On Thu, Sep 21, 2017 at 1:17 PM, Richard Sandiford <richard.sandif...@linaro.org> wrote: > 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.
Hmm, ok. The patch is ok then if you add a comment reflecting the above. Thanks, Richard. > > Thanks, > Richard
