ID: Q67040
6.00 6.00a 6.00ax 7.00 | 6.00 6.00a | 1.00 1.50
MS-DOS | OS/2 | WINDOWS
kbtool kbbuglist
The information in this article applies to:
- Microsoft C for MS-DOS, versions 6.0, 6.0a, and 6.0ax
- Microsoft C for OS/2, versionS 6.0 and 6.0a
- Microsoft C/C++ for MS-DOS, version 7.0
- Microsoft Visual C++ for Windows, versions 1.0 and 1.5
In most cases, a variable declared with the register storage-class in Microsoft C versions 6.0, 6.0a, 6.0ax, and C/C++ version 7.0 will not be allocated to a register if loop optimization (/Ol) is enabled. Instead, the compiler will place the variable on the stack.
The register storage will be allocated only if the function contains an inline assembly (_asm) block, because the inline assembly takes precedence over optimization. This is reflected by the following warning message, which is generated when /Ol is used on a function with an _asm block:
warning C4204: in-line assembler precludes global optimizations
Microsoft has confirmed this to be a problem in the Microsoft products listed at the beginning of this article. We are researching this problem and will post new information here as it becomes available.
This is not a problem in the 32-bit compilers.
The sample program below demonstrates both of the situations described above. If the program is compiled as is with /Ol, the register declared variable "i" will not be put in a register; it will be handled exactly like "j", which is not declared with register storage-class.
If the line at the end of main() with the _asm block is uncommented, then "i" will be allocated storage in a register and the C4204 warning will be displayed if the warning level is set at 3 or 4 (/W3 or /W4). To see the difference, you can generate an assembly listing with /Fa, or you can compile and link the program with CodeView information and then view the mixed source and assembly listing in the debugger.
/* Compile options needed: /Ol /Fc /W3
*/
int func(void);
void main(void)
{
register int i = 7;
int j = 9;
while ( i < 10 ) {
i += func();
j += func();
}
/* Uncomment the following line to have i put in a register */
/* _asm xor i,i */
}
int func(void)
{
return (1);
}
Additional reference words: 1.00 1.50 6.00 6.00a 6.00ax 7.00 8.00 8.00c KBCategory: kbtool kbbuglist KBSubcategory: CLIss Keywords : kb16bitonly
Last Reviewed: July 18, 1997