author: Andres Salomon <dilinger@debian.org>
description: revert compile-time lockfree check; switch back to a runtime check.

This reverts the following:

"commit ce8ce6f81eff8a84e2ea59930cb995b1107181e2 (origin/chromium/5572)
Author: Kimmo Kinnunen <kkinnunen@apple.com>
Date:   Tue Jan 31 13:06:47 2023 +0200

    Remove ASSERT from AtomicSerial, use static_assert
    
    ASSERT is not constexpr, while the AtomicSerial constructor was.
    This would cause compile errors.
    
    Bug: angleproject:7989
    Change-Id: Ib6a438d4c055378d4f2f667285b0d2e99f2522ad
    Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/4205892
"

The problem is that this is a compile-time check to ensure that a std::atomic<uint64_t>
doesn't require a mutex, but for i386 we can't guarantee that; instead, i386 checks
at runtime for the relevant CPU instructions. So we need to revert back to a runtime
assertion.

Since this is in ANGLE rather than Chromium, hopefully there's a chance they'll take a
fix upstream; probably a revert combined with dropping the "constexpr".

--- a/third_party/angle/src/libANGLE/renderer/serial_utils.h
+++ b/third_party/angle/src/libANGLE/renderer/serial_utils.h
@@ -119,6 +119,7 @@ class Serial final
 class AtomicQueueSerial final
 {
   public:
+    constexpr AtomicQueueSerial() : mValue(kInvalid) { ASSERT(mValue.is_lock_free()); }
     AtomicQueueSerial &operator=(const Serial &other)
     {
         mValue.store(other.mValue, std::memory_order_release);
@@ -127,9 +128,8 @@ class AtomicQueueSerial final
     Serial getSerial() const { return Serial(mValue.load(std::memory_order_consume)); }
 
   private:
+    std::atomic<uint64_t> mValue;
     static constexpr uint64_t kInvalid = 0;
-    std::atomic<uint64_t> mValue       = kInvalid;
-    static_assert(decltype(mValue)::is_always_lock_free, "Must always be lock free");
 };
 
 // Used as default/initial serial