[llvm-dev] __builtin_expect hint ignored (original) (raw)

Hans Wennborg via llvm-dev llvm-dev at lists.llvm.org
Mon Apr 24 15:12:46 PDT 2017

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 case2(); > long case3(); > long case1(); > long caseelse(); > long test(long a) { _> switch (builtinexpect(a, 2)) { > case 2: return case2(); > case 3: return case3(); > case 1: return case1(); > case 0: > case 4: > case 5: return caseelse(); _> default: builtinunreachable(); > } > } _> ________________________ > LLVM Developers mailing list > llvm-dev at lists.llvm.org > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

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