Re: strict aliasing warning

[Andrew Haley <aph at redhat dot com>]

Joel Sherrill writes:
 > Silvius Rus wrote:
 > >
 > > I wrote some code (not released yet) that improves the accuracy of 
 > > -Wstrict-aliasing using tree-ssa-alias information.  The primary idea 
 > > was to tell the programmer "go fix the types of variables x and y at 
 > > lines ..." when -fstrict-aliasing breaks their code.
 > >
 > > It occurred to me that part of this code could be used as a 
 > > preconditioner to aggressive optimization that would normally require 
 > > -fstrict-aliasing, so this more aggressive optimization can then be 
 > > performed selectively on individual functions even with 
 > > -fno-strict-aliasing on the command line.  I fear a little though that 
 > > the functions which are provably free of cross-type aliasing might 
 > > also not benefit much from -fstrict-aliasing, but I have very little 
 > > experience with C compilers and GCC.  Is this something worth pursuing?
 > >
 > How reliable is this detection and warning?  Is it ready for testers yet?
 > 
 > I ask because we have found a case where code in RTEMS breaks when
 > strict-aliasing is enabled.  We are having discussions on how to
 > effectively perform an audit on RTEMS for other breakages.  Right
 > now, the best idea is Ralf's to diff the assembly generated for
 > each file compiled with and without strict-aliasing.  If there is a
 > difference, we will have to review it.  This eliminates a lot of
 > the code base but it is still generating a lot of cases to examine
 > by hand.

Ewww, that's scary.  All it tells you is which code breaks today; even
a minor bug fix in gcc might make a difference.

Nonetheless, this is a really interesting idea, and I'm sure we'd all
be very interested to see the results.

Andrew.