Sorry to be so late in joining this discussion. I'm the person who
originally notified Mark Mitchell about Microsoft's compiler performing
this same optimization under certain conditions. Since mailing Mark on
the subject I tried also VC++ 2008 and it behaves exactly like its
predecessor. Here's my test program, directly derived from Mark's test case:
int f(char *buf, int len) {
len = 1 << 30;
if (buf + len < buf)
return 1;
return 0;
}
int main()
{
char * b = "0123456789";
for ( int l = 0; l < 1 << 30; ++l )
f(b, l);
}
This is the command line shown by the IDE:
/Ox /GL /D "WIN32" /D "_CONSOLE" /D "_UNICODE" /D "UNICODE" /Gm /EHsc
/MDd /GS- /Za /FAs /Fa"Debug\\" /Fo"Debug\\" /Fd"Debug\vc90.pdb" /W4 /c
/Wp64 /Zi /TP .\cert.cpp
This is the assembler listing generated by VC++ 2008. VC++ 2005's
listing differs only in the first line:
; Listing generated by Microsoft (R) Optimizing Compiler Version
15.00.21022.08
TITLE d:\src\cert\cert.cpp
.686P
.XMM
include listing.inc
.model flat
$SG-5 DB '0123456789', 00H
PUBLIC ?f@@[EMAIL PROTECTED] ; f
; Function compile flags: /Ogtpy
; File d:\src\cert\cert.cpp
_TEXT SEGMENT
?f@@[EMAIL PROTECTED] PROC ; f
; 2 : len = 1 << 30;
; 3 : if (buf + len < buf)
; 4 : return 1;
; 5 :
; 6 :
; 7 : return 0;
xor eax, eax
; 8 : }
ret 0
?f@@[EMAIL PROTECTED] ENDP ; f
_TEXT ENDS
PUBLIC _main
; Function compile flags: /Ogtpy
_TEXT SEGMENT
_main PROC
; 12 : char * b = "0123456789";
; 13 : for ( int l = 0; l < 1 << 30; ++l )
; 14 : f(b, l);
; 15 : }
xor eax, eax
ret 0
_main ENDP
_TEXT ENDS
END
I can make my project files available if anybody is interested.
I also tested CodeGear (Borland)'s C++ Builder 2007 compiler and as far
as I can tell it doesn't perform this optimization. Maybe we should all
switch to their product ;-)
Cheers,
Nicola Musatti
--
Nicola.Musatti <at> gmail <dot> com
Home: http://nicola.musatti.googlepages.com/home
Blog: http://wthwdik.wordpress.com/
