Richard Biener <richard.guent...@gmail.com> writes: > On Wed, Sep 20, 2017 at 2:18 PM, Richard Sandiford > <richard.sandif...@linaro.org> wrote: >> 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? > > Hmm, is the complication with trying variants necessary or could we simply do > > if (!CONSTANT_CLASS_P (@0)) > (bitop @0 (bitop @1 @2)) > > and be done with it?
That's enough to fix the oscillation, but there are cases that benefit from trying the other combinations. E.g. if @1 and @2 are both INTEGER_CSTs, it's still worth folding them. POLY_CST | INTEGER_CST is supported for some values, so if @0 | @1 doesn't fold, it's worth trying @0 | @2 instead. Thanks, Richard
