[Bug tree-optimization/57343] [4.8/4.9 Regression] wrong code on x86_64-linux at -Os and above

[rakdver at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57343

--- Comment #7 from Zdenek Dvorak <rakdver at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #4)
> We then fall to
> 
>   if (multiple_of_p (TREE_TYPE (c), c, s))
>     {
>       /* If C is an exact multiple of S, then its value will be reached
> before
>          the induction variable overflows (unless the loop is exited in some
>          other way before).  Note that the actual induction variable in the
>          loop (which ranges from base to final instead of from 0 to C) may
>          overflow, in which case BNDS.up will not be giving a correct upper
>          bound on C; thus, BNDS_U_VALID had to be computed in advance.  */
>       no_overflow = true;
>       exit_must_be_taken = true;
> 
> which ends up with no_overflow = true and things going downhill from here.

This is the problem -- multiple_of_p claims that (var * 100) is a multiple of
100, which is not the case if the multiplication overflows.  I am not sure
whether this is an expected behavior of multiple_of_p, or whether this could
cause problems in its other uses.

A conservative fix here would be to replace the multiple_of_p test here by some
safer tests -- important special cases are:
1) both c and s are constants and c is a multiple of s
2) s = 1