combine: Allow combining two insns to two insns

This patch allows combine to combine two insns into two.  This helps
in many cases, by reducing instruction path length, and also allowing
further combinations to happen.  PR85160 is a typical example of code
that it can improve.

This patch does not allow such combinations if either of the original
instructions was a simple move instruction.  In those cases combining
the two instructions increases register pressure without improving the
code.  With this move test register pressure does no longer increase
noticably as far as I can tell.

(At first I also didn't allow either of the resulting insns to be a
move instruction.  But that is actually a very good thing to have, as
should have been obvious).


	PR rtl-optimization/85160
	* combine.c (is_just_move): New function.
	(try_combine): Allow combining two instructions into two if neither of
	the original instructions was a move.

From-SVN: r263067

gcc/ChangeLog

+2018-07-30  Segher Boessenkool  <segher@kernel.crashing.org>
+
+	PR rtl-optimization/85160
+	* combine.c (is_just_move): New function.
+	(try_combine): Allow combining two instructions into two if neither of
+	the original instructions was a move.
+
 2018-07-30  Alexander Monakov  <amonakov@ispras.ru>
 
 	PR target/86673

gcc/combine.c

@@ -2604,6 +2604,17 @@ can_split_parallel_of_n_reg_sets (rtx_insn *insn, int n)
   return true;
 }
 
+/* Return whether X is just a single set, with the source
+   a general_operand.  */
+static bool
+is_just_move (rtx x)
+{
+  if (INSN_P (x))
+    x = PATTERN (x);
+
+  return (GET_CODE (x) == SET && general_operand (SET_SRC (x), VOIDmode));
+}
+
 /* Try to combine the insns I0, I1 and I2 into I3.
    Here I0, I1 and I2 appear earlier than I3.
    I0 and I1 can be zero; then we combine just I2 into I3, or I1 and I2 into
@@ -2668,6 +2679,7 @@ try_combine (rtx_insn *i3, rtx_insn *i2, rtx_insn *i1, rtx_insn *i0,
   int swap_i2i3 = 0;
   int split_i2i3 = 0;
   int changed_i3_dest = 0;
+  bool i2_was_move = false, i3_was_move = false;
 
   int maxreg;
   rtx_insn *temp_insn;
@@ -3059,6 +3071,10 @@ try_combine (rtx_insn *i3, rtx_insn *i2, rtx_insn *i1, rtx_insn *i0,
       return 0;
     }
 
+  /* Record whether i2 and i3 are trivial moves.  */
+  i2_was_move = is_just_move (i2);
+  i3_was_move = is_just_move (i3);
+
   /* Record whether I2DEST is used in I2SRC and similarly for the other
      cases.  Knowing this will help in register status updating below.  */
   i2dest_in_i2src = reg_overlap_mentioned_p (i2dest, i2src);
@@ -4014,8 +4030,10 @@ try_combine (rtx_insn *i3, rtx_insn *i2, rtx_insn *i1, rtx_insn *i0,
 	   && XVECLEN (newpat, 0) == 2
 	   && GET_CODE (XVECEXP (newpat, 0, 0)) == SET
 	   && GET_CODE (XVECEXP (newpat, 0, 1)) == SET
-	   && (i1 || set_noop_p (XVECEXP (newpat, 0, 0))
-		  || set_noop_p (XVECEXP (newpat, 0, 1)))
+	   && (i1
+	       || set_noop_p (XVECEXP (newpat, 0, 0))
+	       || set_noop_p (XVECEXP (newpat, 0, 1))
+	       || (!i2_was_move && !i3_was_move))
 	   && GET_CODE (SET_DEST (XVECEXP (newpat, 0, 0))) != ZERO_EXTRACT
 	   && GET_CODE (SET_DEST (XVECEXP (newpat, 0, 0))) != STRICT_LOW_PART
 	   && GET_CODE (SET_DEST (XVECEXP (newpat, 0, 1))) != ZERO_EXTRACT