Re: [Qemu-devel] [PATCH 2/2] target-mips: reimplement SC instruction and use cmpxchg

[Leon Alrae]

On Fri, Sep 16, 2016 at 09:48:51AM -0700, Richard Henderson wrote:
> On 09/15/2016 01:44 AM, Leon Alrae wrote:
> > /* Store conditional */
> >+static void gen_st_cond(DisasContext *ctx, int rt, int base, int offset,
> >+                        int size)
> > {
> >+    TCGv addr, t0, val;
> >+    TCGLabel *l1 = gen_new_label();
> >+    TCGLabel *l2 = gen_new_label();
> >+    TCGLabel *done = gen_new_label();
> >
> >-#ifdef CONFIG_USER_ONLY
> >     t0 = tcg_temp_local_new();
> >+    addr = tcg_temp_local_new();
> >+    /* check the alignment of the address */
> >+    gen_base_offset_addr(ctx, addr, base, offset);
> >+    tcg_gen_andi_tl(t0, addr, size - 1);
> 
> You shouldn't have to test the alignment here, as the alignment
> should have been tested during the load-locked, and the (aligned)
> address will be compared.

This is to satisfy the requirement that unaligned SC generates Address
Error exception. But I agree that in practice this doesn't seem
particularly useful since LL will do that.

> 
> 
> >+    /* compare the address against that of the preceeding LL */
> >+    tcg_gen_brcond_tl(TCG_COND_EQ, addr, cpu_lladdr, l2);
> >+    tcg_gen_movi_tl(t0, 0);
> >+    tcg_gen_br(done);
> ...
> >+#ifdef TARGET_MIPS64
> >+    case 8: /* SCD */
> >+        tcg_gen_atomic_cmpxchg_i64(t0, addr, cpu_llval, val,
> >+                                   ctx->mem_idx, MO_TEQ);
> >         break;
> > #endif
> >-    case OPC_SC:
> >-    case R6_OPC_SC:
> >-        op_st_sc(t1, t0, rt, ctx);
> >+    case 4: /* SC */
> >+        {
> >+            TCGv_i32 val32 = tcg_temp_new_i32();
> >+            TCGv_i32 llval32 = tcg_temp_new_i32();
> >+            TCGv_i32 old32 = tcg_temp_new_i32();
> >+            tcg_gen_trunc_tl_i32(val32, val);
> >+            tcg_gen_trunc_tl_i32(llval32, cpu_llval);
> >+
> >+            tcg_gen_atomic_cmpxchg_i32(old32, addr, llval32, val32,
> >+                                       ctx->mem_idx, MO_TESL);
> >+            tcg_gen_ext_i32_tl(t0, old32);
> 
> You can use tcg_gen_atomic_cmpxchg_tl so that you do not need to do
> all of this truncation yourself.  Which means that if you replace
> the size parameter with a TCGMemOp parameter (MO_TEQ vs MO_TESL) you
> can make all this code common.

Ah, yes.

> 
> Further, local temporaries are less than ideal and should be avoided
> if possible.  Using them results in an extra store into the local
> stack frame.
> 
> We can avoid this for addr by noting that once you have compared
> addr to cpu_lladdr, you can free addr and use cpu_lladdr in the
> actual cmpxchg.

Ok. I'll correct in v2.

Thanks,
Leon