PR target/17101: question about powerpc s<cond> expanders

[Nathan Sidwell]

PR 17101 is a problem with boolean operations.  rs6000.md contains
seq, sne, sgt ... expanders, but the signed ones are explicitly
disabled for non-POWER (i.e. POWERPC) targets. for instance

(define_expand "sgt"
   [(clobber (match_operand:SI 0 "gpc_reg_operand" ""))]
   ""
   "
{
   if (! rs6000_compare_fp_p
       && (! TARGET_POWER || rs6000_compare_op1 == const0_rtx))
     FAIL;

   rs6000_emit_sCOND (GT, operands[0]);
   DONE;
}")

why is that?  The unsigned variants are not so encumbered.

Also, the signed variants cut out comparisons with zero -- why
do the unsigned ones not do so? Oversight?  In addition, some of
the special cases seem ineffective at best, pessimizing at worst.
Examining each in detail I find the following assembler for 'V OP 0'

.seq: // baseline of 3 insns using the condition regs
         cmpwi 7,3,0
         mfcr 3
         rlwinm 3,3,31,1

.sne: // hm, four insns emitted
         srawi 0,3,31
         xor 3,0,3
         subf 3,3,0
         srwi 3,3,31

.sge: // 2 insns, this is better.
         nor 3,3,3
         srwi 3,3,31

.sgt: // 3 insns, no better
         srawi 0,3,31
         subf 0,3,0
         srwi 0,0,31

.sle: // 3 insns, no better
         addi 0,3,-1
         or 0,0,3
         srwi 0,0,31

.slt: // 1 insn, yay!
         srwi 3,3,31

comments?

nathan
-- 
Nathan Sidwell    ::   http://www.codesourcery.com   ::     CodeSourcery LLC
nathan@codesourcery.com    ::     http://www.planetfall.pwp.blueyonder.co.uk