Re: PING: [PATCH] Fix PR 20972

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

Bernd Schmidt wrote:
> one that is initialized.  However, a register may not only appear to be 
> uninitialized, it can actually be used uninitialized, as in Josh's 
> testcase.

I think this should be fixed by not having any used uninitialized 
registers.  We already have some code in flow that tries to initialize 
registers that are used uninitialized.  See the 
initialize_uninitialized_subregs function.  It could be expanded to 
handle cases like this.

In this particular case, the input testcase is invalid, it uses an 
uninitialized variable, so it doesn't really matter how we handle it. 
However, we also get similar code for bitfield references due to the 
fact that tree-ssa decomposes structure assignments, and the middle-end 
doesn't handle the resulting trees efficiently.

We get two benefits from initialized registers that are used uninitialized:
1) We get better optimization, because we can now simplify operations 
using the uninitialized registers.  This is particularly useful for 
structure field assignments, as they may simplify from a bit-field 
insertion to a simple move.
2) We avoid IA-64 illegal instruction faults.  Any uninitialized 
register is a register that might have the NaT bit set, and if we use 
that register in an instruction with non-reversible side-effects (like 
store or load), then we get an illegal instruction fault.

I have a PR with an IA-64 and x86-64 testcases for this, see PR 21111.
--
Jim Wilson, GNU Tools Support, http://www.SpecifixInc.com