Re: Fix PR rtl-optimization/84071

[Richard Sandiford <richard dot sandiford at linaro dot org>]

Eric Botcazou <ebotcazou@adacore.com> writes:
>> Tested on SPARC64/Linux and ARM/EABI, applied on mainline and 7 branch.
>
> As discussed in the audit trail, this beefs up the internal documentation 
> about WORD_REGISTER_OPERATIONS.
>
> Tested with 'make doc', applied on mainline and 7 branch.
>
>
> 2018-01-31  Eric Botcazou  <ebotcazou@adacore.com>
>
> 	PR rtl-optimization/84071
> 	* doc/tm.texi.in (WORD_REGISTER_OPERATIONS): Add explicit case.
> 	* doc/tm.texi: Regenerate.
>
> Index: doc/tm.texi.in
> ===================================================================
> --- doc/tm.texi.in	(revision 257227)
> +++ doc/tm.texi.in	(working copy)
> @@ -7376,8 +7376,12 @@ is in effect.
>
>  @defmac WORD_REGISTER_OPERATIONS
>  Define this macro to 1 if operations between registers with integral mode
> -smaller than a word are always performed on the entire register.
> -Most RISC machines have this property and most CISC machines do not.
> +smaller than a word are always performed on the entire register.  To be
> +more explicit, if you start with a pair of @code{word_mode} registers with
> +known values and you do a subword, for example @code{QImode}, addition on
> +the low part of the registers, then the compiler may consider that the
> +result has a known value in @code{word_mode} too if the macro is defined
> +to 1.  Most RISC machines have this property and most CISC machines do not.

By QImode addition, do you mean:

   (set (subreg:QI (reg:SI X1) N)
   	(plus:QI (subreg:QI (reg:SI X2) N)
        	 (subreg:QI (reg:SI X3) N)))

?  I thought the point was instead that the target expected such ops
to be done on word_mode, even if the values involved are naturally QImode:

   (set (subreg:SI (reg:QI Y1) 0)
   	(plus:SI (subreg:SI (reg:QI Y2) 0)
        	 (subreg:SI (reg:QI Y3) 0)))

Most RISC/WORD_REGISTER_OPERATIONS targets wouldn't provide QImode
addition patterns, so the first insn seems unlikely.

Thanks,
Richard