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Re: warn for dead function calls [4/4] stor-layout.c fallout
On 26 July 2016 at 17:07, Richard Biener <email@example.com> 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))
>> 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)
>> 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))
> 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 ?