size() {
+  return __builtin_wasm_tls_size();
+  // WEBASSEMBLY32: call i32 @llvm.wasm.tls.size.i32()
+  // WEBASSEMBLY64: call i64 @llvm.wasm.tls.size.i64()
+}
+
 void throw(void *obj) {
   return __builtin_wasm_throw(0, obj);
   // WEBASSEMBLY32: call void @llvm.wasm.throw(i32 0, i8* %{{.*}})