ircd::ctx: Add noexcept specifications; tweak inlines.

include/ircd/ctx/prof.h

@@ -32,7 +32,7 @@ namespace ircd::ctx::prof
 	enum class event :uint8_t;
 	struct ticker;
 
-	uint64_t cycles() noexcept;
+	ulong cycles() noexcept;
 	string_view reflect(const event &);
 
 	// totals
@@ -92,14 +92,28 @@ struct ircd::ctx::prof::ticker
 	std::array<uint64_t, num_of<prof::event>()> event {{0}};
 };
 
-inline uint64_t
+/// Calculate the current reference cycle count (TSC) for the current
+/// execution epoch/slice. This involves one RDTSC sample which is provided
+/// by ircd::prof/ircd::prof::x86 (or for some other platform), and then
+/// subtracting away the prior RDTSC sample which the context system makes
+/// at the start of each execution slice.
+extern inline ulong
+__attribute__((flatten, always_inline, gnu_inline, artificial))
 ircd::ctx::prof::cur_slice_cycles()
 noexcept
 {
-    return cycles() - cur_slice_start();
+	const auto stop(cycles());
+	const auto start(cur_slice_start());
+    return stop - start;
 }
 
-extern inline uint64_t
+/// Sample the current TSC directly with an rdtsc; this is a convenience
+/// wrapper leading to the ircd::prof interface which contains the platform
+/// specific methods to efficiently sample a cycle count.
+///
+/// Developers are advised to obtain cycle counts from this_ctx::cycles()
+/// which accumulates the cycle count for a specific ctx's execution only.
+extern inline ulong
 __attribute__((flatten, always_inline, gnu_inline, artificial))
 ircd::ctx::prof::cycles()
 noexcept

include/ircd/ctx/this_ctx.h

@@ -16,14 +16,12 @@ namespace ircd::ctx {
 inline namespace this_ctx
 {
 	struct ctx &cur() noexcept;                  ///< Assumptional reference to *current
-	const uint64_t &id();                        // Unique ID for cur ctx
+	const uint64_t &id() noexcept;               // Unique ID for cur ctx
-	string_view name();                          // Optional label for cur ctx
+	string_view name() noexcept;                 // Optional label for cur ctx
-
 	ulong cycles() noexcept;                     // misc profiling related
-
 	bool interruption_requested() noexcept;      // interruption(cur())
-	void interruption_point();                   // throws if interruption_requested()
 
+	void interruption_point();                   // throws if interruption_requested()
 	void wait();                                 // Returns when context is woken up.
 	void yield();                                // Allow other contexts to run before returning.
 
@@ -131,16 +129,35 @@ ircd::ctx::this_ctx::wait(const duration &d)
 	return duration_cast<duration>(ret);
 }
 
-inline ulong
+/// View the name of the currently running context, or "*" if no context is
+/// currently running.
+inline ircd::string_view
+ircd::ctx::this_ctx::name()
+noexcept
+{
+	return current? name(cur()) : "*"_sv;
+}
+
+/// Calculate the current TSC (reference cycle count) accumulated for this
+/// context only. This is done by first calculating a cycle count for the
+/// current slice/epoch (see: ctx/prof.h) which is where the RDTSC sample
+/// occurs. This count is added to an accumulator value saved in the ctx
+/// structure. The accumulator value is updated at the end of each execution
+/// slice, thus giving us the cycle count for this ctx only, up to this point.
+extern inline ulong
+__attribute__((flatten, always_inline, gnu_inline, artificial))
 ircd::ctx::this_ctx::cycles()
 noexcept
 {
-	return cycles(cur()) + prof::cur_slice_cycles();
+	const auto slice(prof::cur_slice_cycles());
+	const auto accumulated(cycles(cur()));
+	return accumulated + slice;
 }
 
 /// Reference to the currently running context. Call if you expect to be in a
 /// context. Otherwise use the ctx::current pointer.
 inline ircd::ctx::ctx &
+__attribute__((always_inline))
 ircd::ctx::this_ctx::cur()
 noexcept
 {

ircd/ctx.cc

@@ -738,19 +738,12 @@ noexcept
 /// Returns unique ID of currently running context
 const uint64_t &
 ircd::ctx::this_ctx::id()
+noexcept
 {
 	static const uint64_t zero{0};
 	return current? id(cur()) : zero;
 }
 
-/// Returns optional developer-given name for currently running context
-ircd::string_view
-ircd::ctx::this_ctx::name()
-{
-	static const string_view nada{"*"};
-	return current? name(cur()) : nada;
-}
-
 //
 // uinterruptible
 //