Question about SmallVector implementation detail

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#1

Hello,

I have just a little question about the SmallVector implemention.

In SmallVectorImpl, the following method is currently implemented as:

void push_back(const T &Elt) {
if (this->EndX < this->CapacityX) {
Retry:
new (this->end()) T(Elt);
this->setEnd(this->end()+1);
return;
}
this->grow();
goto Retry;
}

~/llvm-project/llvm/include/llvm/ADT/SmallVector.h:327 (svn rev: 107560)
This function was wrote/last modified by lattner.

Why a goto?
For CPU branch prediction in favor of the positive condition result?
I’am interested by the reason of that, that’s look curious for me.

Thank you,

#2

It's a micro-optimization for code layout and code size. The idea is that we want this to get inlined, so tail duplicating the if condition would be bad. This would be a good candidate for __builtin_expect as well.

In the big picture, it probably doesn't matter much either way :slight_smile:

-Chris

#3

Using __builtin_expect this can be simplified to

  void push_back(const T &Elt) {
    if (unlikely(this->EndX >= this->CapacityX))
      this->grow();
    new (this->end()) T(Elt);
    this->setEnd(this->end()+1);
  }