Merge pull request #20925 from alexrp/windows-tls

`std`: Some Windows TLS cleanup and fixes

Author: andrewrk

lib/std/start_windows_tls.zig renamed to lib/std/os/windows/tls.zig

@@ -1,14 +1,15 @@
 const std = @import("std");
 const builtin = @import("builtin");
+const windows = std.os.windows;
 
 export var _tls_index: u32 = std.os.windows.TLS_OUT_OF_INDEXES;
-export var _tls_start: u8 linksection(".tls") = 0;
-export var _tls_end: u8 linksection(".tls$ZZZ") = 0;
-export var __xl_a: std.os.windows.PIMAGE_TLS_CALLBACK linksection(".CRT$XLA") = null;
-export var __xl_z: std.os.windows.PIMAGE_TLS_CALLBACK linksection(".CRT$XLZ") = null;
+export var _tls_start: ?*anyopaque linksection(".tls") = null;
+export var _tls_end: ?*anyopaque linksection(".tls$ZZZ") = null;
+export var __xl_a: windows.PIMAGE_TLS_CALLBACK linksection(".CRT$XLA") = null;
+export var __xl_z: windows.PIMAGE_TLS_CALLBACK linksection(".CRT$XLZ") = null;
 
 comptime {
-    if (builtin.target.cpu.arch == .x86 and builtin.zig_backend != .stage2_c) {
+    if (builtin.cpu.arch == .x86 and builtin.abi == .msvc and builtin.zig_backend != .stage2_c) {
         // The __tls_array is the offset of the ThreadLocalStoragePointer field
         // in the TEB block whose base address held in the %fs segment.
         asm (
@@ -19,8 +20,6 @@ comptime {
 }
 
 // TODO this is how I would like it to be expressed
-// TODO also note, ReactOS has a +1 on StartAddressOfRawData and AddressOfCallBacks. Investigate
-// why they do that.
 //export const _tls_used linksection(".rdata$T") = std.os.windows.IMAGE_TLS_DIRECTORY {
 //    .StartAddressOfRawData = @intFromPtr(&_tls_start),
 //    .EndAddressOfRawData = @intFromPtr(&_tls_end),
@@ -31,18 +30,21 @@ comptime {
 //};
 // This is the workaround because we can't do @intFromPtr at comptime like that.
 pub const IMAGE_TLS_DIRECTORY = extern struct {
-    StartAddressOfRawData: *anyopaque,
-    EndAddressOfRawData: *anyopaque,
-    AddressOfIndex: *anyopaque,
-    AddressOfCallBacks: *anyopaque,
+    StartAddressOfRawData: *?*anyopaque,
+    EndAddressOfRawData: *?*anyopaque,
+    AddressOfIndex: *u32,
+    AddressOfCallBacks: [*:null]windows.PIMAGE_TLS_CALLBACK,
     SizeOfZeroFill: u32,
     Characteristics: u32,
 };
 export const _tls_used linksection(".rdata$T") = IMAGE_TLS_DIRECTORY{
     .StartAddressOfRawData = &_tls_start,
     .EndAddressOfRawData = &_tls_end,
     .AddressOfIndex = &_tls_index,
-    .AddressOfCallBacks = @as(*anyopaque, @ptrCast(&__xl_a)),
+    // __xl_a is just a global variable containing a null pointer; the actual callbacks sit in
+    // between __xl_a and __xl_z. So we need to skip over __xl_a here. If there are no callbacks,
+    // this just means we point to __xl_z (the null terminator).
+    .AddressOfCallBacks = @as([*:null]windows.PIMAGE_TLS_CALLBACK, @ptrCast(&__xl_a)) + 1,
     .SizeOfZeroFill = 0,
     .Characteristics = 0,
 };

lib/std/start.zig

@@ -176,7 +176,7 @@ fn _DllMainCRTStartup(
     lpReserved: std.os.windows.LPVOID,
 ) callconv(std.os.windows.WINAPI) std.os.windows.BOOL {
     if (!builtin.single_threaded and !builtin.link_libc) {
-        _ = @import("start_windows_tls.zig");
+        _ = @import("os/windows/tls.zig");
     }
 
     if (@hasDecl(root, "DllMain")) {
@@ -434,7 +434,7 @@ fn _start() callconv(.Naked) noreturn {
 fn WinStartup() callconv(std.os.windows.WINAPI) noreturn {
     @setAlignStack(16);
     if (!builtin.single_threaded and !builtin.link_libc) {
-        _ = @import("start_windows_tls.zig");
+        _ = @import("os/windows/tls.zig");
     }
 
     std.debug.maybeEnableSegfaultHandler();
@@ -445,7 +445,7 @@ fn WinStartup() callconv(std.os.windows.WINAPI) noreturn {
 fn wWinMainCRTStartup() callconv(std.os.windows.WINAPI) noreturn {
     @setAlignStack(16);
     if (!builtin.single_threaded and !builtin.link_libc) {
-        _ = @import("start_windows_tls.zig");
+        _ = @import("os/windows/tls.zig");
     }
 
     std.debug.maybeEnableSegfaultHandler();