Fix twolf -funroll-loops -O3 miscompilation (a semi-latent web.c bug)

[Steven Bosscher]

On Tue, Oct 16, 2012 at 1:38 PM, Paolo Bonzini wrote:
>>> Bottom line: This is a bug in df_kill_notes.
>
> Yep, and it could cause wrong code generation, couldn't it?  Because the
> new (reg:DI 72) is obviously not equal to (plus:DI (reg:DI 72)
> (const_int 24)), but it could be propagated to subsequent insns.

Right.


> So I think this patch should be backported to all release branches after
> regstrapping.

Agreed, but let's wait a few days at least to see what happens with
the patch on the trunk.


> Ok after bootstrap, regtest and checking that it actually fixes the bug.

I made sure of that before posting the patch ;-)

Bootstrap&regtest is running on x86_64-unknown-linux-gnu and
powerpc64-unknown-linux-gnu. I'll commit the patch tomorrow if there
are no test suite regressions.

Well done to us all for probing until we found the root cause of this bug!

Ciao!
Steven