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From: "paulmckrcu at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/96327] Inefficient increment through pointer to volatile on x86
Date: Mon, 27 Jul 2020 01:56:18 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D96327

--- Comment #1 from Paul McKenney <paulmckrcu at gmail dot com> ---
This manifests on GCC trunk (see the godbolt.org URL), but was first noted =
in
gcc version 7.5.0.  This is specific to x86, but might apply to any other
architecture that provides increment-memory instructions.  This behavior do=
es
not seem to be affected by GCC options.

This can be reproduced by placing the sample code in a file "rrl.c" and
running:

cc -o rrl rrl.c

This completes successfully with no error or warnings.

Running "cc -o rrl rrl.c --save-temps" generates the following file:

# 1 "rrl.c"
# 1 "<built-in>"
# 1 "<command-line>"
# 31 "<command-line>"
# 1 "/usr/include/stdc-predef.h" 1 3 4
# 32 "<command-line>" 2
# 1 "rrl.c"
struct task {
    int other;
    int rcu_count;
};

struct task *current;

void rcu_read_lock()
{
    (*(volatile int*)&current->rcu_count)++;
}=