[RFC] Check number of uses in simplify_cond_using_ranges().

[Dominik Vogt]

On Thu, Nov 03, 2016 at 04:29:05PM +0100, Eric Botcazou wrote:
> > Is VRP the right pass to do this optimisation or should a later
> > pass rather attempt to eliminate the new use of b_5 instead?  Uli
> > has brought up the idea a mini "sign extend elimination" pass that
> > checks if the result of a sign extend could be replaced by the
> > original quantity in all places, and if so, eliminate the ssa
> > name.
> 
> Did you try to enable -free on s390x?  It's a RTL pass.

Just tried it, but it does not seem to make a difference:

  Trying to eliminate extension:
  (insn 8 6 10 2 (set (reg:SI 1 %r1 [orig:63 b+-3 ] [63])
          (zero_extend:SI (reg/v:QI 2 %r2 [orig:60 b ] [60]))) y.c:9
  1258 {*zero_extendqisi2_extimm}
       (nil))
  Elimination opportunities = 1 realized = 0

I guess when in Rtl format, the code is already too convoluted to
be cleaned up.  Before combine - which should make use of some
advanced s390x instruction (risbg) but doesn't - Rtl is this:

(insn 2 4 3 2 (set (reg/v:DI 62 [ a+-7 ])
        (reg:DI 2 %r2 [ a+-7 ])) y.c:3 1074 {*movdi_64}
# (SImode)(a & 63)
(insn 6 3 8 2 (parallel [
            (set (subreg:SI (reg/v:QI 60 [ b ]) 0)
                (and:SI (subreg/s/v:SI (reg/v:DI 62 [ a+-7 ]) 4)
                    (const_int 63 [0x3f])))
            (clobber (reg:CC 33 %cc))
        ]) y.c:7 1526 {*andsi3_zarch}
(insn 8 6 9 2 (set (reg:SI 63 [ b+-3 ])
        (zero_extend:SI (reg/v:QI 60 [ b ]))) y.c:9 1258
{*zero_extendqisi2_extimm}
# Compare quantity in SImode
(insn 9 8 10 2 (set (reg:CCU 33 %cc)
        (compare:CCU (reg:SI 63 [ b+-3 ])
            (const_int 9 [0x9]))) y.c:9 1047 {*cmpsi_ccu}
(jump_insn 10 9 11 2 (set (pc)
        (if_then_else (leu (reg:CCU 33 %cc)
                (const_int 0 [0]))
            (label_ref:DI 18)
            (pc))) y.c:9 1706 {*cjump_64}
# Extend to DImode
(insn 12 11 13 3 (set (reg:DI 64 [ l ])
        (zero_extend:DI (reg/v:QI 60 [ b ]))) y.c:8 1257
{*zero_extendqidi2_extimm}
# Load DImode function argument for call to bar()
(insn 13 12 14 3 (set (reg:DI 2 %r2)
        (reg:DI 64 [ l ])) y.c:10 1074 {*movdi_64}
...

Ciao

Dominik ^_^  ^_^

-- 

Dominik Vogt
IBM Germany