What is the explanation for "warning: assuming that the loop is not infinite"

Question

I had just taken the decision to change as many variables from unsigned to int and upon recompiling the code in question, was greeted by this warning message:

freespace_state.c:203: warning: assuming that the loop is not infinite

The line in question:

for (x = startx; x <= endx; ++x, ++xptr)

This loop is 60 lines of code (inc white space/brackets etc), and has a goto within it, and at least one occurrence of continue.

In this case, I think I am appreciative that GCC is assuming this loop is not infinite, because, it should never loop indefinitely.

What is GCC trying to tell me here?

^{The grammar of the warning is almost suggestive that the warning should be taken within the context of some other warning, but there are none within that context.}

[edit] It's all completely my own fault. I stole some optimization and warning options from a question on here somewhere without really understanding them, and had since forgotten about them.

See Mark Rushakoff's answer, and in addition, I have also used -Wunsafe-loop-optimizations to explicitly warn if GCC is making assumptions about a loop. See http://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html

Answer 1

I think that GCC is telling you that it can't determine that the loop is not infinite and is carrying on with compilation regardless. It's a warning, not an error, something you may want to think about.

Answer 2

According to this GCC patch from 2005, it appears that GCC is performing an "unsafe loop optimization" (and you are being warned because -funsafe-loop-optimizations is set). If the loop is infinite, this particular optimization will fail somehow.

Since you said it is a terminating loop, it sounds as though you have nothing to worry about.

Another relevant part of the patch:

@opindex Wunsafe-loop-optimizations
Warn if the loop cannot be optimized because the compiler could not
assume anything on the bounds of the loop indices.  With
@option{-funsafe-loop-optimizations} warn if the compiler made
+such assumptions.