[Winpcap-users] adding an outstanding performance optimization to the JIT filter

[Sebastian Gottschalk]

Hello there.

In the JIT filter source it, it mentions that MOV reg,0 should be replaced
with XOR reg,reg - so I simply implemented this. I also added the TEST
instruction, which is much more efficient than CMP if one only wants to
check for (in)equality, also a TEST against an immediate of zero can be
replaced by a test against the register itself.

Here's the diff:

--- jitter.h ---
/// xor dr32,sr32
#define XORrd(dr32, sr32) \
	emitm(&stream, 0x33, 1); \
	emitm(&stream, 3 << 6 | (dr32 & 0x7) << 3 | (sr32 & 0x7), 1);

/// test dr32, sr32
#define TESTrd(dr32, sr32) \
    emitm(&stream, 0x85, 1); \
    emitm(&stream,  3 << 6 | (dr32 & 0x7) << 3 | sr32 & 0x7, 1);

/// test dr32, i32

#define TESTid(dr32, i32) \
    if (dr32 == EAX){ \
    emitm(&stream, 0xa9, 1); \
    emitm(&stream,  i32, 4);} \
    else{ \
    emitm(&stream, 0x85, 1); \
    emitm(&stream,  0x1c << 3 | (dr32 & 0x7), 1);\
    emitm(&stream,  i32, 4);}

------

jitter.c:

@@ -389 +389 @@
-				CMPid(EAX, ins->k)
+				TESTid(EAX, ins->k)
@@ -421 +421 @@
-				CMPrd(EAX, EDX)
+				TESTrd(EAX, EDX)
@@ -457 +457 @@
-				CMPid(EDX, 0)
+				TESTid(EDX, EDX)

------

jitter.c:

s/MOVid\(EAX,0//XORrd(EAX,EAX/g
s/MOVid\(EDX,0//XORrd(EDX,EDX/g

Could someone verify this for correctness?


I also have a question: Where does the filter execution routine check for
DIVISION_THROUGH_ZERO exceptions, and how could we possibly help with
non-conditional backward jumps? After all, a malicious filter program might
crash the system!


Greetings,
Sebastian Gottschalk