Improved handling of MINUS_EXPR in bit CCP.

This patch improves the bit bounds for MINUS_EXPR during tree-ssa's
conditional constant propagation (CCP) pass (and as an added bonus
adds support for POINTER_DIFF_EXPR).

The pessimistic assumptions made by the current algorithm are
demonstrated by considering 1 - (x&1).  Intuitively this should
have possible values 0 and 1, and therefore an unknown mask of 1.
Alas by treating subtraction as a negation followed by addition,
the second operand first becomes 0 or -1, with an unknown mask
of all ones, which results in the addition containing no known bits.

Improved bounds are achieved by using the same approach used for
PLUS_EXPR, determining the result with the minimum number of borrows,
the result from the maximum number of borrows, and examining the bits
they have in common.  One additional benefit of this approach
is that it is applicable to POINTER_DIFF_EXPR, where previously the
negation of a pointer didn't/doesn't make sense.

A more convincing example, where a transformation missed by .032t.cpp
isn't caught a few passes later by .038t.evrp, is the expression
(7 - (x&5)) & 2, which (in the new test case) currently survives the
tree-level optimizers but with this patch is now simplified to the
constant value 2.

2021-08-17  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	* tree-ssa-ccp.c (bit_value_binop) [MINUS_EXPR]: Use same
	algorithm as PLUS_EXPR to improve subtraction bit bounds.
	[POINTER_DIFF_EXPR]: Treat as synonymous with MINUS_EXPR.

gcc/testsuite/ChangeLog
	* gcc.dg/tree-ssa/ssa-ccp-40.c: New test case.

diff --git a/gcc/testsuite/gcc.dg/tree-ssa/ssa-ccp-40.c b/gcc/testsuite/gcc.dg/tree-ssa/ssa-ccp-40.c
new file mode 100644 (file)
index 0000000..aa7349e
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-ssa/ssa-ccp-40.c
@@ -0,0 +1,11 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -fdump-tree-optimized" } */
+
+int foo(int x)
+{
+  int p = 7;
+  int q = p - (x & 5);
+  return q & 2;
+}
+
+/* { dg-final { scan-tree-dump "return 2;" "optimized" } } */

diff --git a/gcc/tree-ssa-ccp.c b/gcc/tree-ssa-ccp.c
index 8e4d8aeb69ba426d37eb7347294799a060315ad7..1a63ae5f10446f2d6ce85be99da3ad2265d9990e 100644 (file)
--- a/gcc/tree-ssa-ccp.c
+++ b/gcc/tree-ssa-ccp.c
@@ -1458,7 +1458,7 @@ bit_value_binop (enum tree_code code, signop sgn, int width,
                 widest_int *val, widest_int *mask,
                 signop r1type_sgn, int r1type_precision,
                 const widest_int &r1val, const widest_int &r1mask,
-                signop r2type_sgn, int r2type_precision,
+                signop r2type_sgn, int r2type_precision ATTRIBUTE_UNUSED,
                 const widest_int &r2val, const widest_int &r2mask)
 {
   bool swap_p = false;
@@ -1505,7 +1505,7 @@ bit_value_binop (enum tree_code code, signop sgn, int width,
            }
          else
            {
-             if (wi::neg_p (shift))
+             if (wi::neg_p (shift, r2type_sgn))
                {
                  shift = -shift;
                  if (code == RROTATE_EXPR)
@@ -1542,7 +1542,7 @@ bit_value_binop (enum tree_code code, signop sgn, int width,
            }
          else
            {
-             if (wi::neg_p (shift))
+             if (wi::neg_p (shift, r2type_sgn))
                break;
              if (code == RSHIFT_EXPR)
                {
@@ -1582,13 +1582,16 @@ bit_value_binop (enum tree_code code, signop sgn, int width,
       }
 
     case MINUS_EXPR:
+    case POINTER_DIFF_EXPR:
       {
-       widest_int temv, temm;
-       bit_value_unop (NEGATE_EXPR, r2type_sgn, r2type_precision, &temv, &temm,
-                         r2type_sgn, r2type_precision, r2val, r2mask);
-       bit_value_binop (PLUS_EXPR, sgn, width, val, mask,
-                        r1type_sgn, r1type_precision, r1val, r1mask,
-                        r2type_sgn, r2type_precision, temv, temm);
+       /* Subtraction is derived from the addition algorithm above.  */
+       widest_int lo = wi::bit_and_not (r1val, r1mask) - (r2val | r2mask);
+       lo = wi::ext (lo, width, sgn);
+       widest_int hi = (r1val | r1mask) - wi::bit_and_not (r2val, r2mask);
+       hi = wi::ext (hi, width, sgn);
+       *mask = r1mask | r2mask | (lo ^ hi);
+       *mask = wi::ext (*mask, width, sgn);
+       *val = lo;
        break;
       }