[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

llvm-svn: 366272

1 files changed, 81 insertions, 0 deletions

diff --git a/lld/test/wasm/tls.ll b/lld/test/wasm/tls.ll
new file mode 100644
index 00000000000..b570d467568
--- /dev/null
+++ b/lld/test/wasm/tls.ll
@@ -0,0 +1,81 @@
+; RUN: llc -mattr=+bulk-memory -filetype=obj %s -o %t.o
+
+target triple = "wasm32-unknown-unknown"
+
+@tls1 = thread_local(localexec) global i32 1, align 4
+@no_tls = global i32 0, align 4
+@tls2 = thread_local(localexec) global i32 1, align 4
+
+define i32* @tls1_addr() {
+  ret i32* @tls1
+}
+
+define i32* @tls2_addr() {
+  ret i32* @tls2
+}
+
+; RUN: wasm-ld -no-gc-sections --shared-memory --max-memory=131072 --no-entry -o %t.wasm %t.o
+; RUN: obj2yaml %t.wasm | FileCheck %s
+
+; RUN: wasm-ld -no-gc-sections --shared-memory --max-memory=131072 --no-merge-data-segments --no-entry -o %t.wasm %t.o
+; RUN: obj2yaml %t.wasm | FileCheck %s
+
+; CHECK:      - Type:            GLOBAL
+; CHECK-NEXT:   Globals:
+; CHECK-NEXT:     - Index:           0
+; CHECK-NEXT:       Type:            I32
+; CHECK-NEXT:       Mutable:         true
+; CHECK-NEXT:       InitExpr:
+; CHECK-NEXT:         Opcode:          I32_CONST
+; CHECK-NEXT:         Value:           66576
+; CHECK-NEXT:     - Index:           1
+; CHECK-NEXT:       Type:            I32
+; CHECK-NEXT:       Mutable:         true
+; CHECK-NEXT:       InitExpr:
+; CHECK-NEXT:         Opcode:          I32_CONST
+; CHECK-NEXT:         Value:           0
+; CHECK-NEXT:     - Index:           2
+; CHECK-NEXT:       Type:            I32
+; CHECK-NEXT:       Mutable:         false
+; CHECK-NEXT:       InitExpr:
+; CHECK-NEXT:         Opcode:          I32_CONST
+; CHECK-NEXT:         Value:           8
+
+
+; CHECK:      - Type:            CODE
+; CHECK-NEXT:   Functions:
+; CHECK-NEXT:     - Index:           0
+; CHECK-NEXT:       Locals:          []
+; CHECK-NEXT:       Body:            0B
+; CHECK-NEXT:     - Index:           1
+; CHECK-NEXT:       Locals:          []
+; CHECK-NEXT:       Body:            20002401200041004108FC0800000B
+
+; Expected body of __wasm_init_tls:
+;   local.get 0
+;   global.set  1
+;   local.get 0
+;   i32.const 0
+;   i32.const 8
+;   memory.init 0, 0
+;   end
+
+; CHECK-NEXT:     - Index:           2
+; CHECK-NEXT:       Locals:          []
+; CHECK-NEXT:       Body:            2381808080004180808080006A0B
+
+; Expected body of tls1_addr:
+;   global.get 1
+;   i32.const 0
+;   i32.add
+;   end
+
+; CHECK-NEXT:     - Index:           3
+; CHECK-NEXT:       Locals:          []
+; CHECK-NEXT:       Body:            2381808080004184808080006A0B
+
+; Expected body of tls1_addr:
+;   global.get 1
+;   i32.const 4
+;   i32.add
+;   end