Re: [PATCH 2/4] gcc/arc: Remove load_update_operand predicate

[Joern Wolfgang Rennecke <gnu at amylaar dot uk>]

On 16/12/15 00:15, Andrew Burgess wrote:
>   
>
> 	* config/arc/arc.md (*loadqi_update): Use 'memory_operand' and fix
> 	RTL pattern to include the plus.
> 	(*load_zeroextendqisi_update): Likewise.
> 	(*load_signextendqisi_update): Likewise.
> 	(*loadhi_update): Likewise.
> 	(*load_zeroextendhisi_update): Likewise.
> 	(*load_signextendhisi_update): Likewise.
> 	(*loadsi_update): Likewise.
> 	(*loadsf_update): Likewise.
> 	* config/arc/predicates.md (load_update_operand): Delete.

Store_update_operand has the very same problem, so it would make sense 
to fix that
in the same check-in.
FWIW, while using "memory_operand" makes for simple source code, it 
introduces
duplicated checks (and they are appropriate only because the the update 
and some of the
non-update addressing modes on the ARC are different modes of the same 
encoding).
It checks that the inside of the MEM is a valid memory address, which is 
redundant
with the pattern-provided checks that there's a plus with appropriate 
base and index/update
operand inside.
Also, by using memory_operand, you are adding a check to reject volatile 
memory operands
during most optimization passes.
Note that ARC's move_src_operand and move_dest_operand are fine with 
volatile MEMs
irrespective of the setting of volatile_ok.
Problems with volatiles can rally only be expected if there are multiple 
MEMs in a single pattern,
that might alias, arithmetic on MEM that might result from 
simplifications using a different set of
MEMs, or if the machine instructions that a pattern corresponds to are 
intrinsically unsuitable for
volatile.
Needlessly rejecting volatile MEMs will reduce optimization potential; 
this tends to be more
visible on embedded platforms than with embedded code than with typical 
workstation code.

Less reliance on this addressing modes orthogonality, no change of 
volatile behaviour,
and maybe a slightly faster compiler, would be to just have an 
"update_operand" or
"any_mem_operand" predicate checking that the inside is a MEM. and leave 
the address
processing entirely to the instruction pattern and its operand predicates.