From 419c423d3aeca754e47e1ce1bf707735603a90a3 Mon Sep 17 00:00:00 2001 From: Vladimir Palevich Date: Wed, 9 Aug 2023 01:34:05 +0300 Subject: libstdc++: fix memory clobbering in std::vector [PR110879] Fix ordering to prevent clobbering of class members by a call to deallocate in _M_realloc_insert and _M_default_append. Because of recent changes in _M_realloc_insert and _M_default_append, calls to deallocate were ordered after assignment to class members of std::vector (in the guard destructor), which is causing said members to be call-clobbered. This is preventing further optimization, the compiler is unable to move memory read out of a hot loop in this case. This patch reorders the call to before assignments by putting guard in its own block. Plus a new testsuite for this case. I'm not very happy with the new testsuite, but I don't know how to properly test this. PR libstdc++/110879 libstdc++-v3/ChangeLog: * include/bits/vector.tcc (_M_realloc_insert): End guard lifetime just before assignment to class members. (_M_default_append): Likewise. gcc/testsuite/ChangeLog: * g++.dg/pr110879.C: New test. Signed-off-by: Vladimir Palevich --- gcc/testsuite/g++.dg/pr110879.C | 16 ++++++++++++++++ 1 file changed, 16 insertions(+) create mode 100644 gcc/testsuite/g++.dg/pr110879.C (limited to 'gcc') diff --git a/gcc/testsuite/g++.dg/pr110879.C b/gcc/testsuite/g++.dg/pr110879.C new file mode 100644 index 0000000..7f0a0a8 --- /dev/null +++ b/gcc/testsuite/g++.dg/pr110879.C @@ -0,0 +1,16 @@ +// { dg-do compile } +// { dg-options "-O3 -fdump-tree-optimized" } + +#include + +std::vector f(std::size_t n) { + std::vector res; + for (std::size_t i = 0; i < n; ++i) { + res.push_back(i); + } + return res; +} + +// Reads of _M_finish should be optimized out. +// This regex matches all reads from res variable except for _M_end_of_storage field. +// { dg-final { scan-tree-dump-not "=\\s*\\S*res_(?!\\S*_M_end_of_storage;)" "optimized" } } -- cgit v1.1