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Re: Need help with PR71976 combine.c::get_last_value returns a wrong result
On Wed, Jul 27, 2016 at 09:14:27PM +0200, Georg-Johann Lay wrote:
> >diff --git a/gcc/combine.c b/gcc/combine.c
> >index 77e0d2b..dec6226 100644
> >--- a/gcc/combine.c
> >+++ b/gcc/combine.c
> >@@ -9977,6 +9977,9 @@ reg_num_sign_bit_copies_for_combine (const_rtx x,
> >machine_mode mode,
> > return NULL;
> > }
> >+ if (GET_MODE_PRECISION (rsp->last_set_mode) != GET_MODE_PRECISION
> >+ return NULL;
> > tem = get_last_value (x);
> > if (tem != 0)
> > return tem;
> But the problem is inside get_last_value. You'd have to add such a
> check at /all/ call sites of that function. Using a value for a hard
> reg that's been set in a smaller mode than the mode of get_last_value is
> just wrong.
Things like nonzero_bits explicitly deal with it. For MODE_INT values
at least; the code looks a bit shaky.
> For example combine tracks bits which are known to be zero, and if
> get_last_value is used in a similar situation, then the conclusion about
> zero-bits is also wrong.
How so? I don't see it.
> Would be interesting to patch get_last_value so that it issues some
> diagnostic if
> if (regno < FIRST_PSEUDO_REGISTER
> && (GET_MODE_PRECISION (rsp->last_set_mode)
> < GET_MODE_PRECISION (GET_MODE (x))))
> and then run the testsuite against that compiler. I would expect that
> the condition doesn't even trigger on x86.
Why restrict this to hard regs at all? Yes I know it isn't supposed
to happen for pseudos, but that also means you do not need to check?