[PATCH][RFC] Poison bitmap_head->obstack

[Jeff Law]

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  <rguenther@suse.de>
> 
> 	* 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.

jeff