[llvm] r212816 - raw_svector_ostream: grow and reserve atomically

[Alp Toker]

On 11/07/2014 19:26, Alexander Kornienko wrote:
> 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?

I'm OK with this change, but any review is welcome. This is a supporting 
commit towards fixing the really old FIXMEs related to scratch buffer 
overallocation.


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

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