[PATCH 0/1] MR7895: Revert "kernel32/tests: Don't use _ReadWriteBarrier on ARM platforms."

[Jinoh Kang (＠iamahuman)]

This reverts commit 504a305a60849704f2d83b55e9aabb26d89edf11.

The choice of weak barrier was by design. The stronger barrier broke the
positive half of the litmus test for FlushProcessWriteBuffers():
> virtual.c:4568: Test failed: expected write-read reordering with compiler barrier
>  only (got 0 reorderings)

FlushProcessWriteBuffers() is used in tandem with a compiler-only
barrier to implement fast synchronization, where only one side is
executed frequently and the other side only occassionally (e.g., garbage
collection).  Prominent applications include .NET and HotSpot JVM.

Although _ReadWriteBarrier() is "marked deprecated," it is in fact used
by Microsoft's own STL library (with explicit suppression of warning) to
implement a compiler-only memory barrier[^1].  The deprecation notice
suggests C++11 atomics as the (only?) alternative.

[^1]: https://github.com/microsoft/STL/blob/b5df16a98934319e2e6864d6036cbe9cd9c74f...

[PATCH 1/1] Revert "kernel32/tests: Don't use _ReadWriteBarrier on ARM platforms."

[Jinoh Kang]

From: Jinoh Kang

This reverts commit 504a305a60849704f2d83b55e9aabb26d89edf11.

The choice of weak barrier was by design. The stronger barrier broke the
positive half of the litmus test for FlushProcessWriteBuffers():
> virtual.c:4568: Test failed: expected write-read reordering with compiler barrier
>  only (got 0 reorderings)

FlushProcessWriteBuffers() is used in tandem with a compiler-only
barrier to implement fast synchronization, where only one side is
executed frequently and the other side only occassionally (e.g., garbage
collection).  Prominent applications include .NET and HotSpot JVM.

Although _ReadWriteBarrier() is "marked deprecated," it is in fact used
by Microsoft's own STL library (with explicit suppression of warning) to
implement a compiler-only memory barrier [1].  The deprecation notice
suggests C++11 atomics as the (only?) alternative.

[1]: https://github.com/microsoft/STL/blob/b5df16a98934319e2e6864d6036cbe9cd9c74f...
---
 dlls/kernel32/tests/virtual.c | 9 +++------
 1 file changed, 3 insertions(+), 6 deletions(-)

diff --git a/dlls/kernel32/tests/virtual.c b/dlls/kernel32/tests/virtual.c
index b83b32dc88c..1c84acfc215 100644
--- a/dlls/kernel32/tests/virtual.c
+++ b/dlls/kernel32/tests/virtual.c
@@ -4469,16 +4469,13 @@ static DWORD CALLBACK sbtest_thread_proc( void *arg )

#ifdef _MSC_VER

+#pragma intrinsic(_ReadWriteBarrier)
+void _ReadWriteBarrier(void);
+
 static void WINAPI compiler_barrier(void)
 {
-#if defined(__i386__) || defined(__x86_64__)
 #pragma warning(suppress:4996)
     _ReadWriteBarrier();
-#elif defined(__arm__)
-    __dmb(_ARM_BARRIER_ISH);
-#elif defined(__aarch64__)
-    __dmb(_ARM64_BARRIER_ISH);
-#endif
 }

#else  /* _MSC_VER */

[Alexandre Julliard (＠julliard)]

> Although _ReadWriteBarrier() is "marked deprecated," it is in fact used by Microsoft's own STL library (with explicit suppression of warning) to implement a compiler-only memory barrier 1. The deprecation notice suggests C++11 atomics as the (only?) alternative.

It breaks the build in MSVC mode on ARM, so that would need to be fixed some other way.

[Jinoh Kang (＠iamahuman)]

On Sat Apr 26 00:51:02 2025 +0000, Alexandre Julliard wrote:
> > Although _ReadWriteBarrier() is "marked deprecated," it is in fact
>
> used by Microsoft's own STL library (with explicit suppression of
> warning) to implement a compiler-only memory barrier 1. The deprecation
> notice suggests C++11 atomics as the (only?) alternative.
> It breaks the build in MSVC mode on ARM, so that would need to be fixed
> some other way.

Sorry. I cannot reproduce this error on my side using this MSVC on wine:

```
Microsoft (R) C/C++ Optimizing Compiler Version 19.43.34810 for ARM
Copyright (C) Microsoft Corporation.  All rights reserved.
```

Do you have an error log for this? Thanks.

[Jacek Caban (＠jacek)]

On Sat Apr 26 00:51:02 2025 +0000, Jinoh Kang wrote:
> Sorry. I cannot reproduce this error on my side using this MSVC on wine:
> Microsoft (R) C/C++ Optimizing Compiler Version 19.43.34810 for ARM
> Copyright (C) Microsoft Corporation.  All rights reserved.
>
> Do you have an error log for this? Thanks.

It's about Clang's MSVC mode, try `configure --with-mingw=clang`.