Re: Problem with the special live analyzer in global alloc

[James E Wilson <wilson at specifix dot com>]

On Tue, 2005-08-23 at 07:44, Bernd Schmidt wrote:
> Jim Wilson once suggested we should just emit insns to make sure every 
> register is initialized and be done with it - problem solved.  I had 
> started to work on that, if people think it's a good idea I can dig that 
> stuff out again.

I'd like this because of an IA-64 specific problem.

IA-64 has Not-a-Thing (NaT) bits, which are used for speculation.  If a
speculative load fails, the NaT bit is set, which indicates that we must
refetch the value before using it.  NaT bits propagate through most
operations, allowing us to speculate a series of instructions instead of
just loads.  However, they will generate an illegal instruction
exception if used in an operation with side-effects, like a store.

So the problem here is that any use of an uninitialized register may
generate an exception, if the instruction has side-effects, and the
uninitialized register just happens to have the NaT bit set.

Mostly we get by because gcc doesn't have speculation support yet, but
it is only a matter of time before someone writes it.  Meanwhile, there
are some hand-written glibc routines that do use speculation, and could
potentially trigger this problem.  This is a disaster waiting to happen
for anyone using gcc on IA-64 machines.

I created PR 21111 for this problem, and it contains an artificial
testcase that demonstrates the problem using bitfield assignments.
-- 
Jim Wilson, GNU Tools Support, http://www.specifix.com