Re: [PATCH] Improve RTL ifcvt for empty else_bb (PR rtl-optimization/80491)

[Jeff Law <law@redhat.com>]

On 04/25/2017 01:31 PM, Jakub Jelinek wrote:
> Hi!
> 
> The following patch is a partial fix for PR80491, an improvement for
> if-conversion if there is empty else_bb.
> 
> What we do right now is in that case we only look at the immediately
> preceeding (non-debug/non-note) instruction before cond_earliest
> and if it is not the set of x, we just turn it into attempt to
> use previous value of x rather than whatever it has been initialized to.
> 
> On the testcases from the PR, we have:
> (insn 7 4 8 2 (set (reg:DI 97 [ _15+8 ])
>          (const_int 0 [0])) 81 {*movdi_internal}
>       (nil))
> (insn 8 7 9 2 (set (reg:DI 104 [ a_11(D)->low ])
>          (mem:DI (reg/v/f:DI 101 [ a ]) [2 a_11(D)->low+0 S8 A64])) 81 {*movdi_internal}
>       (nil))
> (insn 9 8 10 2 (set (reg:DI 105 [ b_12(D)->low ])
>          (mem:DI (reg/v/f:DI 102 [ b ]) [2 b_12(D)->low+0 S8 A64])) 81 {*movdi_internal}
>       (nil))
> (insn 10 9 11 2 (parallel [
>              (set (reg:CCC 17 flags)
>                  (compare:CCC (plus:DI (reg:DI 104 [ a_11(D)->low ])
>                          (reg:DI 105 [ b_12(D)->low ]))
>                      (reg:DI 104 [ a_11(D)->low ])))
>              (set (reg:DI 103)
>                  (plus:DI (reg:DI 104 [ a_11(D)->low ])
>                      (reg:DI 105 [ b_12(D)->low ])))
>          ]) 316 {*adddi3_cc_overflow_1}
>       (expr_list:REG_DEAD (reg:DI 105 [ b_12(D)->low ])
>          (expr_list:REG_DEAD (reg:DI 104 [ a_11(D)->low ])
>              (nil))))
> (jump_insn 11 10 14 2 (set (pc)
>          (if_then_else (ltu (reg:CCC 17 flags)
>                  (const_int 0 [0]))
>              (label_ref 14)
>              (pc))) 617 {*jcc_1}
>       (expr_list:REG_DEAD (reg:CCC 17 flags)
>          (int_list:REG_BR_PROB 4 (nil)))
>   -> 14)
> (code_label 14 11 35 3 3 (nil) [1 uses])
> (note 35 14 15 3 [bb 3] NOTE_INSN_BASIC_BLOCK)
> (insn 15 35 16 3 (set (reg:DI 97 [ _15+8 ])
>          (const_int 1 [0x1])) 81 {*movdi_internal}
>       (nil))
> (code_label 16 15 36 4 2 (nil) [0 uses])
> If insn 7 would come after insn 9 (which doesn't change the behavior, as
> insn 8 and insn 9 don't clobber pseudo 97 and const_int 0 is constant),
> we'd turn that into a setcc pattern, but otherwise we fail.
> 
> This patch let us search for x's setter earlier in the bb.
> During testing I found that modified_in_p/modified_in_between_p don't
> actually take into account calls that could change MEMs, so the patch
> handles that too.
> 
> Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
> 
> During bootstrap/regtest it seems to trigger over 10000 times, but
> my statistics collection only noted cases where it found an earlier setter
> and the noce_process_if_block has been successful, has not attempted to
> check if insn_b/set_b was NULL whether it would otherwise fail (but even
> in cases where it doesn't fail perhaps with the patch it can generate
> better code).
> 
> 2017-04-25  Jakub Jelinek  <jakub@redhat.com>
> 
> 	PR rtl-optimization/80491
> 	* ifcvt.c (noce_process_if_block): When looking for x setter
> 	with missing else_bb, don't check only the insn right before
> 	cond_earliest, but look for the last insn that x is modified in
> 	within the same bb.
OK.

jeff