227 lines
9.5 KiB
C
227 lines
9.5 KiB
C
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//--------------------------------------------------------------------------------------
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// DXUTLockFreePipe.h
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//
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// See the "Lockless Programming Considerations for Xbox 360 and Microsoft Windows"
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// article in the DirectX SDK for more details.
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//
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// http://msdn2.microsoft.com/en-us/library/bb310595.aspx
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//
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// XNA Developer Connection
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// Copyright (C) Microsoft Corporation. All rights reserved.
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//--------------------------------------------------------------------------------------
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#pragma once
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#include <sal.h>
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#ifdef _XBOX_VER
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// Prevent the CPU from rearranging loads
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// and stores, sufficiently for read-acquire
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// and write-release.
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#define DXUTImportBarrier __lwsync
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#define DXUTExportBarrier __lwsync
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#else
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#pragma pack(push)
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#pragma pack(8)
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#include <windows.h>
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#pragma pack (pop)
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extern "C"
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void _ReadWriteBarrier();
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#pragma intrinsic(_ReadWriteBarrier)
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// Prevent the compiler from rearranging loads
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// and stores, sufficiently for read-acquire
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// and write-release. This is sufficient on
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// x86 and x64.
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#define DXUTImportBarrier _ReadWriteBarrier
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#define DXUTExportBarrier _ReadWriteBarrier
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#endif
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//
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// Pipe class designed for use by at most two threads: one reader, one writer.
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// Access by more than two threads isn't guaranteed to be safe.
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//
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// In order to provide efficient access the size of the buffer is passed
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// as a template parameter and restricted to powers of two less than 31.
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//
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template <BYTE cbBufferSizeLog2> class DXUTLockFreePipe
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{
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public:
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DXUTLockFreePipe() : m_readOffset( 0 ),
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m_writeOffset( 0 )
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{
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}
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DWORD GetBufferSize() const
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{
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return c_cbBufferSize;
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}
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__forceinline unsigned long BytesAvailable() const
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{
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return m_writeOffset - m_readOffset;
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}
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bool __forceinline Read( void* pvDest, unsigned long cbDest )
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{
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// Store the read and write offsets into local variables--this is
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// essentially a snapshot of their values so that they stay constant
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// for the duration of the function (and so we don't end up with cache
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// misses due to false sharing).
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DWORD readOffset = m_readOffset;
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DWORD writeOffset = m_writeOffset;
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// Compare the two offsets to see if we have anything to read.
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// Note that we don't do anything to synchronize the offsets here.
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// Really there's not much we *can* do unless we're willing to completely
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// synchronize access to the entire object. We have to assume that as we
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// read, someone else may be writing, and the write offset we have now
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// may be out of date by the time we read it. Fortunately that's not a
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// very big deal. We might miss reading some data that was just written.
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// But the assumption is that we'll be back before long to grab more data
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// anyway.
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//
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// Note that this comparison works because we're careful to constrain
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// the total buffer size to be a power of 2, which means it will divide
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// evenly into ULONG_MAX+1. That, and the fact that the offsets are
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// unsigned, means that the calculation returns correct results even
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// when the values wrap around.
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DWORD cbAvailable = writeOffset - readOffset;
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if( cbDest > cbAvailable )
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{
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return false;
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}
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// The data has been made available, but we need to make sure
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// that our view on the data is up to date -- at least as up to
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// date as the control values we just read. We need to prevent
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// the compiler or CPU from moving any of the data reads before
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// the control value reads. This import barrier serves this
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// purpose, on Xbox 360 and on Windows.
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// Reading a control value and then having a barrier is known
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// as a "read-acquire."
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DXUTImportBarrier();
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unsigned char* pbDest = ( unsigned char* )pvDest;
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unsigned long actualReadOffset = readOffset & c_sizeMask;
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unsigned long bytesLeft = cbDest;
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//
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// Copy from the tail, then the head. Note that there's no explicit
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// check to see if the write offset comes between the read offset
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// and the end of the buffer--that particular condition is implicitly
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// checked by the comparison with AvailableToRead(), above. If copying
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// cbDest bytes off the tail would cause us to cross the write offset,
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// then the previous comparison would have failed since that would imply
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// that there were less than cbDest bytes available to read.
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//
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unsigned long cbTailBytes = min( bytesLeft, c_cbBufferSize - actualReadOffset );
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memcpy( pbDest, m_pbBuffer + actualReadOffset, cbTailBytes );
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bytesLeft -= cbTailBytes;
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if( bytesLeft )
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{
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memcpy( pbDest + cbTailBytes, m_pbBuffer, bytesLeft );
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}
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// When we update the read offset we are, effectively, 'freeing' buffer
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// memory so that the writing thread can use it. We need to make sure that
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// we don't free the memory before we have finished reading it. That is,
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// we need to make sure that the write to m_readOffset can't get reordered
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// above the reads of the buffer data. The only way to guarantee this is to
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// have an export barrier to prevent both compiler and CPU rearrangements.
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DXUTExportBarrier();
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// Advance the read offset. From the CPUs point of view this is several
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// operations--read, modify, store--and we'd normally want to make sure that
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// all of the operations happened atomically. But in the case of a single
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// reader, only one thread updates this value and so the only operation that
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// must be atomic is the store. That's lucky, because 32-bit aligned stores are
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// atomic on all modern processors.
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//
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readOffset += cbDest;
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m_readOffset = readOffset;
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return true;
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}
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bool __forceinline Write( const void* pvSrc, unsigned long cbSrc )
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{
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// Reading the read offset here has the same caveats as reading
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// the write offset had in the Read() function above.
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DWORD readOffset = m_readOffset;
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DWORD writeOffset = m_writeOffset;
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// Compute the available write size. This comparison relies on
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// the fact that the buffer size is always a power of 2, and the
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// offsets are unsigned integers, so that when the write pointer
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// wraps around the subtraction still yields a value (assuming
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// we haven't messed up somewhere else) between 0 and c_cbBufferSize - 1.
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DWORD cbAvailable = c_cbBufferSize - ( writeOffset - readOffset );
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if( cbSrc > cbAvailable )
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{
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return false;
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}
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// It is theoretically possible for writes of the data to be reordered
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// above the reads to see if the data is available. Improbable perhaps,
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// but possible. This barrier guarantees that the reordering will not
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// happen.
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DXUTImportBarrier();
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// Write the data
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const unsigned char* pbSrc = ( const unsigned char* )pvSrc;
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unsigned long actualWriteOffset = writeOffset & c_sizeMask;
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unsigned long bytesLeft = cbSrc;
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// See the explanation in the Read() function as to why we don't
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// explicitly check against the read offset here.
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unsigned long cbTailBytes = min( bytesLeft, c_cbBufferSize - actualWriteOffset );
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memcpy( m_pbBuffer + actualWriteOffset, pbSrc, cbTailBytes );
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bytesLeft -= cbTailBytes;
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if( bytesLeft )
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{
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memcpy( m_pbBuffer, pbSrc + cbTailBytes, bytesLeft );
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}
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// Now it's time to update the write offset, but since the updated position
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// of the write offset will imply that there's data to be read, we need to
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// make sure that the data all actually gets written before the update to
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// the write offset. The writes could be reordered by the compiler (on any
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// platform) or by the CPU (on Xbox 360). We need a barrier which prevents
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// the writes from being reordered past each other.
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//
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// Having a barrier and then writing a control value is called "write-release."
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DXUTExportBarrier();
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// See comments in Read() as to why this operation isn't interlocked.
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writeOffset += cbSrc;
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m_writeOffset = writeOffset;
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return true;
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}
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private:
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// Values derived from the buffer size template parameter
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//
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const static BYTE c_cbBufferSizeLog2 = min( cbBufferSizeLog2, 31 );
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const static DWORD c_cbBufferSize = ( 1 << c_cbBufferSizeLog2 );
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const static DWORD c_sizeMask = c_cbBufferSize - 1;
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// Leave these private and undefined to prevent their use
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DXUTLockFreePipe( const DXUTLockFreePipe& );
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DXUTLockFreePipe& operator =( const DXUTLockFreePipe& );
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// Member data
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//
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BYTE m_pbBuffer[c_cbBufferSize];
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// Note that these offsets are not clamped to the buffer size.
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// Instead the calculations rely on wrapping at ULONG_MAX+1.
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// See the comments in Read() for details.
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volatile DWORD __declspec( align( 4 ) ) m_readOffset;
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volatile DWORD __declspec( align( 4 ) ) m_writeOffset;
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};
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