[llvm-dev] __builtin_expect hint ignored

[Hans Wennborg via llvm-dev]

Yes, the switch lowering code doesn't handle when the fall-through is
the most likely case.

On Mon, Apr 24, 2017 at 3:27 PM, Adam Nowacki <nowak at tepeserwery.pl> wrote:
> Looks only partially fixed. If expected value is for fallthrough case (either 0, 4 or 5) there are still 3 comparisons to reach it.
>
> On 2017-04-24 23:41, Vedant Kumar 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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>>