From 9173e457bed6af0d058e9f338af31a716a415f83 Mon Sep 17 00:00:00 2001
From: Alexey Spiridonov <lesha@meta.com>
Date: Wed, 30 Oct 2024 03:53:43 -0700
Subject: [PATCH] `if constexpr` shortcut for `CancellationToken::merge(x)`

Summary: This takes the `merge1` microbench (stacked) from 74ns to 24ns without regressing anything. This is helpful for generic programs, where the pack MIGHT have 1 element, and there's no reason to shortcut this on the client side.

Reviewed By: Orvid

Differential Revision: D65044520

fbshipit-source-id: e2f91bbc69439d5759d79d8a5391f5ba3e0239ed
---
 folly/CancellationToken-inl.h        | 17 +++++++++++------
 folly/test/CancellationTokenTest.cpp |  6 ++++++
 2 files changed, 17 insertions(+), 6 deletions(-)

diff --git a/folly/CancellationToken-inl.h b/folly/CancellationToken-inl.h
index 9d4ffad7d66..f28720b7f91 100644
--- a/folly/CancellationToken-inl.h
+++ b/folly/CancellationToken-inl.h
@@ -448,12 +448,17 @@ std::pair<CancellationSource, std::tuple<Data...>*> CancellationSource::create(
 
 template <typename... Ts>
 inline CancellationToken CancellationToken::merge(Ts&&... tokens) {
-  bool canBeCancelled = detail::variadicDisjunction(tokens.canBeCancelled()...);
-  return canBeCancelled
-      ? CancellationToken(
-            detail::FixedMergingCancellationState<sizeof...(Ts)>::create(
-                std::forward<Ts>(tokens)...))
-      : CancellationToken();
+  if constexpr (sizeof...(Ts) == 1) {
+    return (tokens, ...);
+  } else {
+    bool canBeCancelled =
+        detail::variadicDisjunction(tokens.canBeCancelled()...);
+    return canBeCancelled
+        ? CancellationToken(
+              detail::FixedMergingCancellationState<sizeof...(Ts)>::create(
+                  std::forward<Ts>(tokens)...))
+        : CancellationToken();
+  }
 }
 
 } // namespace folly
diff --git a/folly/test/CancellationTokenTest.cpp b/folly/test/CancellationTokenTest.cpp
index 8a058c9c1ca..e76394b23b4 100644
--- a/folly/test/CancellationTokenTest.cpp
+++ b/folly/test/CancellationTokenTest.cpp
@@ -293,6 +293,12 @@ TEST(CancellationTokenTest, MergedToken) {
   EXPECT_FALSE(token.canBeCancelled());
 }
 
+TEST(CancellationTokenTest, Merging1TokenIsEfficient) {
+  CancellationSource src;
+  CancellationToken tok = src.getToken();
+  EXPECT_TRUE(tok == CancellationToken::merge(tok));
+}
+
 TEST(CancellationTokenTest, TokenWithData) {
   struct Guard {
     int& counter;