RandomX: added parameter for scratchpad prefetch mode

`scratchpad_prefetch_mode` can have 4 values:
0: off
1: use `prefetcht0` instruction (default, same as previous XMRig versions)
2: use `prefetchnta` instruction (faster on Coffee Lake and a few other CPUs)
3: use `mov` instruction

src/crypto/rx/Rx.cpp

@@ -32,6 +32,7 @@
 #include "base/io/log/Log.h"
 #include "crypto/rx/RxConfig.h"
 #include "crypto/rx/RxQueue.h"
+#include "crypto/randomx/randomx.h"
 
 
 namespace xmrig {
@@ -99,6 +100,8 @@ bool xmrig::Rx::init(const T &seed, const RxConfig &config, const CpuConfig &cpu
         return true;
     }
 
+    randomx_set_scratchpad_prefetch_mode(config.scratchpadPrefetchMode());
+
     if (isReady(seed)) {
         return true;
     }

src/crypto/rx/RxConfig.cpp

@@ -57,6 +57,8 @@ static const char *kCacheQoS    = "cache_qos";
 static const char *kNUMA        = "numa";
 #endif
 
+static const char *kScratchpadPrefetchMode = "scratchpad_prefetch_mode";
+
 static const std::array<const char *, RxConfig::ModeMax> modeNames = { "auto", "fast", "light" };
 
 
@@ -118,6 +120,11 @@ bool xmrig::RxConfig::read(const rapidjson::Value &value)
         }
 #       endif
 
+        const int mode = Json::getInt(value, kScratchpadPrefetchMode, static_cast<int>(m_scratchpadPrefetchMode));
+        if ((mode >= ScratchpadPrefetchOff) && (mode < ScratchpadPrefetchMax)) {
+            m_scratchpadPrefetchMode = static_cast<ScratchpadPrefetchMode>(mode);
+        }
+
         return true;
     }
 
@@ -171,6 +178,8 @@ rapidjson::Value xmrig::RxConfig::toJSON(rapidjson::Document &doc) const
     }
 #   endif
 
+    obj.AddMember(StringRef(kScratchpadPrefetchMode), static_cast<int>(m_scratchpadPrefetchMode), allocator);
+
     return obj;
 }
 

src/crypto/rx/RxConfig.h

@@ -50,6 +50,14 @@ public:
         ModeMax
     };
 
+    enum ScratchpadPrefetchMode : uint32_t {
+        ScratchpadPrefetchOff,
+        ScratchpadPrefetchT0,
+        ScratchpadPrefetchNTA,
+        ScratchpadPrefetchMov,
+        ScratchpadPrefetchMax,
+    };
+
     bool read(const rapidjson::Value &value);
     rapidjson::Value toJSON(rapidjson::Document &doc) const;
 
@@ -68,6 +76,8 @@ public:
     inline bool cacheQoS() const        { return m_cacheQoS; }
     inline Mode mode() const            { return m_mode; }
 
+    inline ScratchpadPrefetchMode scratchpadPrefetchMode() const { return m_scratchpadPrefetchMode; }
+
 #   ifdef XMRIG_FEATURE_MSR
     const char *msrPresetName() const;
     const MsrItems &msrPreset() const;
@@ -94,6 +104,8 @@ private:
     int m_threads       = -1;
     Mode m_mode         = AutoMode;
 
+    ScratchpadPrefetchMode m_scratchpadPrefetchMode = ScratchpadPrefetchT0;
+
 #   ifdef XMRIG_FEATURE_HWLOC
     std::vector<uint32_t> m_nodeset;
 #   endif