Charles Curley - Software Engineer, Writer

A Bug Notification

This is a memo I emailed out to other software engineers describing a bug I had found in a compiler routine. I had to document it in detail in order to be sure the other engineers were seeing this and not something else. It turned out that no-one else in the company was using the ANSI C standard's method of copying structures.

Also, even though the project was written entirely in ANSI C, my skills with assembly language helped me to identify this bug precisely. The assembler is Motorola 68hc11, by the way.

The names have been change to protect the guilty.

I may have come across a bug in Xyz Compiler's code routines, and want to know if anyone else has seen this. The problem shows up when you a) have structures large enough that the compiler lays down a block move instead of using registers, and b) have auto variables on the stack which are used after the first structure move described by a). The compiler lays down a jsr to an assembly routine, c_movstr, to copy a structure. If you are using memcpy(), you won't see it because memcpy() and c_movestr don't share any code. The bug appears to be in how c_movestr saves the processor registers: c_movestr: pshx pshy blah, blah.... pulx puly rts This has the effect of swapping the x and y registers, which going in to the routine contain the stack frame and the destination, respectively. So if you never refer to any stack variables after using this routine, the error doesn't show up. It may not show up under certain other circumstances, such as the destination being the lowest variable on the stack frame. The work-around is to use memcpy(). This is why I checked to see that c_movstr and memcpy() don't share any code. If you have struct foo { stuff; nonsense; } bar, baz; .... baz = bar; substitute memcpy (&baz, &bar, sizeof(struct foo)); Has anyone else seen this? I'd like to confirm this before I squawk at Xyz Company. -- C^2