From 235f655cab0f13dc4f310852f52685530b7b2ef9 Mon Sep 17 00:00:00 2001
From: Muhammad Zahalqa <m@tryfinally.com>
Date: Sat, 13 Jun 2020 11:43:02 +0300
Subject: [PATCH] Optimize rdbEncodeInteger using bit operations

use sign extension mov instructions into 8/16/32 bits int registers to check if value fits
into 8/16/32 bit width.
```
int function(long long value){
    struct SignExtendBits{
        long long bits8: 8;
        long long bits16: 16;
        long long bits32: 32;
    } v;

    v.bits8 = value;
    if ( v.bits8 == value ) return 1;
    m.bits16 = value;
    if ( m.bits16 == value ) return 2;
    m.bits32 = value;
    if ( m.bits32 == value ) return 3;

    return 0;
}

Compiles to compact code even with -O1 :
function(long long):
        movsx rcx, dil
        mov eax, 1
        cmp rcx, rdi
        je .LBB1_3
        movsx rcx, di
        mov eax, 2
        cmp rcx, rdi
        je .LBB1_3
        movsxd rcx, edi
        xor eax, eax
        cmp rcx, rdi
        sete al
        shl eax, 2
.LBB1_3:
        ret
```
---
 src/rdb.cpp | 25 +++++++++++++++++++------
 1 file changed, 19 insertions(+), 6 deletions(-)

diff --git a/src/rdb.cpp b/src/rdb.cpp
index ea82cdbd8..a4034b549 100644
--- a/src/rdb.cpp
+++ b/src/rdb.cpp
@@ -239,25 +239,38 @@ uint64_t rdbLoadLen(rio *rdb, int *isencoded) {
  * representation is stored in the buffer pointer to by "enc" and the string
  * length is returned. Otherwise 0 is returned. */
 int rdbEncodeInteger(long long value, unsigned char *enc) {
-    if (value >= -(1<<7) && value <= (1<<7)-1) {
+    struct SignExtendBits{        
+        long long bits8:  8;        
+        long long bits16: 16;
+        long long bits32: 32;
+    } v;
+    
+    v.bits8 = value;
+    if (v.bits8 == value) {
         enc[0] = (RDB_ENCVAL<<6)|RDB_ENC_INT8;
-        enc[1] = value&0xFF;
+        enc[1] = v.bits8;
         return 2;
-    } else if (value >= -(1<<15) && value <= (1<<15)-1) {
+    }
+
+    v.bits16 = value;
+    if (v.bits16 == value) {
         enc[0] = (RDB_ENCVAL<<6)|RDB_ENC_INT16;
         enc[1] = value&0xFF;
         enc[2] = (value>>8)&0xFF;
         return 3;
-    } else if (value >= -((long long)1<<31) && value <= ((long long)1<<31)-1) {
+    }
+
+    v.bits32 = value;
+    if (v.bits32 == value) {
         enc[0] = (RDB_ENCVAL<<6)|RDB_ENC_INT32;
         enc[1] = value&0xFF;
         enc[2] = (value>>8)&0xFF;
         enc[3] = (value>>16)&0xFF;
         enc[4] = (value>>24)&0xFF;
         return 5;
-    } else {
-        return 0;
     }
+
+    return 0;    
 }
 
 /* Loads an integer-encoded object with the specified encoding type "enctype".