On 10/14/23 03:01, Paolo Bonzini wrote:
+static void gen_CMPccXADD(DisasContext *s, CPUX86State *env, X86DecodedInsn
*decode)
+{
+ TCGv z_tl = tcg_constant_tl(0);
+ TCGLabel *label_top = gen_new_label();
+ TCGLabel *label_bottom = gen_new_label();
+ TCGv oldv = tcg_temp_new();
+ TCGv memv = tcg_temp_new();
+ TCGv newv = tcg_temp_new();
+ TCGv cmpv = tcg_temp_new();
+ TCGv tmp_cc = tcg_temp_new();
+
+ TCGv cmp_lhs, cmp_rhs;
+ MemOp ot, ot_full;
+
+ int jcc_op = (decode->b >> 1) & 7;
+ static const uint8_t cond[16] = {
TCGCond.
+ TCG_COND_NE, /* o, just test OF=1 */
+ TCG_COND_EQ, /* no, just test OF=0 */
+ TCG_COND_LTU, /* b */
+ TCG_COND_GEU, /* ae (nb) */
+ TCG_COND_EQ, /* z */
+ TCG_COND_NE, /* nz */
+ TCG_COND_LEU, /* be */
+ TCG_COND_GTU, /* a (nbe) */
+ TCG_COND_LT, /* s, compares result against 0 */
+ TCG_COND_GE, /* ns, compares result against 0 */
+ TCG_COND_NE, /* p, just test PF=1 */
+ TCG_COND_EQ, /* np, just test PF=0 */
+ TCG_COND_LT, /* l */
+ TCG_COND_GE, /* ge (nl) */
+ TCG_COND_LE, /* le */
+ TCG_COND_GT, /* g (nle) */
+ };
You don't need the full table here:
cond = cond_table[jcc_op];
if (decode->b & 1)
cond = tcg_invert_cond(cond)
+ /* Compute comparison result but do not clobber cc_* yet. */
+ switch (jcc_op) {
+ case JCC_O:
+ case JCC_P:
+ tcg_gen_sub_tl(s->T0, memv, cmpv);
+ gen_helper_cc_compute_all(tmp_cc, s->T0, cmpv, z_tl,
+ tcg_constant_i32(CC_OP_SUBB + ot));
+ decode->cc_src = tmp_cc;
+ set_cc_op(s, CC_OP_EFLAGS);
+
+ tcg_gen_andi_tl(s->T0, tmp_cc, (jcc_op == JCC_O ? CC_O : CC_P));
+ cmp_lhs = s->T0, cmp_rhs = z_tl;
I'm not keen on the weight of the helper function within a cmpxchg loop.
I think you should compute these two cases explicitly:
JCC_O:
// Need operands sign-extended.
// cond_table[JCC_O] = TCG_COND_LT -- sign bit set.
tcg_gen_xor_tl(tmp, cmpv, memv);
tcg_gen_xor_tl(cmp_lhs, cmpv, s->T0);
tcg_gen_and_tl(cmp_lhs, cmp_lhs, tmp);
cmp_rhs = z_tl;
break;
JCC_P:
// cond_table[JCC_P] = TCG_COND_EQ -- even parity.
tcg_gen_ext8u_tl(cmp_lhs, s->T0);
tcg_gen_ctpop_tl(cmp_lhs, cmp_lhs);
tcg_gen_andi_tl(cmp_lhs, cmp_lhs, 1);
cmp_rhs = z_tl;
break;
+ cc_sub:
+ decode->cc_dst = s->T0;
+ decode->cc_src = cmpv;
+ decode->cc_srcT = memv;
+ set_cc_op(s, CC_OP_SUBB + ot);
+ break;
At which point this is common to all cases.
+ }
+
+ /* Compute new value: if condition does not hold, just store back memv */
+ tcg_gen_add_tl(newv, memv, s->T1);
+ tcg_gen_movcond_tl(cond[decode->b & 15], newv, cmp_lhs, cmp_rhs, newv,
memv);
+ tcg_gen_atomic_cmpxchg_tl(oldv, s->A0, memv, newv, s->mem_index, ot_full);
+
+ /* Exit unconditionally if cmpxchg succeeded. */
+ tcg_gen_brcond_tl(TCG_COND_EQ, oldv, memv, label_bottom);
+
+ /* Try again if there was actually a store to make. */
+ tcg_gen_brcond_tl(cond[decode->b & 15], cmp_lhs, cmp_rhs, label_top);
I'm tempted to have this unlikely case sync the pc and exit the tb.
This would restart the current instruction after testing for exit request.
But I suppose we have plenty of other places with unbounded cmpxchg loops...
r~
