On 26 July 2016 at 17:07, Richard Biener <rguent...@suse.de> wrote: > On Tue, 26 Jul 2016, Prathamesh Kulkarni wrote: > >> The following is an interesting case which broke stor-layout.c. >> The patch warned for the following call to be dead from >> bit_field_mode_iterator::next_mode() to get_mode_alignment (): >> >> /* Stop if the mode requires too much alignment. */ >> if (GET_MODE_ALIGNMENT (m_mode) > m_align >> && SLOW_UNALIGNED_ACCESS (m_mode, m_align)) >> break; >> >> GET_MODE_ALIGNMENT (MODE) is just #defined as get_mode_alignment (MODE) >> in machmode.h >> >> SLOW_UNALIGNED_ACCESS (MODE, ALIGN) is #defined to STRICT_ALIGNMENT >> in defaults.h, and i386.h sets STRICT_ALIGNMENT to 0. >> So essentially it comes down to: >> >> if (get_mode_alignment (m_mode) > m_align && 0) >> break; >> >> which clearly makes get_mode_alignment(m_mode) a dead call >> since it's a pure function. >> However if a target overrides SLOW_UNALIGNED_ACCESS(mode, align) >> and sets it to some runtime value, then the call won't be dead for that >> target. >> >> Should we split the above in two different if conditions ? >> if (GET_MODE_ALIGNMENT (m_mode) > m_align) >> if (SLOW_UNALIGNED_ACCESS (m_mode, m_align)) >> break; > > I'm surprised it's only one case that you hit ;) Be prepared for > other targets to be broken similarly. > > This hints at the general issue of issueing warnings after optimization, > they can easily become false positives. Hmm, this would indeed give rise to such false positives :/ I wonder whether we should restrict the warning only for cases when the call is outermost expression ? Not sure how to go about that. Maybe add a new flag to tree_exp for CALL_EXPR say OUTERMOST_CALL_P, which is set by FE when the call is determined to be outermost expression ?
Thanks, Prathamesh > > Richard.