natch.pd tries to reassociate two bit operations if both of them have constant operands. However, with the polynomial integers added later, there's no guarantee that a bit operation on two integers can be folded at compile time. This means that the pattern can trigger for operations on three constants, and as things stood could endlessly oscillate between the two associations.
This patch keeps the existing pattern for the normal case of a non-constant first operand. When all three operands are constant it tries to find a pair of constants that do fold. If none do, it keeps the original expression as-was. Tested on aarch64-linux-gnu, x86_64-linux-gnu and powerpc64le-linus-gnu. OK to install? Richard 2017-09-20 Richard Sandiford <richard.sandif...@linaro.org> Alan Hayward <alan.hayw...@arm.com> David Sherwood <david.sherw...@arm.com> gcc/ * match.pd: Handle bit operations involving three constants and try to fold one pair. Index: gcc/match.pd =================================================================== --- gcc/match.pd 2017-09-16 21:38:21.106513157 +0100 +++ gcc/match.pd 2017-09-20 13:17:10.552389270 +0100 @@ -1017,7 +1017,14 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT) (for bitop (bit_and bit_ior bit_xor) (simplify (bitop (bitop @0 CONSTANT_CLASS_P@1) CONSTANT_CLASS_P@2) - (bitop @0 (bitop @1 @2)))) + (if (!CONSTANT_CLASS_P (@0)) + (bitop @0 (bitop @1 @2)) + (with { tree cst1 = const_binop (bitop, type, @0, @2); } + (if (cst1) + (bitop @1 { cst1; }) + (with { tree cst2 = const_binop (bitop, type, @1, @2); } + (if (cst2) + (bitop @0 { cst2; })))))))) /* Try simple folding for X op !X, and X op X with the help of the truth_valued_p and logical_inverted_value predicates. */