[Bug c/39403] Excessive optimization issue

[casmyu at gmail dot com]

--- Comment #3 from casmyu at gmail dot com  2009-03-10 01:23 ---
Hi:
I have changed the code as you said, but it is more error now.

  1 #include stdio.h
  2
  3 int main()
  4 {
  5 char src[30] = {This is a test message.\n};
  6 char dst[30];
  7 int len = 25;
  8
  9 __asm__ __volatile__(
 10 cld\n\t
 11 rep movsb
 12 :
 13 //  : (src), D(dst), c(len)
 14 : +c(len), +D(dst), +S(src)
 15 );
 16 printf(%s\t%d\n, dst, len);
 17 return 0;
 18 }

[root][~]# gcc -O2 -o bugtest bugtest.c
bugtest.c: In function ¡®main¡¯:
bugtest.c:15: error: input operand constraint contains ¡®+¡¯
bugtest.c:15: error: input operand constraint contains ¡®+¡¯
bugtest.c:15: error: input operand constraint contains ¡®+¡¯
bugtest.c:9: error: input operand constraint contains ¡®+¡¯
bugtest.c:9: error: input operand constraint contains ¡®+¡¯
bugtest.c:9: error: input operand constraint contains ¡®+¡¯

Please help to re-check this issue.
Thank you!

(In reply to comment #2)
 Subject: Re:  Excessive optimization issue
 
 
 
 Sent from my iPhone
 
 On Mar 9, 2009, at 8:36 AM, rguenth at gcc dot gnu dot org
 gcc-bugzi...@gcc.gnu.org 
   wrote:
 
 
 
  --- Comment #1 from rguenth at gcc dot gnu dot org  2009-03-09  
  15:36 ---
  You need to specify that the registers are clobbered by the asm.   
  The only
  way to do that is to use output constraints (+D, +c, etc.) on  
  proper
  temporaries.
 
   int lent = len;
   char *dstt = dst;
   char *srct = src;
   __asm__ __volatile__(
cld\n\t
rep movsb
: +c (lent), +D(dstt), +S(src)
   );
  Otherwise GCC thinks the registers still hold the original value.
 
 Oh and mark this inline-ask as clobbering memory.
 
 
 
 
  -- 
 
  rguenth at gcc dot gnu dot org changed:
 
What|Removed |Added
  --- 
  --- 
  --
  Status|UNCONFIRMED |RESOLVED
  Resolution||INVALID
 
 
  http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39403
 
 


-- 

casmyu at gmail dot com changed:

   What|Removed |Added

 CC||casmyu at gmail dot com
 Status|RESOLVED|UNCONFIRMED
 Resolution|INVALID |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39403

[Bug c/39403] New: Excessive optimization issue

[casmyu at gmail dot com]

My OS and compiler information as follows:
[root][~]# uname -a
Linux debian 2.6.26-1-686 #1 SMP Sat Jan 10 18:29:31 UTC 2009 i686 GNU/Linux
[root][~]# gcc --version
gcc (Debian 4.3.2-1.1) 4.3.2
Copyright (C) 2008 Free Software Foundation, Inc.

A test C code with inline assembly language as follows:
  1 #include stdio.h
  2
  3 int main()
  4 {
  5 char src[30] = {This is a test message.\n};
  6 char dst[30];
  7 int len = 25;
  8
  9 __asm__ __volatile__(
 10 cld\n\t
 11 rep movsb
 12 :
 13 : S(src), D(dst), c(len)
 14 );
 15 printf(%s\t%d\n, dst, len);
 16 return 0;
 17 }

compile the code like this:
[root][~]# gcc -O2 -o bugtest bugtest.c

then run the program, this issue will be re-produced.

I disassembled the binary and found that, when add O2 switch, before the
printf() function invoked, the edi register will be push into the stack instead
of the address of output. So actually, the output of printf is part of the
dst's tail and all the src, not the dst string.
But if did not set O2 switch, this issue will be disappeared.
So I think this is an excessive optimization issue of gcc.
Thank you!


-- 
   Summary: Excessive optimization issue
   Product: gcc
   Version: 4.3.2
Status: UNCONFIRMED
  Severity: normal
  Priority: P3
 Component: c
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: casmyu at gmail dot com
 GCC build triplet: gcc (Debian 4.3.2-1.1) 4.3.2
  GCC host triplet: Debian 5.0
GCC target triplet: Debian 5.0


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39403