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From: "marc.glisse at normalesup dot org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug rtl-optimization/50677] New: volatile forces load into register
Date: Sun, 09 Oct 2011 12:52:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50677

             Bug #: 50677
           Summary: volatile forces load into register
    Classification: Unclassified
           Product: gcc
           Version: 4.7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: marc.glisse@normalesup.org
              Host: x86_64-linux-gnu


Compiling this simple program (-Ofast):

void f(int volatile*i){++*i;}

produces this code:

    movl    (%rdi), %eax
    addl    $1, %eax
    movl    %eax, (%rdi)

(or incl %eax for the central line with -Os).

However, if I remove "volatile", I get the nicer:

    addl    $1, (%rdi)

(or incl (%rdi) with -Os).

The second version seems legal to me even in the volatile case, is that wrong?

There might be a relation to this thread:
http://gcc.gnu.org/ml/gcc/2011-10/msg00006.html
(no volatile there, but a failure to fuse load+add+store)

This is particularly noticable because people (wrongly) use volatile for
threaded code and the 3 instruction version is likely even more racy than the
one with a single instruction.

(sorry if the category is wrong, I just picked one with "optimization" in the
name...)