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Re: [PATCH][RFC] Remove RTL loop unswitching


> 
> This removes RTL loop unswitching (see last years discussion about
> compile-time issues of that pass).  RTL loop unswitching is
> enabled together with GIMPLE loop unswitching at -O3 and by
> -floop-unswitch.  It's clearly the wrong place to do high-level
> loop transforms these days, and the cost of maintainance doesn't
> outweight the questionable benefit.
> 
> Thus the following patch removes it.
> 
> Bootstrap / regtest pending on x86_64-unknown-linux-gnu (I hope
> for testsuite fallout).
> 
> Any objections?

Not really, I am all for moving more of loop stuff to trees.
Did you performed some benchmarks? (I remember I did in 2012
but completely forgot the outcome).

On related note, shall I try to update the following?
http://gcc.gnu.org/ml/gcc-patches/2013-11/msg02270.html

Honza
> 
> Thanks,
> Richard.
> 
> 2014-04-15  Richard Biener  <rguenther@suse.de>
> 
> 	* Makefile.in (OBJS): Remove loop-unswitch.o.
> 	* loop-unswitch.c: Delete.
> 	* tree-pass.h (make_pass_rtl_unswitch): Remove.
> 	* passes.def (pass_rtl_unswitch): Likewise.
> 	* loop-init.c (gate_rtl_unswitch): Likewise.
> 	(rtl_unswitch): Likewise.
> 	(pass_data_rtl_unswitch): Likewise.
> 	(pass_rtl_unswitch): Likewise.
> 	(make_pass_rtl_unswitch): Likewise.
> 	* rtl.h (reversed_condition): Likewise.
> 	(compare_and_jump_seq): Likewise.
> 	* loop-iv.c (reversed_condition): Move here from loop-unswitch.c
> 	and make static.
> 	* loop-unroll.c (compare_and_jump_seq): Likewise.
> 
> Index: gcc/Makefile.in
> ===================================================================
> --- gcc/Makefile.in	(revision 209410)
> +++ gcc/Makefile.in	(working copy)
> @@ -1294,7 +1294,6 @@ OBJS = \
>  	loop-invariant.o \
>  	loop-iv.o \
>  	loop-unroll.o \
> -	loop-unswitch.o \
>  	lower-subreg.o \
>  	lra.o \
>  	lra-assigns.o \
> Index: gcc/tree-pass.h
> ===================================================================
> --- gcc/tree-pass.h	(revision 209410)
> +++ gcc/tree-pass.h	(working copy)
> @@ -512,7 +512,6 @@ extern rtl_opt_pass *make_pass_outof_cfg
>  extern rtl_opt_pass *make_pass_loop2 (gcc::context *ctxt);
>  extern rtl_opt_pass *make_pass_rtl_loop_init (gcc::context *ctxt);
>  extern rtl_opt_pass *make_pass_rtl_move_loop_invariants (gcc::context *ctxt);
> -extern rtl_opt_pass *make_pass_rtl_unswitch (gcc::context *ctxt);
>  extern rtl_opt_pass *make_pass_rtl_unroll_and_peel_loops (gcc::context *ctxt);
>  extern rtl_opt_pass *make_pass_rtl_doloop (gcc::context *ctxt);
>  extern rtl_opt_pass *make_pass_rtl_loop_done (gcc::context *ctxt);
> Index: gcc/passes.def
> ===================================================================
> --- gcc/passes.def	(revision 209410)
> +++ gcc/passes.def	(working copy)
> @@ -341,7 +341,6 @@ along with GCC; see the file COPYING3.
>        PUSH_INSERT_PASSES_WITHIN (pass_loop2)
>  	  NEXT_PASS (pass_rtl_loop_init);
>  	  NEXT_PASS (pass_rtl_move_loop_invariants);
> -	  NEXT_PASS (pass_rtl_unswitch);
>  	  NEXT_PASS (pass_rtl_unroll_and_peel_loops);
>  	  NEXT_PASS (pass_rtl_doloop);
>  	  NEXT_PASS (pass_rtl_loop_done);
> Index: gcc/loop-init.c
> ===================================================================
> --- gcc/loop-init.c	(revision 209410)
> +++ gcc/loop-init.c	(working copy)
> @@ -518,61 +518,7 @@ make_pass_rtl_move_loop_invariants (gcc:
>  }
>  
>  
> -/* Loop unswitching for RTL.  */
> -static bool
> -gate_rtl_unswitch (void)
> -{
> -  return flag_unswitch_loops;
> -}
> -
> -static unsigned int
> -rtl_unswitch (void)
> -{
> -  if (number_of_loops (cfun) > 1)
> -    unswitch_loops ();
> -  return 0;
> -}
> -
> -namespace {
> -
> -const pass_data pass_data_rtl_unswitch =
> -{
> -  RTL_PASS, /* type */
> -  "loop2_unswitch", /* name */
> -  OPTGROUP_LOOP, /* optinfo_flags */
> -  true, /* has_gate */
> -  true, /* has_execute */
> -  TV_LOOP_UNSWITCH, /* tv_id */
> -  0, /* properties_required */
> -  0, /* properties_provided */
> -  0, /* properties_destroyed */
> -  0, /* todo_flags_start */
> -  TODO_verify_rtl_sharing, /* todo_flags_finish */
> -};
> -
> -class pass_rtl_unswitch : public rtl_opt_pass
> -{
> -public:
> -  pass_rtl_unswitch (gcc::context *ctxt)
> -    : rtl_opt_pass (pass_data_rtl_unswitch, ctxt)
> -  {}
> -
> -  /* opt_pass methods: */
> -  bool gate () { return gate_rtl_unswitch (); }
> -  unsigned int execute () { return rtl_unswitch (); }
> -
> -}; // class pass_rtl_unswitch
> -
> -} // anon namespace
> -
> -rtl_opt_pass *
> -make_pass_rtl_unswitch (gcc::context *ctxt)
> -{
> -  return new pass_rtl_unswitch (ctxt);
> -}
> -
> -
> -/* Loop unswitching for RTL.  */
> +/* Loop unrolling and peeling for RTL.  */
>  static bool
>  gate_rtl_unroll_and_peel_loops (void)
>  {
> Index: gcc/loop-iv.c
> ===================================================================
> --- gcc/loop-iv.c	(revision 209410)
> +++ gcc/loop-iv.c	(working copy)
> @@ -1732,6 +1732,21 @@ canon_condition (rtx cond)
>    return cond;
>  }
>  
> +/* Reverses CONDition; returns NULL if we cannot.  */
> +
> +static rtx
> +reversed_condition (rtx cond)
> +{
> +  enum rtx_code reversed;
> +  reversed = reversed_comparison_code (cond, NULL);
> +  if (reversed == UNKNOWN)
> +    return NULL_RTX;
> +  else
> +    return gen_rtx_fmt_ee (reversed,
> +			   GET_MODE (cond), XEXP (cond, 0),
> +			   XEXP (cond, 1));
> +}
> +
>  /* Tries to use the fact that COND holds to simplify EXPR.  ALTERED is the
>     set of altered regs.  */
>  
> Index: gcc/loop-unroll.c
> ===================================================================
> --- gcc/loop-unroll.c	(revision 209410)
> +++ gcc/loop-unroll.c	(working copy)
> @@ -1060,6 +1060,59 @@ split_edge_and_insert (edge e, rtx insns
>    return bb;
>  }
>  
> +/* Prepare a sequence comparing OP0 with OP1 using COMP and jumping to LABEL if
> +   true, with probability PROB.  If CINSN is not NULL, it is the insn to copy
> +   in order to create a jump.  */
> +
> +static rtx
> +compare_and_jump_seq (rtx op0, rtx op1, enum rtx_code comp, rtx label, int prob,
> +		      rtx cinsn)
> +{
> +  rtx seq, jump, cond;
> +  enum machine_mode mode;
> +
> +  mode = GET_MODE (op0);
> +  if (mode == VOIDmode)
> +    mode = GET_MODE (op1);
> +
> +  start_sequence ();
> +  if (GET_MODE_CLASS (mode) == MODE_CC)
> +    {
> +      /* A hack -- there seems to be no easy generic way how to make a
> +	 conditional jump from a ccmode comparison.  */
> +      gcc_assert (cinsn);
> +      cond = XEXP (SET_SRC (pc_set (cinsn)), 0);
> +      gcc_assert (GET_CODE (cond) == comp);
> +      gcc_assert (rtx_equal_p (op0, XEXP (cond, 0)));
> +      gcc_assert (rtx_equal_p (op1, XEXP (cond, 1)));
> +      emit_jump_insn (copy_insn (PATTERN (cinsn)));
> +      jump = get_last_insn ();
> +      gcc_assert (JUMP_P (jump));
> +      JUMP_LABEL (jump) = JUMP_LABEL (cinsn);
> +      LABEL_NUSES (JUMP_LABEL (jump))++;
> +      redirect_jump (jump, label, 0);
> +    }
> +  else
> +    {
> +      gcc_assert (!cinsn);
> +
> +      op0 = force_operand (op0, NULL_RTX);
> +      op1 = force_operand (op1, NULL_RTX);
> +      do_compare_rtx_and_jump (op0, op1, comp, 0,
> +			       mode, NULL_RTX, NULL_RTX, label, -1);
> +      jump = get_last_insn ();
> +      gcc_assert (JUMP_P (jump));
> +      JUMP_LABEL (jump) = label;
> +      LABEL_NUSES (label)++;
> +    }
> +  add_int_reg_note (jump, REG_BR_PROB, prob);
> +
> +  seq = get_insns ();
> +  end_sequence ();
> +
> +  return seq;
> +}
> +
>  /* Unroll LOOP for which we are able to count number of iterations in runtime
>     LOOP->LPT_DECISION.TIMES times.  The transformation does this (with some
>     extra care for case n < 0):
> Index: gcc/rtl.h
> ===================================================================
> --- gcc/rtl.h	(revision 209410)
> +++ gcc/rtl.h	(working copy)
> @@ -2743,10 +2743,6 @@ extern unsigned int variable_tracking_ma
>  extern void get_mode_bounds (enum machine_mode, int, enum machine_mode,
>  			     rtx *, rtx *);
>  
> -/* In loop-unswitch.c  */
> -extern rtx reversed_condition (rtx);
> -extern rtx compare_and_jump_seq (rtx, rtx, enum rtx_code, rtx, int, rtx);
> -
>  /* In loop-iv.c  */
>  extern rtx canon_condition (rtx);
>  extern void simplify_using_condition (rtx, rtx *, bitmap);


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