On Mon, Aug 9, 2021 at 10:13 AM Roger Sayle <ro...@nextmovesoftware.com> wrote: > > > 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?
OK. Thanks, Richard. > > 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 > -- >
