On Tue, Jul 26, 2016 at 02:14:49PM +0200, Georg-Johann Lay wrote:
> >>which returns const0_rtx because reg 18 is set in insn 43 to const0_rtx.
> >>Total outcome is that the right shift of reg:DI 18 is transformed to a
> >>no-op move and cancelled out in the remainder.
> >
> >Why does num_sign_bit_copies return something bigger than 8?
>
> Because it thinks the last value was const0_rtx (which is incorrect) and
Why is that incorrect? See insn 43. Is there another insn I missed?
> The problem might be that get_last_value() is called for reg:DI 18 and
> combine's internal bookkeeping is incorrect
>
> struct reg_stat_type {
> /* Record last point of death of (hard or pseudo) register n. */
> rtx_insn *last_death;
>
> /* Record last point of modification of (hard or pseudo) register n. */
> rtx_insn *last_set;
>
> .last_set is the set for reg:QI 18 but later insns also change hard reg:DI
> 18, for example the set of reg:QI 19. This means that the information in
> reg_stat_type does not tell the whole story for hard regs.
>
> One fix could be to get the mode precision of the SET from the last_set
> insn and only use the information if that mode is at least as wide as the
> mode of the register for which get_last_value is called. reg_stat_type has
> .last_set_mode for this purpose (QImode in the test case).
Yes, last_set_mode, and there is last_set_sign_bit_copies too. Are those
correct?
> >We do have a value, and it is for this bb.
>
> But not for the complete hard register; it's only for reg:QI 18 which is
> one byte of reg:DI 18,
The rest of the hard register bits are set to unspecified values.
> thus the code should never reach this point in the
> first place and return earlier. For example:
>
> Index: combine.c
> ===================================================================
> --- combine.c (revision 238701)
> +++ combine.c (working copy)
> @@ -13206,6 +13206,13 @@ get_last_value (const_rtx x)
> && DF_INSN_LUID (rsp->last_set) >= subst_low_luid)
> return 0;
>
> + /* If the lookup is for a hard register make sure that value contains at
> least
> + as many bits as x does. */
> +
> + if (regno < FIRST_PSEUDO_REGISTER
> + && GET_MODE_PRECISION (rsp->last_set_mode) < GET_MODE_PRECISION
> (GET_MODE (x)))
> + return 0;
> +
> /* If the value has all its registers valid, return it. */
> if (get_last_value_validate (&value, rsp->last_set, rsp->last_set_label,
> 0))
> return value;
That might be a bit harsh.
First things first: is the information recorded correct?
Segher
