Hi!
On Mon, Sep 10, 2018 at 12:59:19PM +0200, Ilya Leoshkevich wrote:
> Consider the following RTL:
>
> (code_label 11 10 26 4 2 (nil) [1 uses])
> (note 26 11 12 4 [bb 4] NOTE_INSN_BASIC_BLOCK)
> (insn 12 26 15 4 (set (reg:SI 65)
> (if_then_else:SI (eq (reg:CCZ 33 %cc)
> (const_int 0 [0]))
> (const_int 1 [0x1])
> (const_int 0 [0]))) "pr80080-4.c":9 1674 {*movsicc})
> (insn 15 12 16 4 (parallel [
> (set (reg:CCZ 33 %cc)
> (compare:CCZ (reg:SI 65)
> (const_int 0 [0])))
> (clobber (scratch:SI))
> ]) "pr80080-4.c":9 1216 {*tstsi_cconly_extimm})
> (jump_insn 16 15 17 4 (set (pc)
> (if_then_else (ne (reg:CCZ 33 %cc)
> (const_int 0 [0]))
> (label_ref:DI 23)
> (pc))) "pr80080-4.c":9 1897 {*cjump_64})
>
> Combine simplifies this into:
>
> (code_label 11 10 26 4 2 (nil) [1 uses])
> (note 26 11 12 4 [bb 4] NOTE_INSN_BASIC_BLOCK)
> (note 12 26 15 4 NOTE_INSN_DELETED)
> (note 15 12 16 4 NOTE_INSN_DELETED)
> (jump_insn 16 15 17 4 (set (pc)
> (if_then_else (eq (reg:CCZ 33 %cc)
> (const_int 0 [0]))
> (label_ref:DI 23)
> (pc))) "pr80080-4.c":9 1897 {*cjump_64})
>
> opening up the possibility to perform jump threading. Since this
> happens infrequently, perform jump threading only when there is a
> changed basic block, whose sole side effect is a trailing jump.
So this happens because now there is *only* a conditional jump in this BB?
Could you please show generated code before and after this patch?
I mean generated assembler code. What -S gives you.
> +/* Return true iff the only side effect of BB is its trailing jump_insn. */
> +
> +static bool
> +is_single_jump_bb (basic_block bb)
> +{
> + rtx_insn *end = BB_END (bb);
> + rtx_insn *insn;
> +
> + if (!JUMP_P (end))
> + return false;
> +
> + for (insn = BB_HEAD (bb); insn != end; insn = NEXT_INSN (insn))
> + if (INSN_P (insn) && side_effects_p (PATTERN (insn)))
> + return false;
> + return true;
> +}
Hrm, so it is more than that.
Why does the existing jump threading not work for you; should it happen
at another time?
Segher
