diff --git a/src/lj_asm_x86.h b/src/lj_asm_x86.h
index fe3cac68..b7ce4b06 100644
--- a/src/lj_asm_x86.h
+++ b/src/lj_asm_x86.h
@@ -866,12 +866,13 @@ static void asm_intrin_opcode(ASMState *as, IRIns *ir, IntrinsInfo *ininfo)
   checkmclim(as);
 }
 
-void asm_intrin_results(ASMState *as, IRIns *ir, CIntrinsic* intrins, IntrinsInfo* ininfo)
+int asm_intrin_results(ASMState *as, IRIns *ir, CIntrinsic* intrins, IntrinsInfo* ininfo)
 {
   IRRef results[LJ_INTRINS_MAXREG];
   RegSet evict = 0, outset = 0, aout = 0;
   int32_t i = intrin_regmode(intrins) ? intrins->dyninsz : 0;
   int32_t dynout = intrin_dynrout(intrins) ? 1 : 0;
+  int used = 0;
 
   /* Gather the output register IR instructions */
   if (intrins->outsz > 0) {
@@ -883,6 +884,7 @@ void asm_intrin_results(ASMState *as, IRIns *ir, CIntrinsic* intrins, IntrinsInf
       results[n] = (IRRef)(irret - as->ir);
 
       if (ra_used(irret)) {
+        used++;
         if (n >= dynout && irret->r == reg_rid(ininfo->inregs[n])) {
           rset_set(aout, irret->r);
         }
@@ -895,6 +897,10 @@ void asm_intrin_results(ASMState *as, IRIns *ir, CIntrinsic* intrins, IntrinsInf
     }
   }
   
+  if (!used && !intrin_sideeff(intrins)) {
+    /* IR is dead code */
+    return 0;
+  }
   evict = ininfo->modset;
 
   /* Check what registers need evicting for fixed input registers */
@@ -938,6 +944,8 @@ void asm_intrin_results(ASMState *as, IRIns *ir, CIntrinsic* intrins, IntrinsInf
       ra_destreg(as, irret, r);
     }
   }
+
+  return 1;
 }
 
 static void asm_intrinsic(ASMState *as, IRIns *ir, IRIns *asmend)
@@ -983,7 +991,10 @@ static void asm_intrinsic(ASMState *as, IRIns *ir, IRIns *asmend)
   }
   lua_assert(n == 0);
 
-  asm_intrin_results(as, ir, intrins, &ininfo);
+  /* If there is no users of our results skip emitting */
+  if (!asm_intrin_results(as, ir, intrins, &ininfo)) {
+    goto exit;
+  }
 
   if (intrin_regmode(intrins)) {
     asm_intrin_opcode(as, ir, &ininfo);
@@ -1004,6 +1015,7 @@ static void asm_intrinsic(ASMState *as, IRIns *ir, IRIns *asmend)
   }
 
   asm_asmsetupargs(as, &ininfo);
+exit:
   if (ininfo.asmend) {
     /* Skip over our IR_INTRN since were emitting from the tail */
     as->curins = (IRRef)(ir - as->ir);
diff --git a/src/lj_crecord.c b/src/lj_crecord.c
index 75fafa13..c60cf471 100644
--- a/src/lj_crecord.c
+++ b/src/lj_crecord.c
@@ -1398,8 +1398,9 @@ void crec_call_intrins(jit_State *J, RecordFFData *rd, CType *func)
     }
   }
   
-  /* Intrinsics are assumed to always have side effects */
-  J->needsnap = 1;
+  if (intrin_sideeff(intrins)) {
+    J->needsnap = 1;
+  }
   rd->nres = intrins->outsz;
 }
 #else
diff --git a/src/lj_intrinsic.c b/src/lj_intrinsic.c
index 7a39abb7..c140a708 100644
--- a/src/lj_intrinsic.c
+++ b/src/lj_intrinsic.c
@@ -310,6 +310,11 @@ static int parse_opmode(const char *op, MSize len)
       case 'U':
         flags |= INTRINSFLAG_IMMB;
         break;
+      case 'S':
+        flags |= INTRINSFLAG_MEMORYSIDE;
+      case 's':
+        flags |= INTRINSFLAG_HASSIDE;
+        break;
       case 'C':
         flags |= INTRINSFLAG_CALLED;
         break;
diff --git a/src/lj_intrinsic.h b/src/lj_intrinsic.h
index bcf4618e..438174c2 100644
--- a/src/lj_intrinsic.h
+++ b/src/lj_intrinsic.h
@@ -61,6 +61,8 @@ typedef enum INTRINSFLAGS {
   INTRINSFLAG_LARGEOP  = 0x800,
   /* Opcode is commutative allowing the input registers to be swapped to allow better fusing */
   INTRINSFLAG_ISCOMM = 0x1000,
+  /* Instruction has non obvious side effects */
+  INTRINSFLAG_HASSIDE  = 0x2000, 
  
   /* Opcode uses ymm registers */
   INTRINSFLAG_VEX256   = 0x4000,
@@ -89,6 +91,8 @@ typedef struct AsmHeader {
 #define intrin_setregmode(intrins, mode) \
   (intrins)->flags = ((intrins)->flags & ~INTRINSFLAG_REGMODEMASK)|(mode)
 #define intrin_iscomm(intrins) ((intrins)->flags & INTRINSFLAG_ISCOMM)
+/* Has side effects that may not be to memory */
+#define intrin_sideeff(intrins) ((intrins)->flags & INTRINSFLAG_HASSIDE)
 
 #define intrin_getopextb(intrins) ((intrins)->out[3])
 #define intrin_setopextb(intrins, opext) \
diff --git a/tests/intrinsic_spec.lua b/tests/intrinsic_spec.lua
index 58db64a1..80f39bf5 100644
--- a/tests/intrinsic_spec.lua
+++ b/tests/intrinsic_spec.lua
@@ -478,7 +478,7 @@ context("__mcode", function()
   end)
   
   it("prefix byte", function() 
-    assert_cdef([[void atomicadd(int32_t* nptr, int32_t n) __mcode("01mRIP", 0xF0);]], "atomicadd")
+    assert_cdef([[void atomicadd(int32_t* nptr, int32_t n) __mcode("01mRIPS", 0xF0);]], "atomicadd")
     
     local sum = 0   
     local function checker(i, jsum)
@@ -498,7 +498,7 @@ context("__mcode", function()
   
   if ffi.arch == "x64" then
     it("prefix64", function()
-      assert_cdef([[void atomicadd64(int64_t* nptr, int64_t n) __mcode("01mRIP", 0xF0);]], "atomicadd64")
+      assert_cdef([[void atomicadd64(int64_t* nptr, int64_t n) __mcode("01mRIPS", 0xF0);]], "atomicadd64")
       
       local sum = 0
       local function checker(i, jsum)
@@ -516,7 +516,7 @@ context("__mcode", function()
   end
 
   it("prefix and imm byte", function() 
-    assert_cdef([[void atomicadd1(int32_t* nptr) __mcode("830mIUP", 0xF0, 0x01);]], "atomicadd1")
+    assert_cdef([[void atomicadd1(int32_t* nptr) __mcode("830mIUPS", 0xF0, 0x01);]], "atomicadd1")
     
     local function checker(i, jsum)
       if(jsum ~= i) then 
@@ -565,11 +565,39 @@ context("__mcode", function()
     assert_exit(10, test_idiv, 10, 5)
   end)
   
+  it("side effects(mode)", function()
+    assert_cdef([[void add1_noside(int32_t* nptr) __mcode("830mIU", 0x01);]], "add1_noside") 
+    assert_cdef([[void add1_side(int32_t* nptr) __mcode("830mIUs", 0x01);]], "add1_side")
+    
+    local numptr = ffi.new("int32_t[2]", 0)
+
+    local function checker(i, n)
+      assert(n == i)
+      assert(numptr[0] >= numptr[1])
+    end
+  
+    local function test_sideff(i)
+      ffi.C.add1_side(numptr)
+      ffi.C.add1_noside(numptr+1)
+      return numptr[0]
+    end
+  
+    assert_jitchecker(checker, test_sideff)   
+    assert_greater_than(numptr[0], numptr[1])
+    
+    numptr[0] = 0
+    numptr[1] = 0
+    --test directly as JIT'ed 
+    test_sideff()
+    assert_equal(numptr[0], 1)
+    assert_equal(numptr[1], 0)
+  end)
+  
   it("prefetch", function()
-    assert_cdef([[void prefetch0(void* mem) __mcode("0F181mI")]], "prefetch0")
-    assert_cdef([[void prefetch1(void* mem) __mcode("0F182mI")]], "prefetch1")
-    assert_cdef([[void prefetch2(void* mem) __mcode("0F183mI")]], "prefetch2")
-    assert_cdef([[void prefetchnta(void* mem) __mcode("0F180mI")]], "prefetchnta")
+    assert_cdef([[void prefetch0(void* mem) __mcode("0F181mIs")]], "prefetch0")
+    assert_cdef([[void prefetch1(void* mem) __mcode("0F182mIs")]], "prefetch1")
+    assert_cdef([[void prefetch2(void* mem) __mcode("0F183mIs")]], "prefetch2")
+    assert_cdef([[void prefetchnta(void* mem) __mcode("0F180mIs")]], "prefetchnta")
 
     local asm = ffi.C
     local kmem = ffi.new("int[4]")