Auto merge of #108089 - Zoxc:windows-tls, r=bjorn3

Support TLS access into dylibs on Windows

This allows access to `#[thread_local]`  in upstream dylibs on Windows by introducing a MIR shim to return the address of the thread local. Accesses that go into an upstream dylib will call the MIR shim to get the address of it.

`convert_tls_rvalues` is introduced in `rustc_codegen_ssa` which rewrites MIR TLS accesses to dummy calls which are replaced with calls to the MIR shims when the dummy calls are lowered to backend calls.

A new `dll_tls_export` target option enables this behavior with a `false` value which is set for Windows platforms.

This fixes https://github.com/rust-lang/rust/issues/84933.

2 files changed, 4 insertions, 48 deletions

diff --git a/library/std/src/sys/common/thread_local/fast_local.rs b/library/std/src/sys/common/thread_local/fast_local.rs
index 2addcc4a759..e229eb16aa1 100644
--- a/library/std/src/sys/common/thread_local/fast_local.rs
+++ b/library/std/src/sys/common/thread_local/fast_local.rs
@@ -10,7 +10,7 @@
 macro_rules! __thread_local_inner {
     // used to generate the `LocalKey` value for const-initialized thread locals
     (@key $t:ty, const $init:expr) => {{
-        #[cfg_attr(not(windows), inline)] // see comments below
+        #[cfg_attr(not(bootstrap), inline)]
         #[deny(unsafe_op_in_unsafe_fn)]
         unsafe fn __getit(
             _init: $crate::option::Option<&mut $crate::option::Option<$t>>,
@@ -77,29 +77,7 @@ macro_rules! __thread_local_inner {
             #[inline]
             fn __init() -> $t { $init }
 
-            // When reading this function you might ask "why is this inlined
-            // everywhere other than Windows?", and that's a very reasonable
-            // question to ask. The short story is that it segfaults rustc if
-            // this function is inlined. The longer story is that Windows looks
-            // to not support `extern` references to thread locals across DLL
-            // boundaries. This appears to at least not be supported in the ABI
-            // that LLVM implements.
-            //
-            // Because of this we never inline on Windows, but we do inline on
-            // other platforms (where external references to thread locals
-            // across DLLs are supported). A better fix for this would be to
-            // inline this function on Windows, but only for "statically linked"
-            // components. For example if two separately compiled rlibs end up
-            // getting linked into a DLL then it's fine to inline this function
-            // across that boundary. It's only not fine to inline this function
-            // across a DLL boundary. Unfortunately rustc doesn't currently
-            // have this sort of logic available in an attribute, and it's not
-            // clear that rustc is even equipped to answer this (it's more of a
-            // Cargo question kinda). This means that, unfortunately, Windows
-            // gets the pessimistic path for now where it's never inlined.
-            //
-            // The issue of "should enable on Windows sometimes" is #84933
-            #[cfg_attr(not(windows), inline)]
+            #[cfg_attr(not(bootstrap), inline)]
             unsafe fn __getit(
                 init: $crate::option::Option<&mut $crate::option::Option<$t>>,
             ) -> $crate::option::Option<&'static $t> {
diff --git a/library/std/src/sys/common/thread_local/os_local.rs b/library/std/src/sys/common/thread_local/os_local.rs
index 6f6560c4aa9..ce74ad3486e 100644
--- a/library/std/src/sys/common/thread_local/os_local.rs
+++ b/library/std/src/sys/common/thread_local/os_local.rs
@@ -10,7 +10,7 @@
 macro_rules! __thread_local_inner {
     // used to generate the `LocalKey` value for const-initialized thread locals
     (@key $t:ty, const $init:expr) => {{
-        #[cfg_attr(not(windows), inline)] // see comments below
+        #[cfg_attr(not(bootstrap), inline)]
         #[deny(unsafe_op_in_unsafe_fn)]
         unsafe fn __getit(
             _init: $crate::option::Option<&mut $crate::option::Option<$t>>,
@@ -49,29 +49,7 @@ macro_rules! __thread_local_inner {
             #[inline]
             fn __init() -> $t { $init }
 
-            // When reading this function you might ask "why is this inlined
-            // everywhere other than Windows?", and that's a very reasonable
-            // question to ask. The short story is that it segfaults rustc if
-            // this function is inlined. The longer story is that Windows looks
-            // to not support `extern` references to thread locals across DLL
-            // boundaries. This appears to at least not be supported in the ABI
-            // that LLVM implements.
-            //
-            // Because of this we never inline on Windows, but we do inline on
-            // other platforms (where external references to thread locals
-            // across DLLs are supported). A better fix for this would be to
-            // inline this function on Windows, but only for "statically linked"
-            // components. For example if two separately compiled rlibs end up
-            // getting linked into a DLL then it's fine to inline this function
-            // across that boundary. It's only not fine to inline this function
-            // across a DLL boundary. Unfortunately rustc doesn't currently
-            // have this sort of logic available in an attribute, and it's not
-            // clear that rustc is even equipped to answer this (it's more of a
-            // Cargo question kinda). This means that, unfortunately, Windows
-            // gets the pessimistic path for now where it's never inlined.
-            //
-            // The issue of "should enable on Windows sometimes" is #84933
-            #[cfg_attr(not(windows), inline)]
+            #[cfg_attr(not(bootstrap), inline)]
             unsafe fn __getit(
                 init: $crate::option::Option<&mut $crate::option::Option<$t>>,
             ) -> $crate::option::Option<&'static $t> {