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. Richard.
