[wide-int] Simplify mult-to-shift conversion

[Richard Sandiford]

Trunk expand_mult uses the following code to handle CONST_DOUBLEs:

	  /* If we are multiplying in DImode, it may still be a win
	     to try to work with shifts and adds.  */
	  if (CONST_DOUBLE_HIGH (scalar_op1) == 0
	      && (CONST_DOUBLE_LOW (scalar_op1) > 0
		  || (CONST_DOUBLE_LOW (scalar_op1) < 0
		      && EXACT_POWER_OF_2_OR_ZERO_P
			   (CONST_DOUBLE_LOW (scalar_op1)))))
	    {
	      coeff = CONST_DOUBLE_LOW (scalar_op1);
	      is_neg = false;
	    }
	  else if (CONST_DOUBLE_LOW (scalar_op1) == 0)
	    {
	      coeff = CONST_DOUBLE_HIGH (scalar_op1);
	      if (EXACT_POWER_OF_2_OR_ZERO_P (coeff))
		{
		  int shift = floor_log2 (coeff) + HOST_BITS_PER_WIDE_INT;
		  if (shift < HOST_BITS_PER_DOUBLE_INT - 1
		      || mode_bitsize <= HOST_BITS_PER_DOUBLE_INT)
		    return expand_shift (LSHIFT_EXPR, mode, op0,
					 shift, target, unsignedp);
		}
	      goto skip_synth;
	    }
	  else
	    goto skip_synth;

So the only non-power-of-2 case that avoids the skip_synth treatment
is a CONST_DOUBLE whose high HWI is 0 and whose low HWI is positive.
I think that's outdated, because even on trunk such constants should be
CONST_INTs instead.  They definitely are on wide-int.  So we only need
to handle the power-of-2 case.

The shift handling for that case can be much simpler.  We're dealing
with a constant in a particular mode, so if exact_log2 returns >= 0,
it's a valid shift for that mode.  No truncation or boundary checks
are needed.  wide ints really clean this up.

The same reasoning applies to simplify_binary_operation_1, except there
we're just saving a redundant upper bound test.

Tested on powerpc64-linux-gnu and by rerunning the assembly comparison.
OK to install?

Thanks,
Richard


Index: gcc/expmed.c
===================================================================
--- gcc/expmed.c	2013-11-09 10:50:12.836314443 +0000
+++ gcc/expmed.c	2013-11-09 10:50:37.333494113 +0000
@@ -3075,30 +3075,11 @@ expand_mult (enum machine_mode mode, rtx
       else if (CONST_DOUBLE_AS_INT_P (scalar_op1))
 #endif
 	{
-	  int p = GET_MODE_PRECISION (mode);
 	  int shift = wi::exact_log2 (std::make_pair (scalar_op1, mode));
-	  /* Perfect power of 2.  */
-	  is_neg = false;
+	  /* Perfect power of 2 (other than 1, which is handled above).  */
 	  if (shift > 0)
-	    {
-	      /* Do the shift count trucation against the bitsize, not
-		 the precision.  See the comment above
-		 wide-int.c:trunc_shift for details.  */
-	      if (SHIFT_COUNT_TRUNCATED)
-		shift &= GET_MODE_BITSIZE (mode) - 1;
-	      /* We could consider adding just a move of 0 to target
-		 if the shift >= p  */
-	      if (shift < p)
-		return expand_shift (LSHIFT_EXPR, mode, op0, 
-				     shift, target, unsignedp);
-	      /* Any positive number that fits in a word.  */
-	      coeff = CONST_WIDE_INT_ELT (scalar_op1, 0);
-	    }
-	  else if (wi::sign_mask (std::make_pair (scalar_op1, mode)) == 0)
-	    {
-	      /* Any positive number that fits in a word.  */
-	      coeff = CONST_WIDE_INT_ELT (scalar_op1, 0);
-	    }
+	    return expand_shift (LSHIFT_EXPR, mode, op0,
+				 shift, target, unsignedp);
 	  else
 	    goto skip_synth;
 	}
Index: gcc/simplify-rtx.c
===================================================================
--- gcc/simplify-rtx.c	2013-11-09 10:50:12.836314443 +0000
+++ gcc/simplify-rtx.c	2013-11-09 10:50:14.046323332 +0000
@@ -2399,7 +2399,7 @@ simplify_binary_operation_1 (enum rtx_co
       if (CONST_SCALAR_INT_P (trueop1))
 	{
 	  val = wi::exact_log2 (std::make_pair (trueop1, mode));
-	  if (val >= 0 && val < GET_MODE_BITSIZE (mode))
+	  if (val >= 0)
 	    return simplify_gen_binary (ASHIFT, mode, op0, GEN_INT (val));
 	}