From a5892302492868246c4612575fa50b4a8f41ef6e Mon Sep 17 00:00:00 2001
From: Amos Bird <amosbird@gmail.com>
Date: Sun, 26 Aug 2018 02:23:26 +0800
Subject: [PATCH] Faster path for varint read.

clickhouse-benchmark <<< 'select count() from s where not ignore(s);'

before this patch:

```

QPS: 0.732, RPS: 2346562049.608, MiB/s: 22378.560, result RPS: 0.732, result MiB/s: 0.000.
0.000%  1.310 sec.
10.000% 1.321 sec.
20.000% 1.327 sec.
30.000% 1.337 sec.
40.000% 1.343 sec.
50.000% 1.359 sec.
60.000% 1.366 sec.
70.000% 1.381 sec.
80.000% 1.400 sec.
90.000% 1.434 sec.
95.000% 1.448 sec.
99.000% 1.489 sec.
99.900% 1.499 sec.
99.990% 1.500 sec.

```

after this patch:

```
QPS: 0.787, RPS: 2524560389.064, MiB/s: 24076.084, result RPS: 0.787, result MiB/s: 0.000.
0.000%  1.228 sec.
10.000% 1.232 sec.
20.000% 1.235 sec.
30.000% 1.241 sec.
40.000% 1.246 sec.
50.000% 1.256 sec.
60.000% 1.265 sec.
70.000% 1.278 sec.
80.000% 1.296 sec.
90.000% 1.321 sec.
95.000% 1.354 sec.
99.000% 1.421 sec.
99.900% 1.453 sec.
99.990% 1.456 sec.
```

I also tried a SSE2 implementation and it's much slower (50%)
---
 dbms/src/IO/VarInt.h | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/dbms/src/IO/VarInt.h b/dbms/src/IO/VarInt.h
index 73a404e037e..25b8013a24c 100644
--- a/dbms/src/IO/VarInt.h
+++ b/dbms/src/IO/VarInt.h
@@ -118,13 +118,15 @@ inline void throwReadAfterEOF()
     throw Exception("Attempt to read after eof", ErrorCodes::ATTEMPT_TO_READ_AFTER_EOF);
 }
 
-inline void readVarUInt(UInt64 & x, ReadBuffer & istr)
+template<bool fast>
+inline void readVarUIntImpl(UInt64 & x, ReadBuffer & istr)
 {
     x = 0;
     for (size_t i = 0; i < 9; ++i)
     {
-        if (istr.eof())
-            throwReadAfterEOF();
+        if constexpr (!fast)
+            if (istr.eof())
+                throwReadAfterEOF();
 
         UInt64 byte = *istr.position();
         ++istr.position();
@@ -135,6 +137,13 @@ inline void readVarUInt(UInt64 & x, ReadBuffer & istr)
     }
 }
 
+inline void readVarUInt(UInt64 & x, ReadBuffer & istr)
+{
+    if (istr.buffer().end() - istr.position() >= 9)
+        return readVarUIntImpl<true>(x, istr);
+    return readVarUIntImpl<false>(x, istr);
+}
+
 
 inline void readVarUInt(UInt64 & x, std::istream & istr)
 {