[libcxx-commits] [PATCH] D72704: Optimize basic_string::operator=(const basic_string&) for SSO assignments

[Martijn Vels via Phabricator via libcxx-commits]

mvels created this revision.
Herald added subscribers: libcxx-commits, christof.
Herald added a project: libc++.
mvels added reviewers: EricWF, mclow.lists.

This change optimizes the operator=() assignment for short strings by direcly copying the raw data from the source into the current instance. This creates an optimized / inlined mempcy up to over 2X faster for short string assignments. With inlining enabled for operator=, performance is up to 6X faster.

Benchmarks 'as is':
name                                    old time/op   new time/op    delta
BM_StringAssignStr_Empty_Opaque         6.05ns ± 2%   3.59ns ± 0%  -40.67%
BM_StringAssignStr_Empty_Transparent    5.15ns ± 0%   3.08ns ± 0%  -40.12%
BM_StringAssignStr_Small_Opaque         7.71ns ± 0%   3.59ns ± 0%  -53.45%
BM_StringAssignStr_Small_Transparent    7.66ns ± 0%   3.09ns ± 0%  -59.66%
BM_StringAssignStr_Large_Opaque         24.1ns ± 0%   24.9ns ± 0%   +3.22%
BM_StringAssignStr_Large_Transparent    22.2ns ± 0%   22.8ns ± 0%   +2.77%
BM_StringAssignStr_Huge_Opaque           315ns ± 6%    320ns ± 5%     ~
BM_StringAssignStr_Huge_Transparent      318ns ± 5%    321ns ± 4%     ~

Benchmarks with partial inlining operator=():
name                                    old time/op   new time/op    delta
BM_StringAssignStr_Empty_Opaque         5.94ns ± 2%   1.95ns ± 0%  -67.21%
BM_StringAssignStr_Empty_Transparent    5.14ns ± 0%   1.04ns ± 1%  -79.73%
BM_StringAssignStr_Small_Opaque         7.69ns ± 0%   1.96ns ± 0%  -74.48%
BM_StringAssignStr_Small_Transparent    7.65ns ± 0%   1.04ns ± 0%  -86.40%
BM_StringAssignStr_Large_Opaque         24.1ns ± 0%   24.5ns ± 0%   +1.61%
BM_StringAssignStr_Large_Transparent    22.2ns ± 0%   21.1ns ± 0%   -4.70%
BM_StringAssignStr_Huge_Opaque           317ns ± 5%    323ns ± 4%     ~
BM_StringAssignStr_Huge_Transparent      318ns ± 5%    320ns ± 5%     ~


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D72704

Files:
  libcxx/include/string


Index: libcxx/include/string
===================================================================
--- libcxx/include/string
+++ libcxx/include/string
@@ -2260,7 +2260,14 @@
     if (this != &__str)
     {
         __copy_assign_alloc(__str);
-        return assign(__str.data(), __str.size());
+        if (__str.__is_long()) {
+            return __assign(__str.data(), __str.size());
+        }
+        if (__is_long()) {
+            __alloc_traits::deallocate(__alloc(), __get_long_pointer(),
+                                       __get_long_cap());
+        }
+        __r_.first().__r = __str.__r_.first().__r;
     }
     return *this;
 }


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