Re: Rationale for an old TRUNCATE patch

[Jeff Law <law@redhat.com>]

Ian Lance Taylor wrote:
> Adam Nemet <anemet@caviumnetworks.com> writes:
>
>   
>> I am trying to understand the checkin by Jeff Law from about 11 years ago:
>>
>>    r19204 | law | 1998-04-14 01:04:21 -0700 (Tue, 14 Apr 1998) | 4 lines
>>    
>>    
>>            * combine.c (simplify_rtx, case TRUNCATE): Respect value of
>>            TRULY_NOOP_TRUNCATION.
>>    
>>    
>>    Index: combine.c
>>    ===================================================================
>>    --- combine.c	(revision 19018)
>>    +++ combine.c	(revision 19204)
>>    @@ -3736,7 +3736,9 @@ simplify_rtx (x, op0_mode, last, in_dest
>>           if (GET_MODE_CLASS (mode) == MODE_PARTIAL_INT)
>>     	break;
>>     
>>    -      if (GET_MODE_BITSIZE (mode) <= HOST_BITS_PER_WIDE_INT)
>>    +      if (GET_MODE_BITSIZE (mode) <= HOST_BITS_PER_WIDE_INT
>>    +	  && TRULY_NOOP_TRUNCATION (GET_MODE_BITSIZE (mode),
>>    +				    GET_MODE_BITSIZE (GET_MODE (XEXP (x, 0)))))
>>     	SUBST (XEXP (x, 0),
>>     	       force_to_mode (XEXP (x, 0), GET_MODE (XEXP (x, 0)),
>>     			      GET_MODE_MASK (mode), NULL_RTX, 0));
>>
>> This optimization simplifies the input to a truncate by only computing bits
>> that won't be eliminated by the truncation.  Normally these are the bits in
>> the output mode mask.  Note that the optimization does not change the truncate
>> into a low-part subreg, which would pretty automatically warrant the
>> TRULY_NOOP_TRUNCATION check.
>>     
>
> I agree that this patch looks wrong in todays compiler.  There should be
> no need to call TRULY_NOOP_TRUNCATION if you are in a TRUNCATE anyhow.
>   
Based on reviewing my old notes, we do have to ensure that combine 
doesn't replace a TRUNCATE with a SUBREG as that can result in having a 
32bit value that isn't sign-extended, which clearly causes MIPS64 ports 
grief.

Jeff