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.
This patch has been tested on x86_64-pc-linux-gnu with "make bootstrap"
and "make -k check" with no new failures.
Ok for mainline?
2021-08-09 Roger Sayle <ro...@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 non-negative constants.
gcc/testsuite/ChangeLog
* gcc.dg/fold-ior-5.c: New test case.
Roger
--
diff --git a/gcc/tree-ssa-ccp.c b/gcc/tree-ssa-ccp.c
index 9ce6214..86ca3ae 100644
--- a/gcc/tree-ssa-ccp.c
+++ b/gcc/tree-ssa-ccp.c
@@ -1340,6 +1340,66 @@ 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. */
@@ -1482,24 +1542,33 @@ 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:
/* { 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" } } */
