Re: missing conditional propagation in cprop.c pass

["Amker.Cheng" <amker dot cheng at gmail dot com>]

On Tue, Sep 27, 2011 at 4:19 PM, Amker.Cheng <amker.cheng@gmail.com> wrote:
> Hi,
> I ran into a case and found conditional (const) propagation is
> mishandled in cprop pass.
> With following insn sequence after cprop1 pass:
> ----------------------------------------------------
> (note 878 877 880 96 [bb 96] NOTE_INSN_BASIC_BLOCK)
>
> (insn 882 881 883 96 (set (reg:CC 24 cc)
> ? ? ? ?(compare:CC (reg:SI 684 [ default_num_contexts ])
> ? ? ? ? ? ?(const_int 0 [0]))) core_main.c:265 211 {*arm_cmpsi_insn}
> ? ? (nil))
>
> (jump_insn 883 882 886 96 (set (pc)
> ? ? ? ?(if_then_else (ne (reg:CC 24 cc)
> ? ? ? ? ? ? ? ?(const_int 0 [0]))
> ? ? ? ? ? ?(label_ref:SI 905)
> ? ? ? ? ? ?(pc))) core_main.c:265 223 {*arm_cond_branch}
> ? ? (expr_list:REG_DEAD (reg:CC 24 cc)
> ? ? ? ?(expr_list:REG_BR_PROB (const_int 9100 [0x238c])
> ? ? ? ? ? ?(nil)))
> ?-> 905)
>
> (note 886 883 49 97 [bb 97] NOTE_INSN_BASIC_BLOCK)
>
> (insn 49 886 0 97 (set (reg/v:SI 291 [ total_errors ])
> ? ? ? ?(reg:SI 684 [ default_num_contexts ])) core_main.c:265 709
> {*thumb2_movsi_insn}
> ? ? (expr_list:REG_DEAD (reg:SI 684 [ default_num_contexts ])
> ? ? ? ?(expr_list:REG_EQUAL (const_int 0 [0])
> ? ? ? ? ? ?(nil))))
> ......
>
> (code_label 905 54 904 47 54 "" [1 uses])
>
> (note 904 905 46 47 [bb 47] NOTE_INSN_BASIC_BLOCK)
>
> (insn 46 904 47 47 (set (reg/v:SI 291 [ total_errors ])
> ? ? ? ?(const_int 0 [0])) core_main.c:265 709 {*thumb2_movsi_insn}
> ? ? (nil))
> ----------------------------------------------------
>
> The insn49 should be propagated with conditional const from insn882
> and jump_insn883, optimized into "r291<-0" as following code, then let
> pre do redundancy elimination work.
> ----------------------------------------------------
> (note 878 877 880 96 [bb 96] NOTE_INSN_BASIC_BLOCK)
>
> (insn 882 881 883 96 (set (reg:CC 24 cc)
> ? ? ? ?(compare:CC (reg:SI 684 [ default_num_contexts ])
> ? ? ? ? ? ?(const_int 0 [0]))) core_main.c:265 211 {*arm_cmpsi_insn}
> ? ? (nil))
>
> (jump_insn 883 882 886 96 (set (pc)
> ? ? ? ?(if_then_else (ne (reg:CC 24 cc)
> ? ? ? ? ? ? ? ?(const_int 0 [0]))
> ? ? ? ? ? ?(label_ref:SI 905)
> ? ? ? ? ? ?(pc))) core_main.c:265 223 {*arm_cond_branch}
> ? ? (expr_list:REG_DEAD (reg:CC 24 cc)
> ? ? ? ?(expr_list:REG_BR_PROB (const_int 9100 [0x238c])
> ? ? ? ? ? ?(nil)))
> ?-> 905)
>
> (note 886 883 49 97 [bb 97] NOTE_INSN_BASIC_BLOCK)
>
> (insn 49 886 0 97 (set (reg/v:SI 291 [ total_errors ])
> ? ? ? ?(const_int 0 [0])) core_main.c:265 709 {*thumb2_movsi_insn}
> ? ? (expr_list:REG_DEAD (reg:SI 684 [ default_num_contexts ])
> ? ? ? ?(expr_list:REG_EQUAL (const_int 0 [0])
> ? ? ? ? ? ?(nil))))
> ......
>
> (code_label 905 54 904 47 54 "" [1 uses])
>
> (note 904 905 46 47 [bb 47] NOTE_INSN_BASIC_BLOCK)
>
> (insn 46 904 47 47 (set (reg/v:SI 291 [ total_errors ])
> ? ? ? ?(const_int 0 [0])) core_main.c:265 709 {*thumb2_movsi_insn}
> ? ? (nil))
> ----------------------------------------------------
>
> The problem is function one_cprop_pass does local const/copy
> propagation pass first, then the global pass, which only handles
> global opportunities.
> Though conditional const information "r684 <- 0" is collected by
> find_implicit_sets, the conditional information is recorded as local
> information of bb 97, and it is not recorded in avout of bb 96, so not
> in avin of bb 97 either.
>
> Unfortunately, the global pass only considers potential opportunities
> from avin of each basic block in function cprop_insn and
> find_avail_set.
>
> That's why the conditional propagation opportunity in bb 97 is missed.
>
> I worked a patch to fix this, and wanna hear more suggestions on this topic.
> Is it a bug or I missed something important?
>
> Thanks
>
> BTW, I'm using gcc mainline which configured for arm-none-eabi target.
>

No Interest? Any tips will be great appreciated, thanks.

-- 
Best Regards.