[Bug tree-optimization/39736] signed overflow in loop induction variable: missing warning and wrong code

["edwintorok at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #5 from edwintorok at gmail dot com  2009-04-13 06:56 -------
(In reply to comment #4)
> (In reply to comment #3)
> > But converting from short to int for the argument to printf should behave as if
> > a short value was converted to int, i.e. the int value should be in range
> > -32768 to 32767, right?
> 
> Usually but not here. Since you compiled with -fstrict-overflow (implicitly via
> -O2) the compiler can assume your short variables will have proper short
> values. As an optimisation the compiler could decide to store short variables
> in wider int variables and to perform short arithmetic using int arithmetic.
> Since you promised not to cause signed overflow those int variables would
> always be the proper sign-extension of the corresponding short variables.
> 
> But your test program does cause signed overflow, so this optimisation changes
> behaviour. It's still not the compiler's fault. If you deliberately cause
> signed overflow, do not compile with -fstrict-overflow.
> 

Understood. Then this is not wrong code, just a missing warning.

(In reply to comment #0)
> There are 2 bugs here:
> - no warning is given with -Wstrict-overflow=5, although -fstrict-overflow
> changes the behaviour of the code

This is still a bug: "It warns about cases where the compiler optimizes based
on the assumption that signed overflow does not occur."

> - from is of type short, so when sign extended to an int, it should only take
> values allowed for a short: -32768 to 32767, not all values allowed for an int
> (like 2253491)

This is not a bug.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39736