Improved handling of division/modulus in bit CCP.

This patch implements support for TRUNC_MOD_EXPR and TRUNC_DIV_EXPR
in tree-ssa's bit CCP pass.  This is mostly for completeness, as the
VRP pass already provides better bounds for these operations, but
seeing mask values of all_ones in my debugging/instrumentation logs
seemed overly pessimistic.  With this patch, the expression X%10
has a nonzero bits of 0x0f (for unsigned X), likewise (X&1)/3 has
a known value of zero, and (X&3)/3 has a nonzero bits mask of 0x1.

2021-08-23  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	* tree-ssa-ccp.c (bit_value_binop) [TRUNC_MOD_EXPR, TRUNC_DIV_EXPR]:
	Provide bounds for unsigned (and signed with non-negative operands)
	division and modulus.

diff --git a/gcc/tree-ssa-ccp.c b/gcc/tree-ssa-ccp.c
index 1a63ae5..1a94aeb 100644 (file)
--- a/gcc/tree-ssa-ccp.c
+++ b/gcc/tree-ssa-ccp.c
@@ -1736,6 +1736,68 @@ bit_value_binop (enum tree_code code, signop sgn, int width,
        break;
       }
 
+    case TRUNC_MOD_EXPR:
+      {
+       widest_int r1max = r1val | r1mask;
+       widest_int r2max = r2val | r2mask;
+       if (sgn == UNSIGNED
+           || (!wi::neg_p (r1max) && !wi::neg_p (r2max)))
+         {
+           /* Confirm R2 has some bits set, to avoid division by zero.  */
+           widest_int r2min = wi::bit_and_not (r2val, r2mask);
+           if (r2min != 0)
+             {
+               /* R1 % R2 is R1 if R1 is always less than R2.  */
+               if (wi::ltu_p (r1max, r2min))
+                 {
+                   *mask = r1mask;
+                   *val = r1val;
+                 }
+               else
+                 {
+                   /* R1 % R2 is always less than the maximum of R2.  */
+                   unsigned int lzcount = wi::clz (r2max);
+                   unsigned int bits = wi::get_precision (r2max) - lzcount;
+                   if (r2max == wi::lshift (1, bits))
+                     bits--;
+                   *mask = wi::mask <widest_int> (bits, false);
+                   *val = 0;
+                 }
+              }
+           }
+       }
+      break;
+
+    case TRUNC_DIV_EXPR:
+      {
+       widest_int r1max = r1val | r1mask;
+       widest_int r2max = r2val | r2mask;
+       if (sgn == UNSIGNED
+           || (!wi::neg_p (r1max) && !wi::neg_p (r2max)))
+         {
+           /* Confirm R2 has some bits set, to avoid division by zero.  */
+           widest_int r2min = wi::bit_and_not (r2val, r2mask);
+           if (r2min != 0)
+             {
+               /* R1 / R2 is zero if R1 is always less than R2.  */
+               if (wi::ltu_p (r1max, r2min))
+                 {
+                   *mask = 0;
+                   *val = 0;
+                 }
+               else
+                 {
+                   widest_int upper = wi::udiv_trunc (r1max, r2min);
+                   unsigned int lzcount = wi::clz (upper);
+                   unsigned int bits = wi::get_precision (upper) - lzcount;
+                   *mask = wi::mask <widest_int> (bits, false);
+                   *val = 0;
+                 }
+              }
+           }
+       }
+      break;
+
     default:;
     }
 }