From: waffl3x <waffl3x@protonmail.com>
To: "libstdc++@gcc.gnu.org" <libstdc++@gcc.gnu.org>
Subject: minor optimization bug in basic_string move assignment
Date: Tue, 03 Jan 2023 23:11:53 +0000 [thread overview]
Message-ID: <G9uH4h4S1h6p8JjtjxWOwtv0Z9UpcZGEsH_6Fw-Z6yM-Io23_8UuINXojQh3EfgNMqzHMytO5gGql_xOom_UouW4nzs0tC3h2T73H2_aIKM=@protonmail.com> (raw)
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Example: https://godbolt.org/z/sKhGqG1qK
https://gcc.gnu.org/git/?p=gcc.git;a=blob;f=libstdc%2B%2B-v3/include/bits/basic_string.h;hb=HEAD#l880
When move assigning to a basic_string, the allocated memory of the moved into string is stored into the source string instead of deallocating it, a good optimization when everything is compatible. However in the case of a stateful allocator (is_always_true() evaluating as false) this optimization is never taken. Unless there is some reason I can't think of that makes equal stateful allocators incompatible here, I believe the if statement on line 880 of basic_string.h should also compare the equality of each strings allocator. The first condition in the function seems to indicate to me that this scenario was being considered and just forgotten about, as the memory doesn't get deallocated immediately if the two allocators are equal. I'll note that because of how everything is handled, this doesn't result in a leak so this bug is still only a minor missed optimization.
mailto:libstdc++@gcc.gnu.org
next reply other threads:[~2023-01-03 23:12 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-01-03 23:11 waffl3x [this message]
2023-01-04 18:20 ` François Dumont
2023-01-17 19:18 ` Jonathan Wakely
2023-01-25 18:38 ` [PATCH] " François Dumont
2023-02-03 14:50 ` Jonathan Wakely
2023-02-04 13:11 ` François Dumont
2023-02-04 13:30 ` Jonathan Wakely
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