From e205755d5eacbd1e0fa38ae4f68e95f0b8c34d98 Mon Sep 17 00:00:00 2001 From: Jason Volk Date: Mon, 7 May 2018 12:38:24 -0700 Subject: [PATCH] ircd::ctx: Add ctx::critical_indicator. --- include/ircd/ctx/ctx.h | 1 + include/ircd/ctx/this_ctx.h | 22 ++++++++++++++++++++++ 2 files changed, 23 insertions(+) diff --git a/include/ircd/ctx/ctx.h b/include/ircd/ctx/ctx.h index 0cce5cc8e..7b501d452 100644 --- a/include/ircd/ctx/ctx.h +++ b/include/ircd/ctx/ctx.h @@ -105,4 +105,5 @@ namespace ircd using ctx::use_future; using ctx::critical_assertion; + using ctx::critical_indicator; } diff --git a/include/ircd/ctx/this_ctx.h b/include/ircd/ctx/this_ctx.h index f09ceb455..72ce6ca72 100644 --- a/include/ircd/ctx/this_ctx.h +++ b/include/ircd/ctx/this_ctx.h @@ -15,6 +15,7 @@ namespace ircd::ctx { /// Interface to the currently running context inline namespace this_ctx { + struct critical_indicator; // Indicates if yielding happened for a section struct critical_assertion; // Assert no yielding for a section struct exception_handler; // Must be present to yield in a handler @@ -91,6 +92,27 @@ class ircd::ctx::this_ctx::critical_assertion #endif }; +/// An instance of critical_indicator reports if context switching happened. +/// +/// A critical_indicator remains true after construction until a context switch +/// has occurred. It then becomes false. This is not an assertion and is +/// available in optimized builds for real use. For example, a context may +/// want to recompute some value after a context switch and opportunistically +/// skip this effort when this indicator shows no switch occurred. +/// +class ircd::ctx::this_ctx::critical_indicator +{ + uint64_t state; + + public: + uint64_t count() const { return yields(cur()) - state; } + operator bool() const { return yields(cur()) == state; } + + critical_indicator() + :state{yields(cur())} + {} +}; + /// An instance of exception_handler must be present to allow a context /// switch inside a catch block. This is due to ABI limitations that stack /// exceptions with thread-local assumptions and don't expect catch blocks