[llvm] r212816 - raw_svector_ostream: grow and reserve atomically

[Alexander Kornienko]

I wonder whether I missed the review thread for this patch or this kind of
patch is generally accepted to be committed without any review?


On Fri, Jul 11, 2014 at 4:02 PM, Alp Toker <alp at nuanti.com> wrote:

> Author: alp
> Date: Fri Jul 11 09:02:04 2014
> New Revision: 212816
>
> URL: http://llvm.org/viewvc/llvm-project?rev=212816&view=rev
> Log:
> raw_svector_ostream: grow and reserve atomically
>
> Including the scratch buffer size in the initial reservation eliminates the
> subsequent malloc+move operation and offers a healthier constant growth
> with
> less memory wastage.
>
> When doing this, take care to avoid invalidating the source buffer.
>
> Modified:
>     llvm/trunk/lib/Support/raw_ostream.cpp
>
> Modified: llvm/trunk/lib/Support/raw_ostream.cpp
> URL:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Support/raw_ostream.cpp?rev=212816&r1=212815&r2=212816&view=diff
>
> ==============================================================================
> --- llvm/trunk/lib/Support/raw_ostream.cpp (original)
> +++ llvm/trunk/lib/Support/raw_ostream.cpp Fri Jul 11 09:02:04 2014
> @@ -729,24 +729,26 @@ void raw_svector_ostream::resync() {
>  }
>
>  void raw_svector_ostream::write_impl(const char *Ptr, size_t Size) {
> -  // If we're writing bytes from the end of the buffer into the
> smallvector, we
> -  // don't need to copy the bytes, just commit the bytes because they are
> -  // already in the right place.
> -  if (Ptr == OS.end()) {
> -    assert(OS.size() + Size <= OS.capacity() && "Invalid write_impl()
> call!");
> -    OS.set_size(OS.size() + Size);
> +  size_t NewSize = OS.size() + Size;
> +  size_t NewReservation = NewSize + 64;
> +
> +  bool NoOverlap = Ptr + Size < OS.begin() || Ptr > OS.begin() +
> OS.capacity();
> +
> +  if (NoOverlap) {
> +    assert(!GetNumBytesInBuffer());
> +    OS.reserve(NewReservation);
> +    memcpy(OS.end(), Ptr, Size);
> +    OS.set_size(NewSize);
> +  } else if (Ptr == OS.end()) {
> +    // Grow the buffer to include the scratch area without copying.
> +    assert(NewSize <= OS.capacity() && "Invalid write_impl() call!");
> +    OS.set_size(NewSize);
> +    OS.reserve(NewReservation);
>    } else {
> -    assert(GetNumBytesInBuffer() == 0 &&
> -           "Should be writing from buffer if some bytes in it");
> -    // Otherwise, do copy the bytes.
> -    OS.append(Ptr, Ptr+Size);
> +    OS.append(Ptr, Ptr + Size);
> +    OS.reserve(NewReservation);
>    }
>
> -  // Grow the vector if necessary.
> -  if (OS.capacity() - OS.size() < 64)
> -    OS.reserve(OS.capacity() * 2);
> -
> -  // Update the buffer position.
>    SetBuffer(OS.end(), OS.capacity() - OS.size());
>  }
>
>
>
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