[Bug rtl-optimization/45235] const volatile read moved out of order

["bigotp at acm dot org" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #4 from bigotp at acm dot org  2010-08-11 22:54 -------
I don't see that the const qualifier should be relevant: doesn't it simply
indicate that the code is not permitted to write through that lvalue?  (FWIW,
the real code uses a memory mapped address and the const qualifier was placed
on the address declaration by the chip manufacturer.  I assume that a write to
the location may be interpreted as a command to the hardware to self-destruct.)

Given that, the existing behavior appears to be a clear violation of the
requirement that volatile accesses not be re-ordered across sequence points.

In concept, I like rguenth's solution (which I haven't tested), as it fits with
the documented semantics of MEM_READONLY_P indicating "not modified during the
lifetime of the program", something that certainly can't be said of any
readable volatile object.  That the RTL flag is stored in a field named
"unchanging" is a lot more clear than "READONLY".  I think that approach
eliminates the need to muck with any of those other places where the same
problem might be occurring.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45235