jump.c

gcc库的原代码,对编程有很大帮助.

		 2) if (...) x = 0; and jumps are expensive,
		 3) x = a; if (...) x = b; and A and B are constants where all
		 the set bits in A are also set in B and jumps are expensive,
		 4) x = a; if (...) x = b; and A and B non-zero, and jumps are
		 more expensive, and
		 5) if (...) x = b; if jumps are even more expensive.  */

	      if (GET_MODE_CLASS (GET_MODE (temp1)) == MODE_INT
		  && ((GET_CODE (temp3) == CONST_INT)
		      /* Make the latter case look like
			 x = x; if (...) x = 0;  */
		      || (temp3 = temp1,
			  ((BRANCH_COST >= 2
			    && temp2 == const0_rtx)
			   || BRANCH_COST >= 3)))
		  /* If B is zero, OK; if A is zero, can only do (1) if we
		     can reverse the condition.  See if (3) applies possibly
		     by reversing the condition.  Prefer reversing to (4) when
		     branches are very expensive.  */
		  && ((reversep = 0, temp2 == const0_rtx)
		      || (temp3 == const0_rtx
			  && (reversep = can_reverse_comparison_p (temp4, insn)))
		      || (BRANCH_COST >= 2
			  && GET_CODE (temp2) == CONST_INT
			  && GET_CODE (temp3) == CONST_INT
			  && ((INTVAL (temp2) & INTVAL (temp3)) == INTVAL (temp2)
			      || ((INTVAL (temp2) & INTVAL (temp3)) == INTVAL (temp3)
				  && (reversep = can_reverse_comparison_p (temp4,
									   insn)))))
		      || BRANCH_COST >= 3)
		  )
		{
		  enum rtx_code code = GET_CODE (temp4);
		  rtx uval, cval, var = temp1;
		  int normalizep;
		  rtx target;

		  /* If necessary, reverse the condition.  */
		  if (reversep)
		    code = reverse_condition (code), uval = temp2, cval = temp3;
		  else
		    uval = temp3, cval = temp2;

		  /* If CVAL is non-zero, normalize to -1.  Otherwise, if UVAL
		     is the constant 1, it is best to just compute the result
		     directly.  If UVAL is constant and STORE_FLAG_VALUE
		     includes all of its bits, it is best to compute the flag
		     value unnormalized and `and' it with UVAL.  Otherwise,
		     normalize to -1 and `and' with UVAL.  */
		  normalizep = (cval != const0_rtx ? -1
				: (uval == const1_rtx ? 1
				   : (GET_CODE (uval) == CONST_INT
				      && (INTVAL (uval) & ~STORE_FLAG_VALUE) == 0)
				   ? 0 : -1));

		  /* We will be putting the store-flag insn immediately in
		     front of the comparison that was originally being done,
		     so we know all the variables in TEMP4 will be valid.
		     However, this might be in front of the assignment of
		     A to VAR.  If it is, it would clobber the store-flag
		     we will be emitting.

		     Therefore, emit into a temporary which will be copied to
		     VAR immediately after TEMP.  */

		  start_sequence ();
		  target = emit_store_flag (gen_reg_rtx (GET_MODE (var)), code,
					    XEXP (temp4, 0), XEXP (temp4, 1),
					    VOIDmode,
					    (code == LTU || code == LEU 
					     || code == GEU || code == GTU),
					    normalizep);
		  if (target)
		    {
		      rtx seq;
		      rtx before = insn;

		      seq = get_insns ();
		      end_sequence ();

		      /* Put the store-flag insns in front of the first insn
			 used to compute the condition to ensure that we
			 use the same values of them as the current 
			 comparison.  However, the remainder of the insns we
			 generate will be placed directly in front of the
			 jump insn, in case any of the pseudos we use
			 are modified earlier.  */

		      emit_insns_before (seq, temp5);

		      start_sequence ();

		      /* Both CVAL and UVAL are non-zero.  */
		      if (cval != const0_rtx && uval != const0_rtx)
			{
			  rtx tem1, tem2;

			  tem1 = expand_and (uval, target, NULL_RTX);
			  if (GET_CODE (cval) == CONST_INT
			      && GET_CODE (uval) == CONST_INT
			      && (INTVAL (cval) & INTVAL (uval)) == INTVAL (cval))
			    tem2 = cval;
			  else
			    {
			      tem2 = expand_unop (GET_MODE (var), one_cmpl_optab,
						  target, NULL_RTX, 0);
			      tem2 = expand_and (cval, tem2,
						 (GET_CODE (tem2) == REG
						  ? tem2 : 0));
			    }

			  /* If we usually make new pseudos, do so here.  This
			     turns out to help machines that have conditional
			     move insns.  */
			  /* ??? Conditional moves have already been handled.
			     This may be obsolete.  */

			  if (flag_expensive_optimizations)
			    target = 0;

			  target = expand_binop (GET_MODE (var), ior_optab,
						 tem1, tem2, target,
						 1, OPTAB_WIDEN);
			}
		      else if (normalizep != 1)
			{
			  /* We know that either CVAL or UVAL is zero.  If
			     UVAL is zero, negate TARGET and `and' with CVAL.
			     Otherwise, `and' with UVAL.  */
			  if (uval == const0_rtx)
			    {
			      target = expand_unop (GET_MODE (var), one_cmpl_optab,
						    target, NULL_RTX, 0);
			      uval = cval;
			    }

			  target = expand_and (uval, target,
					       (GET_CODE (target) == REG
						&& ! preserve_subexpressions_p ()
						? target : NULL_RTX));
			}
		  
		      emit_move_insn (var, target);
		      seq = get_insns ();
		      end_sequence ();
#ifdef HAVE_cc0
		      /* If INSN uses CC0, we must not separate it from the
			 insn that sets cc0.  */
		      if (reg_mentioned_p (cc0_rtx, PATTERN (before)))
			before = prev_nonnote_insn (before);
#endif
		      emit_insns_before (seq, before);

		      delete_insn (temp);
		      next = NEXT_INSN (insn);
		      delete_jump (insn);
		      changed = 1;
		      continue;
		    }
		  else
		    end_sequence ();
		}
	    }

	  /* If branches are expensive, convert
	        if (foo) bar++;    to    bar += (foo != 0);
	     and similarly for "bar--;" 

	     INSN is the conditional branch around the arithmetic.  We set:

	     TEMP is the arithmetic insn.
	     TEMP1 is the SET doing the arithmetic.
	     TEMP2 is the operand being incremented or decremented.
	     TEMP3 to the condition being tested.
	     TEMP4 to the earliest insn used to find the condition.  */

	  if ((BRANCH_COST >= 2
#ifdef HAVE_incscc
	       || HAVE_incscc
#endif
#ifdef HAVE_decscc
	       || HAVE_decscc
#endif
	      )
	      && ! reload_completed
	      && this_is_condjump && ! this_is_simplejump
	      && (temp = next_nonnote_insn (insn)) != 0
	      && (temp1 = single_set (temp)) != 0
	      && (temp2 = SET_DEST (temp1),
		  GET_MODE_CLASS (GET_MODE (temp2)) == MODE_INT)
	      && GET_CODE (SET_SRC (temp1)) == PLUS
	      && (XEXP (SET_SRC (temp1), 1) == const1_rtx
		  || XEXP (SET_SRC (temp1), 1) == constm1_rtx)
	      && rtx_equal_p (temp2, XEXP (SET_SRC (temp1), 0))
	      && ! side_effects_p (temp2)
	      && ! may_trap_p (temp2)
	      /* INSN must either branch to the insn after TEMP or the insn
		 after TEMP must branch to the same place as INSN.  */
	      && (reallabelprev == temp
		  || ((temp3 = next_active_insn (temp)) != 0
		      && simplejump_p (temp3)
		      && JUMP_LABEL (temp3) == JUMP_LABEL (insn)))
	      && (temp3 = get_condition (insn, &temp4)) != 0
	      /* We must be comparing objects whose modes imply the size.
		 We could handle BLKmode if (1) emit_store_flag could
		 and (2) we could find the size reliably.  */
	      && GET_MODE (XEXP (temp3, 0)) != BLKmode
	      && can_reverse_comparison_p (temp3, insn))
	    {
	      rtx temp6, target = 0, seq, init_insn = 0, init = temp2;
	      enum rtx_code code = reverse_condition (GET_CODE (temp3));

	      start_sequence ();

	      /* It must be the case that TEMP2 is not modified in the range
		 [TEMP4, INSN).  The one exception we make is if the insn
		 before INSN sets TEMP2 to something which is also unchanged
		 in that range.  In that case, we can move the initialization
		 into our sequence.  */

	      if ((temp5 = prev_active_insn (insn)) != 0
		  && GET_CODE (temp5) == INSN
		  && (temp6 = single_set (temp5)) != 0
		  && rtx_equal_p (temp2, SET_DEST (temp6))
		  && (CONSTANT_P (SET_SRC (temp6))
		      || GET_CODE (SET_SRC (temp6)) == REG
		      || GET_CODE (SET_SRC (temp6)) == SUBREG))
		{
		  emit_insn (PATTERN (temp5));
		  init_insn = temp5;
		  init = SET_SRC (temp6);
		}

	      if (CONSTANT_P (init)
		  || ! reg_set_between_p (init, PREV_INSN (temp4), insn))
		target = emit_store_flag (gen_reg_rtx (GET_MODE (temp2)), code,
					  XEXP (temp3, 0), XEXP (temp3, 1),
					  VOIDmode,
					  (code == LTU || code == LEU
					   || code == GTU || code == GEU), 1);

	      /* If we can do the store-flag, do the addition or
		 subtraction.  */

	      if (target)
		target = expand_binop (GET_MODE (temp2),
				       (XEXP (SET_SRC (temp1), 1) == const1_rtx
					? add_optab : sub_optab),
				       temp2, target, temp2, 0, OPTAB_WIDEN);

	      if (target != 0)
		{
		  /* Put the result back in temp2 in case it isn't already.
		     Then replace the jump, possible a CC0-setting insn in
		     front of the jump, and TEMP, with the sequence we have
		     made.  */

		  if (target != temp2)
		    emit_move_insn (temp2, target);

		  seq = get_insns ();
		  end_sequence ();

		  emit_insns_before (seq, temp4);
		  delete_insn (temp);

		  if (init_insn)
		    delete_insn (init_insn);

		  next = NEXT_INSN (insn);
#ifdef HAVE_cc0
		  delete_insn (prev_nonnote_insn (insn));
#endif
		  delete_insn (insn);
		  changed = 1;
		  continue;
		}
	      else
		end_sequence ();
	    }

	  /* Simplify   if (...) x = 1; else {...}  if (x) ...
	     We recognize this case scanning backwards as well.

	     TEMP is the assignment to x;
	     TEMP1 is the label at the head of the second if.  */
	  /* ?? This should call get_condition to find the values being
	     compared, instead of looking for a COMPARE insn when HAVE_cc0
	     is not defined.  This would allow it to work on the m88k.  */
	  /* ?? This optimization is only safe before cse is run if HAVE_cc0
	     is not defined and the condition is tested by a separate compare
	     insn.  This is because the code below assumes that the result
	     of the compare dies in the following branch.

	     Not only that, but there might be other insns between the
	     compare and branch whose results are live.  Those insns need
	     to be executed.

	     A way to fix this is to move the insns at JUMP_LABEL (insn)
	     to before INSN.  If we are running before flow, they will
	     be deleted if they aren't needed.   But this doesn't work
	     well after flow.

	     This is really a special-case of jump threading, anyway.  The
	     right thing to do is to replace this and jump threading with
	     much simpler code in cse.

	     This code has been turned off in the non-cc0 case in the
	     meantime.  */

#ifdef HAVE_cc0
	  else if (this_is_simplejump
		   /* Safe to skip USE and CLOBBER insns here
		      since they will not be deleted.  */
		   && (temp = prev_active_insn (insn))
		   && no_labels_between_p (temp, insn)
		   && GET_CODE (temp) == INSN
		   && GET_CODE (PATTERN (temp)) == SET
		   && GET_CODE (SET_DEST (PATTERN (temp))) == REG
		   && CONSTANT_P (SET_SRC (PATTERN (temp)))
		   && (temp1 = next_active_insn (JUMP_LABEL (insn)))
		   /* If we find that the next value tested is `x'
		      (TEMP1 is the insn where this happens), win.  */
		   && GET_CODE (temp1) == INSN
		   && GET_CODE (PATTERN (temp1)) == SET
#ifdef HAVE_cc0
		   /* Does temp1 `tst' the value of x?  */
		   && SET_SRC (PATTERN (temp1)) == SET_DEST (PATTERN (temp))
		   && SET_DEST (PATTERN (temp1)) == cc0_rtx
		   && (temp1 = next_nonnote_insn (temp1))
#else
		   /* Does temp1 compare the value of x against zero?  */
		   && GET_CODE (SET_SRC (PATTERN (temp1))) == COMPARE
		   && XEXP (SET_SRC (PATTERN (temp1)), 1) == const0_rtx
		   && (XEXP (SET_SRC (PATTERN (temp1)), 0)
		       == SET_DEST (PATTERN (temp)))
		   && GET_CODE (SET_DEST (PATTERN (temp1))) == REG
		   && (temp1 = find_next_ref (SET_DEST (PATTERN (temp1)), temp1))
#endif
		   && condjump_p (temp1))
	    {
	      /* Get the if_then_else from the condjump.  */
	      rtx choice = SET_SRC (PATTERN (temp1));
	      if (GET_CODE (choice) == IF_THEN_ELSE)
		{
		  enum rtx_code code = GET_CODE (XEXP (choice, 0));
		  rtx val = SET_SRC (PATTERN (temp));
		  rtx cond
		    = simplify_relational_operation (code, GET_MODE (SET_DEST (PATTERN (temp))),
						     val, const0_rtx);
		  rtx ultimate;

		  if (cond == const_true_rtx)
		    ultimate = XEXP (choice, 1);
		  else if (cond == const0_rtx)
		    ultimate = XEXP (choice, 2);
		  else
		    ultimate = 0;

		  if (ultimate == pc_rtx)
		    ultimate = get_label_after (temp1);
		  else if (ultimate && GET_CODE (ultimate) != RETURN)
		    ultimate = XEXP (ultimate, 0);

		  if (ultimate)
		    changed |= redirect_jump (insn, ultimate);
		}
	    }
#endif

#if 0
	  /* @@ This needs a bit of work before it will be right.

	     Any type of comparison can be accepted for the first and
	     second compare.  When rewriting the first jump, we must
	     compute the what conditions can reach label3, and use the
	     appropriate code.  We can not simply reverse/swap the code
	     of the first jump.  In some cases, the second jump must be
	     rewritten also.

	     For example,