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POWhy does gcc warn "assuming signed overflow" in one case and not in the other
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copied!<p>Consider the following code:</p> <pre><code>$ cat o.c #include <stdio.h> #include <limits.h> int absolute(int i) { int j = i < 0 ? -i : i; if (j<0) /* This is line 6 */ return 0; return j; } int main() { int i = 1; printf("%d %d\n", i, absolute(i)); return 0; } </code></pre> <p>Compiling it with <code>-O2</code> and <code>-Wstrict-overflow</code> produces a warning:</p> <pre><code>$ gcc -O2 -Wall -Wextra -Wstrict-overflow o.c o.c: In function ‘absolute’: o.c:6:6: warning: assuming signed overflow does not occur when simplifying comparison of absolute value and zero [-Wstrict-overflow] </code></pre> <p>Now consider the following which seems functionally equivalent to the above code:</p> <pre><code>$ cat p.c #include <stdio.h> #include <limits.h> int main() { int i = 1; int j = i < 0 ? -i : i; if (j<0) // Changing i to INT_MIN above and changing (j<0) to (j>INT_MAX) // doesn't change the behavior j=0; printf("%d %d\n", i, j); return 0; } </code></pre> <p>Compiling this with the same options doesn't result in any warnings.</p> <pre><code>$ gcc -O2 -Wall -Wextra -Wstrict-overflow p.c $ </code></pre> <p>As commented in the second code, changing the assignment to <code>i=INT_MIN;</code> and the condition to <code>(j>INT_MAX)</code> doesn't warn either.</p> <p>I'm using gcc 4.7.2 on Ubuntu:</p> <pre><code>$ gcc --version gcc (Ubuntu/Linaro 4.7.2-2ubuntu1) 4.7.2 </code></pre> <p>I'm unable to figure the difference in the two cases. Does it have something to do with optimization or is gcc not behaving quite right here?</p>

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