https://github.com/aquynh/capstone/issues/483

arch/X86/X86Disassembler.c

@@ -791,6 +791,17 @@ bool X86_getInstruction(csh ud, const uint8_t *code, size_t code_len,
 	info.offset = address;
 
 	memset(&insn, 0, offsetof(InternalInstruction, reader));
+	//initialize some the necessary values
+	insn.prefixPresent[0x26] = 0;
+	insn.prefixPresent[0x2e] = 0;
+	insn.prefixPresent[0x36] = 0;
+	insn.prefixPresent[0x3e] = 0;
+	insn.prefixPresent[0x64] = 0;
+	insn.prefixPresent[0x65] = 0;
+	insn.prefixPresent[0x66] = 0;
+	insn.prefixPresent[0xf0] = 0;
+	insn.prefixPresent[0xf2] = 0;
+	insn.prefixPresent[0xf3] = 0;
 
 	if (instr->flat_insn->detail) {
 		instr->flat_insn->detail->x86.op_count = 0;

arch/X86/X86DisassemblerDecoder.h

@@ -553,8 +553,6 @@ struct InstructionSpecifier {
 typedef struct InternalInstruction {
   // from here, all members must be initialized to ZERO to work properly
   uint8_t operandSize;
-  /* 1 if the prefix byte corresponding to the entry is present; 0 if not */
-  uint8_t prefixPresent[0x100];
   uint8_t prefix0, prefix1, prefix2, prefix3;
   /* The value of the REX prefix, if present */
   uint8_t rexPrefix;
@@ -580,9 +578,6 @@ typedef struct InternalInstruction {
   /* 1 if the prefix byte, 0xf2 or 0xf3 is xacquire or xrelease */
   bool xAcquireRelease;
 
-  /* contains the location (for use with the reader) of the prefix byte */
-  uint64_t prefixLocations[0x100];
-
   /* The value of the vector extension prefix(EVEX/VEX/XOP), if present */
   uint8_t vectorExtensionPrefix[4];
 
@@ -590,6 +585,11 @@ typedef struct InternalInstruction {
 
   /* Reader interface (C) */
   byteReader_t reader;
+
+  /* 1 if the prefix byte corresponding to the entry is present; 0 if not */
+  uint8_t prefixPresent[0x100];
+  /* contains the location (for use with the reader) of the prefix byte */
+  uint64_t prefixLocations[0x100];
   /* Opaque value passed to the reader */
   const void* readerArg;
   /* The address of the next byte to read via the reader */

tests/test_iter_benchmark.c

@@ -0,0 +1,65 @@
+/* Capstone Disassembler Engine */
+/* By Nguyen Anh Quynh <aquynh@gmail.com>, 2013> */
+
+// This sample code demonstrates the APIs cs_malloc() & cs_disasm_iter().
+#include <stdio.h>
+#include <stdlib.h>
+#include <time.h>
+#include "../myinttypes.h"
+
+#include <capstone.h>
+
+static void test()
+{
+#define X86_CODE32 "\x8d\x4c\x32\x08\x01\xd8\x81\xc6\x34\x12\x00\x00"
+	/* origin version output: time used:2.683000
+	 * modified version output: time used:2.358000
+	 * if don't output format text string,like this:
+	   //handle->printer(&mci, &ss, handle->printer_info);  <-----cs.c line 700
+	   output:time used:1.138000
+	 */
+
+	csh handle;
+	uint64_t address;
+	cs_insn *insn;
+	int i;
+	cs_err err;
+	const uint8_t *code;
+	size_t size;
+
+	err = cs_open(CS_ARCH_X86, CS_MODE_32, &handle);
+	if (err) {
+		printf("Failed on cs_open() with error returned: %u\n", err);
+		return;
+	}
+	cs_option(handle, CS_OPT_SYNTAX, CS_OPT_SYNTAX_INTEL);
+	cs_option(handle, CS_OPT_DETAIL, CS_OPT_ON);
+
+	clock_t start, end;
+	double  timeUsed;
+
+	start = clock();
+	int maxcount = 3400000;
+	insn = cs_malloc(handle);
+	for (i = 0; i < maxcount;) {
+		code = X86_CODE32;
+		address = 0x1000;
+		size = sizeof(X86_CODE32) - 1;
+		while(cs_disasm_iter(handle, &code, &size, &address, insn)) {
+			i++;
+		}
+	}
+	cs_free(insn, 1);
+	cs_close(&handle);
+	end = clock();
+	timeUsed = (double)(end - start) / CLOCKS_PER_SEC;
+	printf("time used:%f\n", timeUsed);
+	getchar();
+}
+
+int main()
+{
+	test();
+
+	return 0;
+}