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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.
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