Improved handling of MULT_EXPR in bit CCP.

This patch allows GCC to constant fold (i | (i<<16)) | ((i<<24) | (i<<8)),
where i is an unsigned char, or the equivalent (i*65537) | (i*16777472), to
i*16843009.  The trick is to teach tree_nonzero_bits which bits may be
set in the result of a multiplication by a constant given which bits are
potentially set in the operands.  This allows the optimizations recently
added to match.pd to catch more cases.

The required mask/value pair from a multiplication may be calculated using
a classical shift-and-add algorithm, given we already have implementations
for both addition and shift by constant.  To keep this optimization "cheap",
this functionality is only used if the constant multiplier has a few bits
set (unless flag_expensive_optimizations), and we provide a special case
fast-path implementation for the common case where the (non-constant)
operand has no bits that are guaranteed to be set.  I have no evidence
that this functionality causes performance issues, it's just that sparse
multipliers provide the largest benefit to CCP.

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

gcc/ChangeLog
	* tree-ssa-ccp.c (bit_value_mult_const): New helper function to
	calculate the mask-value pair result of a multiplication by an
	unsigned constant.
	(bit_value_binop) [MULT_EXPR]:  Call it from here for
	multiplications by (sparse) non-negative constants.

gcc/testsuite/ChangeLog
	* gcc.dg/fold-ior-5.c: New test case.

diff --git a/gcc/testsuite/gcc.dg/fold-ior-5.c b/gcc/testsuite/gcc.dg/fold-ior-5.c
new file mode 100644 (file)
index 0000000..8de5697
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/fold-ior-5.c
@@ -0,0 +1,17 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -fdump-tree-optimized" } */
+
+unsigned int test_ior(unsigned char i)
+{
+  return (i | (i<<16)) | ((i<<24) | (i<<8));
+}
+
+unsigned int test_xor(unsigned char i)
+{
+  return (i ^ (i<<16)) ^ ((i<<24) ^ (i<<8));
+}
+
+/* { dg-final { scan-tree-dump-not " \\^ " "optimized" } } */
+/* { dg-final { scan-tree-dump-not " \\| " "optimized" } } */
+/* { dg-final { scan-tree-dump-times " \\* 16843009" 2 "optimized" } } */
+

diff --git a/gcc/tree-ssa-ccp.c b/gcc/tree-ssa-ccp.c
index 003c9c2af9df8a58cc2eb86e703f95c9a052ac12..8e4d8aeb69ba426d37eb7347294799a060315ad7 100644 (file)
--- a/gcc/tree-ssa-ccp.c
+++ b/gcc/tree-ssa-ccp.c
@@ -1389,6 +1389,66 @@ bit_value_unop (enum tree_code code, signop type_sgn, int type_precision,
     }
 }
 
+/* Determine the mask pair *VAL and *MASK from multiplying the
+   argument mask pair RVAL, RMASK by the unsigned constant C.  */
+void
+bit_value_mult_const (signop sgn, int width,
+                     widest_int *val, widest_int *mask,
+                     const widest_int &rval, const widest_int &rmask,
+                     widest_int c)
+{
+  widest_int sum_mask = 0;
+
+  /* Ensure rval_lo only contains known bits.  */
+  widest_int rval_lo = wi::bit_and_not (rval, rmask);
+
+  if (rval_lo != 0)
+    {
+      /* General case (some bits of multiplicand are known set).  */
+      widest_int sum_val = 0;
+      while (c != 0)
+       {
+         /* Determine the lowest bit set in the multiplier.  */
+         int bitpos = wi::ctz (c);
+         widest_int term_mask = rmask << bitpos;
+         widest_int term_val = rval_lo << bitpos;
+
+         /* sum += term.  */
+         widest_int lo = sum_val + term_val;
+         widest_int hi = (sum_val | sum_mask) + (term_val | term_mask);
+         sum_mask |= term_mask | (lo ^ hi);
+         sum_val = lo;
+
+         /* Clear this bit in the multiplier.  */
+         c ^= wi::lshift (1, bitpos);
+       }
+      /* Correctly extend the result value.  */
+      *val = wi::ext (sum_val, width, sgn);
+    }
+  else
+    {
+      /* Special case (no bits of multiplicand are known set).  */
+      while (c != 0)
+       {
+         /* Determine the lowest bit set in the multiplier.  */
+         int bitpos = wi::ctz (c);
+         widest_int term_mask = rmask << bitpos;
+
+         /* sum += term.  */
+         widest_int hi = sum_mask + term_mask;
+         sum_mask |= term_mask | hi;
+
+         /* Clear this bit in the multiplier.  */
+         c ^= wi::lshift (1, bitpos);
+       }
+      *val = 0;
+    }
+
+  /* Correctly extend the result mask.  */
+  *mask = wi::ext (sum_mask, width, sgn);
+}
+
+
 /* Apply the operation CODE in type TYPE to the value, mask pairs
    R1VAL, R1MASK and R2VAL, R2MASK representing a values of type R1TYPE
    and R2TYPE and set the value, mask pair *VAL and *MASK to the result.  */
@@ -1533,24 +1593,33 @@ bit_value_binop (enum tree_code code, signop sgn, int width,
       }
 
     case MULT_EXPR:
-      {
-       /* Just track trailing zeros in both operands and transfer
-          them to the other.  */
-       int r1tz = wi::ctz (r1val | r1mask);
-       int r2tz = wi::ctz (r2val | r2mask);
-       if (r1tz + r2tz >= width)
-         {
-           *mask = 0;
-           *val = 0;
-         }
-       else if (r1tz + r2tz > 0)
-         {
-           *mask = wi::ext (wi::mask <widest_int> (r1tz + r2tz, true),
-                            width, sgn);
-           *val = 0;
-         }
-       break;
-      }
+      if (r2mask == 0
+         && !wi::neg_p (r2val, sgn)
+         && (flag_expensive_optimizations || wi::popcount (r2val) < 8))
+       bit_value_mult_const (sgn, width, val, mask, r1val, r1mask, r2val);
+      else if (r1mask == 0
+              && !wi::neg_p (r1val, sgn)
+              && (flag_expensive_optimizations || wi::popcount (r1val) < 8))
+       bit_value_mult_const (sgn, width, val, mask, r2val, r2mask, r1val);
+      else
+       {
+         /* Just track trailing zeros in both operands and transfer
+            them to the other.  */
+         int r1tz = wi::ctz (r1val | r1mask);
+         int r2tz = wi::ctz (r2val | r2mask);
+         if (r1tz + r2tz >= width)
+           {
+             *mask = 0;
+             *val = 0;
+           }
+         else if (r1tz + r2tz > 0)
+           {
+             *mask = wi::ext (wi::mask <widest_int> (r1tz + r2tz, true),
+                              width, sgn);
+             *val = 0;
+           }
+       }
+      break;
 
     case EQ_EXPR:
     case NE_EXPR: