mem-cache: Implement a perfect compressor

Implement a perfect compressor that always manages to compresses data
exactly to its maximum allowed compression ratio. This allows tracking
a compression upper bound.

Change-Id: Ibc68bf2dc84b75207795d5ba6304b9ed6dbeae8f
Signed-off-by: Daniel R. Carvalho <odanrc@yahoo.com.br>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/21160
Tested-by: kokoro <noreply+kokoro@google.com>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>

src/mem/cache/compressors/Compressors.py

@@ -100,6 +100,18 @@ class MultiCompressor(BaseCacheCompressor):
     compressors = VectorParam.BaseCacheCompressor([CPack(), FPCD()],
         "Array of compressors")
 
+class PerfectCompressor(BaseCacheCompressor):
+    type = 'PerfectCompressor'
+    cxx_class = 'PerfectCompressor'
+    cxx_header = "mem/cache/compressors/perfect.hh"
+
+    max_compression_ratio = Param.Int(2,
+        "Maximum compression ratio allowed")
+    compression_latency = Param.Cycles(1,
+        "Number of cycles to perform data compression")
+    decompression_latency = Param.Cycles(1,
+        "Number of cycles to perform data decompression")
+
 class RepeatedQwordsCompressor(BaseDictionaryCompressor):
     type = 'RepeatedQwordsCompressor'
     cxx_class = 'RepeatedQwordsCompressor'

src/mem/cache/compressors/SConscript

@@ -38,5 +38,6 @@ Source('base_delta.cc')
 Source('cpack.cc')
 Source('fpcd.cc')
 Source('multi.cc')
+Source('perfect.cc')
 Source('repeated_qwords.cc')
 Source('zero.cc')

src/mem/cache/compressors/perfect.cc

@@ -0,0 +1,88 @@
+/*
+ * Copyright (c) 2019 Inria
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met: redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer;
+ * redistributions in binary form must reproduce the above copyright
+ * notice, this list of conditions and the following disclaimer in the
+ * documentation and/or other materials provided with the distribution;
+ * neither the name of the copyright holders nor the names of its
+ * contributors may be used to endorse or promote products derived from
+ * this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ *
+ * Authors: Daniel Carvalho
+ */
+
+/** @file
+ * Implementation of a perfect compressor, which compresses data to its
+ * maximum allowed compression ratio.
+ */
+
+#include "mem/cache/compressors/perfect.hh"
+
+#include <algorithm>
+
+#include "debug/CacheComp.hh"
+#include "params/PerfectCompressor.hh"
+
+PerfectCompressor::CompData::CompData(const uint64_t* data,
+    std::size_t num_entries)
+    : CompressionData(), entries(data, data + num_entries)
+{
+}
+
+PerfectCompressor::PerfectCompressor(const Params *p)
+    : BaseCacheCompressor(p),
+      compressedSize(8 * blkSize / p->max_compression_ratio),
+      compressionLatency(p->compression_latency),
+      decompressionLatency(p->decompression_latency)
+{
+}
+
+std::unique_ptr<BaseCacheCompressor::CompressionData>
+PerfectCompressor::compress(const uint64_t* cache_line, Cycles& comp_lat,
+    Cycles& decomp_lat)
+{
+    // Compress every word sequentially
+    std::unique_ptr<BaseCacheCompressor::CompressionData> comp_data(
+        new CompData(cache_line, blkSize/8));
+
+    // Set relevant metadata
+    comp_data->setSizeBits(compressedSize);
+    comp_lat = compressionLatency;
+    decomp_lat = decompressionLatency;
+
+    return comp_data;
+}
+
+void
+PerfectCompressor::decompress(const CompressionData* comp_data,
+    uint64_t* data)
+{
+    // Decompress every entry sequentially
+    const std::vector<uint64_t>& entries =
+        static_cast<const CompData*>(comp_data)->entries;
+    assert(entries.size() == (blkSize/8));
+    std::copy(entries.begin(), entries.end(), data);
+}
+
+PerfectCompressor*
+PerfectCompressorParams::create()
+{
+    return new PerfectCompressor(this);
+}

src/mem/cache/compressors/perfect.hh

@@ -0,0 +1,89 @@
+/*
+ * Copyright (c) 2019 Inria
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met: redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer;
+ * redistributions in binary form must reproduce the above copyright
+ * notice, this list of conditions and the following disclaimer in the
+ * documentation and/or other materials provided with the distribution;
+ * neither the name of the copyright holders nor the names of its
+ * contributors may be used to endorse or promote products derived from
+ * this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ *
+ * Authors: Daniel Carvalho
+ */
+
+/** @file
+ * Definition of a perfect compressor, that always manages to compress to
+ * its maximum compression ratio.
+ */
+
+#ifndef __MEM_CACHE_COMPRESSORS_PERFECT_COMPRESSOR_HH__
+#define __MEM_CACHE_COMPRESSORS_PERFECT_COMPRESSOR_HH__
+
+#include <cstdint>
+#include <memory>
+#include <vector>
+
+#include "base/types.hh"
+#include "mem/cache/compressors/base.hh"
+
+struct PerfectCompressorParams;
+
+class PerfectCompressor : public BaseCacheCompressor
+{
+  protected:
+    class CompData;
+
+    /** Size to which this compressor always compresses. */
+    const int compressedSize;
+
+    /** Number of cycles needed to perform compression. */
+    const Cycles compressionLatency;
+
+    /** Number of cycles needed to perform decompression. */
+    const Cycles decompressionLatency;
+
+    std::unique_ptr<CompressionData> compress(const uint64_t* cache_line,
+        Cycles& comp_lat, Cycles& decomp_lat) override;
+
+    void decompress(const CompressionData* comp_data, uint64_t* data) override;
+
+  public:
+    typedef PerfectCompressorParams Params;
+    PerfectCompressor(const Params *p);
+    ~PerfectCompressor() {};
+};
+
+class PerfectCompressor::CompData : public CompressionData
+{
+  public:
+    /** The original data is simply copied over to this vector. */
+    std::vector<uint64_t> entries;
+
+    /**
+     * Default constructor that creates a copy of the original data.
+     *
+     * @param data The data to be compressed.
+     * @param num_entries The number of qwords in the data.
+     */
+    CompData(const uint64_t* data, std::size_t num_entries);
+    ~CompData() = default;
+};
+
+#endif //__MEM_CACHE_COMPRESSORS_PERFECT_COMPRESSOR_HH__