[llvm-dev] __builtin_expect hint ignored

[Hans Wennborg via llvm-dev]

GCC produces a jump table. Clang doesn't do that for switches with
fewer than 4 cases (the example code effectively has cases 1, 2, 3 and
the "else case"). I don't know if that cut-off is optimal, but it's
not entirely obvious that a jump table would be better in this case.

On Mon, Apr 24, 2017 at 2:41 PM, Vedant Kumar <vsk at apple.com> wrote:
>
> On x86-64, clang-trunk at least checks for the expected case first:
>
>   https://godbolt.org/g/mEfQ4e.
>
> What changed? Well bisection results show that this was fixed by r300440:
>
>   [llvm] [mirror/master] [BPI] Use metadata info before any other heuristics
>
> But the gcc 7 snapshot produces a lookup table. That might be better. CC'ing Marcello and Hans to comment, since IIRC they have both worked on switch lowering.
>
> best
> vedant
>
> > On Apr 18, 2017, at 4:20 PM, Adam Nowacki via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> >
> > https://godbolt.org/g/VvrbFZ
> >
> > For the code below generated assembly is the worst possible - test 1, else test 3, else test 2 with expected value of 2 last ... Whats going on?
> >
> > long case_2();
> > long case_3();
> > long case_1();
> > long case_else();
> > long test(long a) {
> >    switch (__builtin_expect(a, 2)) {
> >        case 2: return case_2();
> >        case 3: return case_3();
> >        case 1: return case_1();
> >        case 0:
> >        case 4:
> >        case 5: return case_else();
> >        default: __builtin_unreachable();
> >    }
> > }
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>