On Wed, Nov 28, 2018 at 9:47 AM Michael Eager <ea...@eagerm.com> wrote:
>
> On 11/28/18 09:10, Jeff Law wrote:
> > On 11/28/18 10:00 AM, Michael Eager wrote:
> >> I have a small test case which generates poor quality code on my target.
> >> Here is the original:
> >>
> >> if (cond1 == 2048 || cond2 == 8)
> >> {
> >> x = x + y;
> >> }
> >> return x;
> >>
> >> This ends up generating a series of instructions to compute a flag with
> >> the result of the condition followed by a single compare with zero and
> >> a jump. Better code would be two compares and two jumps.
> >>
> >> The gimple is
> >>
> >> _1 = cond1 == 2048;
> >> _2 = cond2 == 8;
> >> _3 = _1 | _2;
> >> if (_3 != 0) goto <D.1464>; else goto <D.1465>;
> >> ...
> >>
> >> so this poor code sequence is essentially a correct translation.
> >>
> >> On MIPS, for the same test case, the gimple is different:
> >>
> >> if (cond1 == 2048) goto <D.1491>; else goto <D.1493>;
> >> <D.1493>:
> >> if (cond2 == 8) goto <D.1491>; else goto <D.1492>;
> >> <D.1491>:
> >> ...
> >>
> >> which generates the better code sequence.
> >>
> >> Can someone give me a pointer where to find where the lowering
> >> pass decides to break up a condition into multiple tests? Is
> >> there a target configuration option which I have overlooked or
> >> maybe set incorrectly?
> > BRANCH_COST, which comes into play during generation of the initial
> > trees as well in passes which try to optimize branchy code into
> > straighter code.
>
> Thanks. I did look at BRANCH_COST, and played with the values, but it
> didn't seem to have any affect. I'll take a closer look.
Look at LOGICAL_OP_NON_SHORT_CIRCUIT . By defualt, it is defined as:
(BRANCH_COST (optimize_function_for_speed_p (cfun), \
false) >= 2)
But MIPS defines it as 0.
Changing LOGICAL_OP_NON_SHORT_CIRCUIT to 0 will disable this optimization.
LOGICAL_OP_NON_SHORT_CIRCUIT controls both places where (cond1 == 2048
|| cond2 == 8) would remove the branch.
NOTE I think MIPS defines LOGICAL_OP_NON_SHORT_CIRCUIT incorrectly but
that is a different story.
Thanks,
Andrew Pinski
>
> --
> Michael Eager ea...@eagerm.com
> 1960 Park Blvd., Palo Alto, CA 94306
