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  A Compiler Bug?
  Submitted by



Seeing that we haven't had a TOTD in a while, I'd like to share something that's not quite a tip, but a warning about a compiler bug. The compiler I used is MSVC6, with SP4 and PP.

Not long ago, in the previous TOTD, I recommended a way to do comparisons with unsigned numbers when they are close to zero. The problem is that you can't do something like:

unsigned int i;
for (i = GetNum(); i = 0; --i) {
	printf("i = %u\n", i);
} 



That won't work in any compiler, because "i" is unsigned, and unsigned numbers are ALWAYS ">=0".

So, I was proposing a different solution, which looked more like:

unsigned int i;
for (i = GetNum(); i+1  0; --i) {
	printf("i = %u\n", i);
} 



The problem is that this doesn't work, at least in MSVC. You can try it in this program:

#include <stdio.h

int GetNum();

int main(int argc, char* argv[]) { unsigned int i; for (i = GetNum(); i+1 0; --i) { printf("i = %u\n", i); } printf("Press RETURN\n"); getchar(); return 0; }

int GetNum() { return 5; }



Compile it in RELEASE mode, and you'll see that it only runs the loop once. The reason is that, for the loop comparison, it uses the following code:

0040101D 4E                   dec         esi
0040101E 83 FE FF             cmp         esi,0FFFFFFFFh
00401021 77 EC                ja          main+0Fh (0040100f) 



Which is patently wrong.

In order to work around it, I do something like:

const unsigned int n = GetNum();
unsigned int i;
for (i = n; i <= n; --i) {
	printf("i = %u\n", i);
} 



And it works. Now, it works because the PC does integer arithmetic in 2's complement. If it didn't it'd probably not work. So, I do have two legitimate questions about the standard C++:

1- Is "i+1 > 0" a supported expression when "i" is unsigned? Or is it generally undefined?
2- Is my solution supported in general? Or is it generally undefined?


What do you think?


The zip file viewer built into the Developer Toolbox made use of the zlib library, as well as the zlibdll source additions.

 

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