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From: "H. Peter Anvin" <hpa@zytor.com>
To: Jamie Lokier <jamie@shareable.org>
Cc: 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 21:37:33 -0800	[thread overview]
Message-ID: <3FD55F9D.9070203@zytor.com> (raw)
In-Reply-To: <20031209034935.GA26987@mail.shareable.org>

Jamie Lokier wrote:
> H. Peter Anvin wrote:
> 
>>>It would be nice to have a way to declare an asm like "pure" not
>>>"const", so that it's allowed to read memory but multiple calls can be
>>>eliminated; I don't know of a way to express that.
>>
>>Just specify memory input operands.
> 
> 
> Thanks.  That's even more useful than "pure" because it implies the
> asm only reads the explicitly passed memory operands.
> 
> Memory input operands don't work if you want the asm to read arbitrary
> memory not mentioned in the inputs (like "pure" allows) or traverse
> linked lists.
> 
> (A long time ago there was a question about whether GCC could ever
> copy the value associated with an "m" operand to a stack slot, and
> pass the address of the stack slot.  After all, GCC _will_ copy the
> value if the operand is an "r", and presumably gives mixed results
> with "rm".  We seem to have concluded that it never will).
> 

Sure it will:


int foo(int x)
{
   int y, z;

   asm("movl %1,%0" : "=r" (y) : "r" (x));
   asm("movl %1,%0" : "=r" (z) : "m" (y));

   return z;
}

: smyrno 21 ; gcc -O2 -c testme.c
: smyrno 22 ; objdump -dr testme.o

testme.o:     file format elf32-i386

Disassembly of section .text:

00000000 <foo>:
    0:   55                      push   %ebp			; Make stack frame
    1:   89 e5                   mov    %esp,%ebp		; d:o
    3:   50                      push   %eax			; Allocate stack slot
    4:   8b 45 08                mov    0x8(%ebp),%eax		; Copy to register
    7:   89 c0                   mov    %eax,%eax		; First asm()
    9:   89 45 fc                mov    %eax,0xfffffffc(%ebp) 	; Copy to stack slot
    c:   8b 45 fc                mov    0xfffffffc(%ebp),%eax	; Second asm()
    f:   c9                      leave				; Destroy stack frame
   10:   c3                      ret



  reply	other threads:[~2003-12-09  5:37 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 [this message]
2003-12-09  7:26               ` Linus Torvalds
2003-12-09  7:40                 ` H. Peter Anvin
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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