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Re: [PATCH][RFC] Poison bitmap_head->obstack
On Wed, 5 Dec 2018, Jeff Law wrote:
> On 12/4/18 6:16 AM, Richard Biener wrote:
> > This tries to make bugs like that in PR88317 harder to create by
> > introducing a bitmap_release function that can be used as
> > pendant to bitmap_initialize for non-allocated bitmap heads.
> > The function makes sure to poison the bitmaps obstack member
> > so the obstack the bitmap was initialized with can be safely
> > released.
> > The patch also adds a default constructor to bitmap_head
> > doing the same, but for C++ reason initializes to a
> > all-zero bitmap_obstack rather than 0xdeadbeef because
> > the latter isn't possible in constexpr context (it is
> > by using unions but then things start to look even more ugly).
> > The stage1 compiler might end up with a few extra runtime
> > initializers but constexpr makes sure they'll vanish for
> > later stages.
> > I had to paper over that you-may-not-use-memset-to-zero classes
> > with non-trivial constructors warning in two places and I
> > had to teach gengtype about CONSTEXPR (probably did so in
> > an awkward way - suggestions and pointers into gengtype
> > appreciated).
> > Bootstrapped (with host GCC 4.8) on x86_64-unknown-linux-gnu,
> > testing in progress.
> > The LRA issue seems to be rare enough (on x86_64...) that
> > I didn't trip over it sofar.
> > Comments? Do we want this? Not sure how we can easily
> > discover all bitmap_clear () users that should really
> > use bitmap_release (suggestion for a better name appreciated
> > as well - I thought about bitmap_uninitialize)
> > Richard.
> > 2018-12-04 Richard Biener <email@example.com>
> > * bitmap.c (bitmap_head::crashme): Define.
> > * bitmap.h (bitmap_head): Add constexpr default constructor
> > poisoning the obstack member.
> > (bitmap_head::crashme): Declare.
> > (bitmap_release): New function clearing a bitmap and poisoning
> > the obstack member.
> > * gengtype.c (main): Make it recognize CONSTEXPR.
> > * lra-constraints.c (lra_inheritance): Use bitmap_release
> > instead of bitmap_clear.
> > * ira.c (ira): Work around warning.
> > * regrename.c (create_new_chain): Likewise.
> I don't see enough complexity in here to be concerning -- so if it makes
> it harder to make mistakes, then I'm for it.
Any comment about the -Wclass-memaccess workaround sprinkling around two
(void *) conversions? I didn't dig deep enough to look for a more
appropriate solution, also because there were some issues with older
host compilers and workarounds we installed elsewhere...
Otherwise yes, it makes it harder to do mistakes. I'll probably
use bitmap_head::crashme instead of 0xdeadbeef in bitmap_release.
And of course we'd need to hunt down users of bitmap_clear that
should be bitmap_release instead...