diff --git a/target/i386/helper.h b/target/i386/helper.h
index b7de5429ef..2df8049f91 100644
--- a/target/i386/helper.h
+++ b/target/i386/helper.h
@@ -66,8 +66,6 @@ DEF_HELPER_1(rsm, void, env)
 #endif /* !CONFIG_USER_ONLY */
 
 DEF_HELPER_2(into, void, env, int)
-DEF_HELPER_2(cmpxchg8b_unlocked, void, env, tl)
-DEF_HELPER_2(cmpxchg8b, void, env, tl)
 #ifdef TARGET_X86_64
 DEF_HELPER_2(cmpxchg16b_unlocked, void, env, tl)
 DEF_HELPER_2(cmpxchg16b, void, env, tl)
diff --git a/target/i386/tcg/mem_helper.c b/target/i386/tcg/mem_helper.c
index e3cdafd2d4..814786bb87 100644
--- a/target/i386/tcg/mem_helper.c
+++ b/target/i386/tcg/mem_helper.c
@@ -27,63 +27,6 @@
 #include "tcg/tcg.h"
 #include "helper-tcg.h"
 
-void helper_cmpxchg8b_unlocked(CPUX86State *env, target_ulong a0)
-{
-    uintptr_t ra = GETPC();
-    uint64_t oldv, cmpv, newv;
-    int eflags;
-
-    eflags = cpu_cc_compute_all(env, CC_OP);
-
-    cmpv = deposit64(env->regs[R_EAX], 32, 32, env->regs[R_EDX]);
-    newv = deposit64(env->regs[R_EBX], 32, 32, env->regs[R_ECX]);
-
-    oldv = cpu_ldq_data_ra(env, a0, ra);
-    newv = (cmpv == oldv ? newv : oldv);
-    /* always do the store */
-    cpu_stq_data_ra(env, a0, newv, ra);
-
-    if (oldv == cmpv) {
-        eflags |= CC_Z;
-    } else {
-        env->regs[R_EAX] = (uint32_t)oldv;
-        env->regs[R_EDX] = (uint32_t)(oldv >> 32);
-        eflags &= ~CC_Z;
-    }
-    CC_SRC = eflags;
-}
-
-void helper_cmpxchg8b(CPUX86State *env, target_ulong a0)
-{
-#ifdef CONFIG_ATOMIC64
-    uint64_t oldv, cmpv, newv;
-    int eflags;
-
-    eflags = cpu_cc_compute_all(env, CC_OP);
-
-    cmpv = deposit64(env->regs[R_EAX], 32, 32, env->regs[R_EDX]);
-    newv = deposit64(env->regs[R_EBX], 32, 32, env->regs[R_ECX]);
-
-    {
-        uintptr_t ra = GETPC();
-        int mem_idx = cpu_mmu_index(env, false);
-        MemOpIdx oi = make_memop_idx(MO_TEUQ, mem_idx);
-        oldv = cpu_atomic_cmpxchgq_le_mmu(env, a0, cmpv, newv, oi, ra);
-    }
-
-    if (oldv == cmpv) {
-        eflags |= CC_Z;
-    } else {
-        env->regs[R_EAX] = (uint32_t)oldv;
-        env->regs[R_EDX] = (uint32_t)(oldv >> 32);
-        eflags &= ~CC_Z;
-    }
-    CC_SRC = eflags;
-#else
-    cpu_loop_exit_atomic(env_cpu(env), GETPC());
-#endif /* CONFIG_ATOMIC64 */
-}
-
 #ifdef TARGET_X86_64
 void helper_cmpxchg16b_unlocked(CPUX86State *env, target_ulong a0)
 {
diff --git a/target/i386/tcg/translate.c b/target/i386/tcg/translate.c
index 1175540a2c..a134d63946 100644
--- a/target/i386/tcg/translate.c
+++ b/target/i386/tcg/translate.c
@@ -2976,15 +2976,59 @@ static void gen_sty_env_A0(DisasContext *s, int offset, bool align)
 
 static void gen_cmpxchg8b(DisasContext *s, CPUX86State *env, int modrm)
 {
+    TCGv_i64 cmp, val, old;
+    TCGv Z;
+
     gen_lea_modrm(env, s, modrm);
 
-    if ((s->prefix & PREFIX_LOCK) &&
-        (tb_cflags(s->base.tb) & CF_PARALLEL)) {
-        gen_helper_cmpxchg8b(cpu_env, s->A0);
+    cmp = tcg_temp_new_i64();
+    val = tcg_temp_new_i64();
+    old = tcg_temp_new_i64();
+
+    /* Construct the comparison values from the register pair. */
+    tcg_gen_concat_tl_i64(cmp, cpu_regs[R_EAX], cpu_regs[R_EDX]);
+    tcg_gen_concat_tl_i64(val, cpu_regs[R_EBX], cpu_regs[R_ECX]);
+
+    /* Only require atomic with LOCK; non-parallel handled in generator. */
+    if (s->prefix & PREFIX_LOCK) {
+        tcg_gen_atomic_cmpxchg_i64(old, s->A0, cmp, val, s->mem_index, MO_TEUQ);
     } else {
-        gen_helper_cmpxchg8b_unlocked(cpu_env, s->A0);
+        tcg_gen_nonatomic_cmpxchg_i64(old, s->A0, cmp, val,
+                                      s->mem_index, MO_TEUQ);
     }
-    set_cc_op(s, CC_OP_EFLAGS);
+    tcg_temp_free_i64(val);
+
+    /* Set tmp0 to match the required value of Z. */
+    tcg_gen_setcond_i64(TCG_COND_EQ, cmp, old, cmp);
+    Z = tcg_temp_new();
+    tcg_gen_trunc_i64_tl(Z, cmp);
+    tcg_temp_free_i64(cmp);
+
+    /*
+     * Extract the result values for the register pair.
+     * For 32-bit, we may do this unconditionally, because on success (Z=1),
+     * the old value matches the previous value in EDX:EAX.  For x86_64,
+     * the store must be conditional, because we must leave the source
+     * registers unchanged on success, and zero-extend the writeback
+     * on failure (Z=0).
+     */
+    if (TARGET_LONG_BITS == 32) {
+        tcg_gen_extr_i64_tl(cpu_regs[R_EAX], cpu_regs[R_EDX], old);
+    } else {
+        TCGv zero = tcg_constant_tl(0);
+
+        tcg_gen_extr_i64_tl(s->T0, s->T1, old);
+        tcg_gen_movcond_tl(TCG_COND_EQ, cpu_regs[R_EAX], Z, zero,
+                           s->T0, cpu_regs[R_EAX]);
+        tcg_gen_movcond_tl(TCG_COND_EQ, cpu_regs[R_EDX], Z, zero,
+                           s->T1, cpu_regs[R_EDX]);
+    }
+    tcg_temp_free_i64(old);
+
+    /* Update Z. */
+    gen_compute_eflags(s);
+    tcg_gen_deposit_tl(cpu_cc_src, cpu_cc_src, Z, ctz32(CC_Z), 1);
+    tcg_temp_free(Z);
 }
 
 #ifdef TARGET_X86_64
