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likely / unlikely support #5661

@S-YOU

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@S-YOU

Some performance sensitive libraries use likely / unlikely for branch prediction.

I am discovering crystal like two weeks or so, I have no idea what is the correct way to implement and test.

Here is what I found so far.

https://llvm.org/docs/LangRef.html#id766

declare i1 @llvm.expect.i1(i1 <val>, i1 <expected_val>)
declare i32 @llvm.expect.i32(i32 <val>, i32 <expected_val>)
declare i64 @llvm.expect.i64(i64 <val>, i64 <expected_val>)

Rust: core: add likely and unlikely intrinsics

Something like this in crystal will correctly works?

lib Intrinsics
  fun inspect1 = "llvm.expect.i1"(val : Bool, expected_val : Bool) : Bool
  fun inspect32 = "llvm.expect.i32"(val : Int32, expected_val : Int32) : Int32
  fun inspect64 = "llvm.expect.i64"(val : Int64, expected_val : Int64) : Int64
end
def likely(val)
  Intrinsics.inspect1(val, true)
end

def unlikely(val)
  Intrinsics.inspect1(val, false)
end

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