[C Patch]: pr52543

[Richard Sandiford]

"Georg-Johann Lay" <avr@gjlay.de> writes:
>> This patch takes a different approach to fixing PR52543 than does the 
>> patch in
>> 
>> http://gcc.gnu.org/ml/gcc-patches/2012-03/msg00641.html
>> 
>> This patch transforms the lower-subreg pass(es) from unconditionally 
>> splitting wide moves, zero extensions, and shifts, so that it now takes 
>> into account the target specific costs and only does the transformations 
>> if it is profitable.
>
> As far as I understand the pass, it's not only about splitting these instructions
> but also to the additional benefits of the split, i.e.  AND 0xfffffffe will only need
> one QI operation instead of 1 SI operation that costs 4 QI.
>
> And in fact, the positive benefit of subreg-lowering occurs with bit-wise operations
> like AND, IOR, EOR etc.

Hmm, but in that case, why define andsi3 at all?  optabs applies exactly
that decomposition if you don't:

  /* These can be done a word at a time.  */
  if ((binoptab == and_optab || binoptab == ior_optab || binoptab == xor_optab)
      && mclass == MODE_INT
      && GET_MODE_SIZE (mode) > UNITS_PER_WORD
      && optab_handler (binoptab, word_mode) != CODE_FOR_nothing)

Richard