chenBright commented on code in PR #3713:
URL: https://github.com/apache/brpc/pull/3713#discussion_r4481374858
##########
src/butil/thread_local.h:
##########
@@ -29,6 +30,34 @@
#define BAIDU_THREAD_LOCAL __thread
#endif // _MSC_VER
+// Define a pthread-local variable accessed through name():
+// BUTIL_THREAD_LOCAL(int, counter, 0);
+// ++counter();
+// counter() = 10;
+// Prefix with static for internal linkage or a static class member, or inline
+// for a definition in a header. A declaration in another translation unit is
+// simply `type& name();`. Use a type alias for arrays or types containing
commas.
+//
+// A bthread can resume on a different pthread. Keep TLS address resolution in
+// a non-inlined function so the compiler cannot hoist it across a suspension.
+// The opaque asm output also prevents interprocedural optimization (including
+// LTO) from treating the returned address as invariant. A memory barrier
alone,
+// or an inline function taking &tls as its argument, does not achieve this.
+//
+// The returned reference/pointer is only valid for the current pthread: never
+// retain it across a call that may suspend. In particular, evaluate a possibly
+// suspending RHS before assigning it to name(). Initializers must not suspend.
+// Constant-initialized trivial types have no dynamic initialization/exit hook.
+#define BUTIL_THREAD_LOCAL(type, name, ...) \
+ __attribute__((noinline, unused)) type& name() { \
+ static thread_local type tls_value = __VA_ARGS__; \
+ type* tls_ptr = std::addressof(tls_value); \
+ asm volatile("" : "+r"(tls_ptr) : : "memory"); \
+ return *tls_ptr; \
+ }
Review Comment:
Windows/MSVC support is outside the scope of this PR. The implementation
targets GCC and Clang, consistent with brpc's supported build configurations.
We will keep the current implementation.
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