[Committed] PR21137: Fold ((X>>1)&4) != 0 as (X&8) != 0

[Roger Sayle <roger at eyesopen dot com>]

The following patch resolves PR middle-end/21137 by implementing
some constant folding transformations of ((X >> C1) & C2) eq/ne 0
where C2 is a single bit, i.e. a power of two.  My thanks to Jim
Morrison for his previous attempt at the PR; the patch below fixes
the problems identified in the review of his patch.

The first difficulty is that the test for equality/inequality is
vital in this transformation.  "if ((x >> 2) & 1)" is equivalent
to "if (x & 4)", but "y = ((x >> 2) & 1)" isn't the same as
"y = (x & 4)".  The other refinement is that "if ((x >> 31) & 4)"
needs to be implemented as "if (x < 0)" or "if (0)" depending
upon whether x is signed (an arithmetic shift), or unsigned
(a logical shift) respectively.

The following patch has been tested on x86_64-unknown-linux-gnu
with a full "make bootstrap", all default languages, and regression
tested with a top-level "make -k check" with no new failures.

Committed to mainline as revision 111453.



2006-02-26  Roger Sayle  <roger@eyesopen.com>
	    James A. Morrison  <phython@gcc.gnu.org>

	PR middle-end/21137
	* fold-const.c (fold_binary) <EQ_EXPR>:  Fold ((X>>C1)&C2) eq/ne 0,
	when C2 is a power of two, as either (X&(C2<<C1)) eq/ne 0 if the
	new constant C2<<C1, or as (X<0) or (X,false) depending upon the
	signedness of the shift operation.

	* gcc.dg/fold-eqandshift-1.c: New test case.


Index: fold-const.c
===================================================================
*** fold-const.c	(revision 111428)
--- fold-const.c	(working copy)
*************** fold_binary (enum tree_code code, tree t
*** 9653,9658 ****
--- 9653,9704 ----
  			      fold_convert (newtype, arg1));
  	}

+       /* Fold ((X >> C1) & C2) == 0 and ((X >> C1) & C2) != 0 where
+ 	 C1 is a valid shift constant, and C2 is a power of two, i.e.
+ 	 a single bit.  */
+       if (TREE_CODE (arg0) == BIT_AND_EXPR
+ 	  && TREE_CODE (TREE_OPERAND (arg0, 0)) == RSHIFT_EXPR
+ 	  && TREE_CODE (TREE_OPERAND (TREE_OPERAND (arg0, 0), 1))
+ 	     == INTEGER_CST
+ 	  && integer_pow2p (TREE_OPERAND (arg0, 1))
+ 	  && integer_zerop (arg1))
+ 	{
+ 	  tree itype = TREE_TYPE (arg0);
+ 	  unsigned HOST_WIDE_INT prec = TYPE_PRECISION (itype);
+ 	  tree arg001 = TREE_OPERAND (TREE_OPERAND (arg0, 0), 1);
+
+ 	  /* Check for a valid shift count.  */
+ 	  if (TREE_INT_CST_HIGH (arg001) == 0
+ 	      && TREE_INT_CST_LOW (arg001) < prec)
+ 	    {
+ 	      tree arg01 = TREE_OPERAND (arg0, 1);
+ 	      tree arg000 = TREE_OPERAND (TREE_OPERAND (arg0, 0), 0);
+ 	      unsigned HOST_WIDE_INT log2 = tree_log2 (arg01);
+ 	      /* If (C2 << C1) doesn't overflow, then ((X >> C1) & C2) != 0
+ 		 can be rewritten as (X & (C2 << C1)) != 0.  */
+ 	      if ((log2 + TREE_INT_CST_LOW (arg01)) < prec)
+ 		{
+ 		  tem = fold_build2 (LSHIFT_EXPR, itype, arg01, arg001);
+ 		  tem = fold_build2 (BIT_AND_EXPR, itype, arg000, tem);
+ 		  return fold_build2 (code, type, tem, arg1);
+ 		}
+ 	      /* Otherwise, for signed (arithmetic) shifts,
+ 		 ((X >> C1) & C2) != 0 is rewritten as X < 0, and
+ 		 ((X >> C1) & C2) == 0 is rewritten as X >= 0.  */
+ 	      else if (!TYPE_UNSIGNED (itype))
+ 		return fold_build2 (code == EQ_EXPR ? GE_EXPR : LT_EXPR, type,
+ 				    arg000, build_int_cst (itype, 0));
+ 	      /* Otherwise, of unsigned (logical) shifts,
+ 		 ((X >> C1) & C2) != 0 is rewritten as (X,false), and
+ 		 ((X >> C1) & C2) == 0 is rewritten as (X,true).  */
+ 	      else
+ 		return omit_one_operand (type,
+ 					 code == EQ_EXPR ? integer_one_node
+ 							 : integer_zero_node,
+ 					 arg000);
+ 	    }
+ 	}
+
        /* If this is an NE comparison of zero with an AND of one, remove the
  	 comparison since the AND will give the correct value.  */
        if (code == NE_EXPR


/* PR middle-end/21137  */
/* { dg-do compile } */
/* { dg-options "-O2 -fdump-tree-original" } */
extern void foo();

void test1(int a)
{
  if ((a >> 3) & 1)
    foo ();
}

void test2(int b)
{
  if ((b >> 3) & 4)
    foo ();
}

int test3(int c)
{
  return (c >> 3) & 1;
}

int test4(int d)
{
  return (d >> 3) & 4;
}

#if 0
void test5(int e)
{
  if ((e >> 31) & 64)
    foo();
}

void test6(unsigned int f)
{
  if ((f >> 31) & 64)
    foo();
}
#endif

/* { dg-final { scan-tree-dump-times "\\(a \& 8\\) != 0" 1 "original" } } */
/* { dg-final { scan-tree-dump-times "\\(b \& 32\\) != 0" 1 "original" } } */
/* { dg-final { scan-tree-dump-times "c >> 3 \& 1" 1 "original" } } */
/* { dg-final { scan-tree-dump-times "d >> 3 \& 4" 1 "original" } } */
/* { dg-final { cleanup-tree-dump "original" } } */

Roger
--