From: "H. Peter Anvin" <hpa@zytor.com>
To: Linus Torvalds <torvalds@osdl.org>
Cc: Jamie Lokier <jamie@shareable.org>,
Nikita Danilov <Nikita@Namesys.COM>,
Arnd Bergmann <arnd@arndb.de>,
linux-kernel@vger.kernel.org
Subject: Re: const versus __attribute__((const))
Date: Mon, 08 Dec 2003 23:40:39 -0800 [thread overview]
Message-ID: <3FD57C77.4000403@zytor.com> (raw)
In-Reply-To: <Pine.LNX.4.58.0312082321470.18255@home.osdl.org>
Linus Torvalds wrote:
>
> And as far as I know, it won't. Newer gcc's will _require_ memory
> arguments to be lvalues (well, it will warn if they aren't), and will
> always turn them into addressables of that particular lvalue.
>
Well, I get no warning from the following code, with gcc 3.2.2 and -W -Wall:
int foo(int x)
{
int y, z, w;
asm("movl %1,%0" : "=r" (y) : "r" (x));
asm("movl %1,%0" : "=r" (z) : "m" (y+1));
asm("movl %1,%0" : "=r" (w) : "m" (33));
return z+w;
}
The memory operand is definitely not an lvalue. gcc allocates a stack slot
for the y+1 value as a temporary automatic and passes it in. The value 33
is put in the .rodata segment:
00000000 <foo>:
0: 55 push %ebp
1: 89 e5 mov %esp,%ebp
3: 50 push %eax
4: 8b 45 08 mov 0x8(%ebp),%eax
7: 89 c0 mov %eax,%eax
9: 40 inc %eax
a: 89 45 fc mov %eax,0xfffffffc(%ebp)
d: 8b 15 00 00 00 00 mov 0x0,%edx
f: R_386_32 .rodata.cst4
13: 8b 45 fc mov 0xfffffffc(%ebp),%eax
16: 01 d0 add %edx,%eax
18: c9 leave
19: c3 ret
> This is actually an issue, because some asm code depends on getting the
> proper address - not just the proper value. Things like locks etc care
> _deeply_ about more than just the value.
That would be a very bad thing indeed. Fortunately I think it would take
some rather odd contortions on the part of the compiler for that situation
to occur at all, I would think, and if the gcc people have since been
made aware of it it should be pretty easy for them to make sure it will not
happen.
-hpa
next prev parent reply other threads:[~2003-12-09 7:41 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2003-12-08 15:46 const versus __attribute__((const)) Arnd Bergmann
2003-12-08 17:50 ` H. Peter Anvin
2003-12-08 18:27 ` Nikita Danilov
2003-12-08 18:31 ` H. Peter Anvin
2003-12-09 2:59 ` Jamie Lokier
2003-12-09 3:21 ` H. Peter Anvin
2003-12-09 3:49 ` Jamie Lokier
2003-12-09 5:37 ` H. Peter Anvin
2003-12-09 7:26 ` Linus Torvalds
2003-12-09 7:40 ` H. Peter Anvin [this message]
2003-12-09 11:56 ` Arnd Bergmann
2003-12-09 15:42 ` H. Peter Anvin
2003-12-09 16:44 ` Linus Torvalds
2003-12-09 16:51 ` H. Peter Anvin
2003-12-09 19:15 ` Jamie Lokier
2003-12-09 7:19 ` Linus Torvalds
-- strict thread matches above, loose matches on Subject: below --
2003-12-08 1:19 H. Peter Anvin
2003-12-08 12:32 ` Nikita Danilov
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