From: "Emilio G. Cota" <c...@braap.org>

Emulating LL/SC with cmpxchg is not correct, since it can
suffer from the ABA problem. Portable parallel code, however,
is written assuming only cmpxchg--and not LL/SC--is available.
This means that in practice emulating LL/SC with cmpxchg is
a viable alternative.

The appended emulates LL/SC pairs in ARM with cmpxchg helpers.
This works in both user and system mode. In usermode, it avoids
pausing all other CPUs to perform the LL/SC pair. The subsequent
performance and scalability improvement is significant, as the
plots below show. They plot the throughput of atomic_add-bench
compiled for ARM and executed on a 64-core x86 machine.

Hi-res plots: http://imgur.com/a/aNQpB

               atomic_add-bench: 1000000 ops/thread, [0,1] range

  9 ++---------+----------+----------+----------+----------+----------+---++
    +cmpxchg +-E--+       +          +          +          +          +    |
  8 +Emaster +-H--+                                                       ++
    | |                                                                    |
  7 ++E                                                                   ++
    | |                                                                    |
  6 ++++                                                                  ++
    |  |                                                                   |
  5 ++ |                                                                  ++
  4 ++ |                                                                  ++
    |  |                                                                   |
  3 ++ |                                                                  ++
    |   |                                                                  |
  2 ++  |                                                                 ++
    |H++E+---                                  +++  ---+E+------+E+------+E|
  1 +++     +E+-----+E+------+E+------+E+------+E+--   +++      +++       ++
    ++H+       +    +++   +  +++     ++++       +          +          +    |
  0 ++--H----H-+-----H----+----------+----------+----------+----------+---++
    0          10         20         30         40         50         60
                               Number of threads

                atomic_add-bench: 1000000 ops/thread, [0,2] range

  16 ++---------+----------+---------+----------+----------+----------+---++
     +cmpxchg +-E--+       +         +          +          +          +    |
  14 ++master +-H--+                                                      ++
     | |                                                                   |
  12 ++|                                                                  ++
     | E                                                                   |
  10 ++|                                                                  ++
     | |                                                                   |
   8 ++++                                                                 ++
     |E+|                                                                  |
     |  |                                                                  |
   6 ++ |                                                                 ++
     |   |                                                                 |
   4 ++  |                                                                ++
     |  +E+---       +++      +++              +++           ---+E+------+E|
   2 +H+     +E+------E-------+E+-----+E+------+E+------+E+--            +++
     + |        +    +++   +         ++++       +          +          +    |
   0 ++H-H----H-+-----H----+---------+----------+----------+----------+---++
     0          10         20        30         40         50         60
                                Number of threads

               atomic_add-bench: 1000000 ops/thread, [0,128] range

  70 ++---------+----------+---------+----------+----------+----------+---++
     +cmpxchg +-E--+       +         +          +       ++++          +    |
  60 ++master +-H--+                                 ----E------+E+-------++
     |                                        -+E+---   +++     +++      +E|
     |                                +++ ---- +++                       ++|
  50 ++                       +++  ---+E+-                                ++
     |                        -E---                                        |
  40 ++                    ---+++                                         ++
     |               +++---                                                |
     |              -+E+                                                   |
  30 ++      +++----                                                      ++
     |       +E+                                                           |
  20 ++ +++--                                                             ++
     |  +E+                                                                |
     |+E+                                                                  |
  10 +E+                                                                  ++
     +          +          +         +          +          +          +    |
   0 +HH-H----H-+-----H----+---------+----------+----------+----------+---++
     0          10         20        30         40         50         60
                                Number of threads

              atomic_add-bench: 1000000 ops/thread, [0,1024] range

  120 ++---------+---------+----------+---------+----------+----------+---++
      +cmpxchg +-E--+      +          +         +          +          +    |
      | master +-H--+                                                    ++|
  100 ++                                                              ----E+
      |                                                 +++  ---+E+---   ++|
      |                                                --E---   +++        |
   80 ++                                           ---- +++               ++
      |                                     ---+E+-                        |
   60 ++                              -+E+--                              ++
      |                       +++ ---- +++                                 |
      |                      -+E+-                                         |
   40 ++              +++----                                             ++
      |      +++   ---+E+                                                  |
      |     -+E+---                                                        |
   20 ++ +E+                                                              ++
      |+E+++                                                               |
      +E+        +         +          +         +          +          +    |
    0 +HH-H---H--+-----H---+----------+---------+----------+----------+---++
      0          10        20         30        40         50         60
                                Number of threads

[rth: Enforce alignment for ldrexd.]

Reviewed-by: Alex Bennée <alex.ben...@linaro.org>
Signed-off-by: Emilio G. Cota <c...@braap.org>
Message-Id: <1467054136-10430-23-git-send-email-c...@braap.org>
Signed-off-by: Richard Henderson <r...@twiddle.net>
---
 target-arm/translate.c | 136 +++++++++++++++----------------------------------
 1 file changed, 42 insertions(+), 94 deletions(-)

diff --git a/target-arm/translate.c b/target-arm/translate.c
index bcd2958..2bcc97b 100644
--- a/target-arm/translate.c
+++ b/target-arm/translate.c
@@ -7676,47 +7676,27 @@ static void gen_logicq_cc(TCGv_i32 lo, TCGv_i32 hi)
     tcg_gen_or_i32(cpu_ZF, lo, hi);
 }
 
-/* Load/Store exclusive instructions are implemented by remembering
-   the value/address loaded, and seeing if these are the same
-   when the store is performed. This should be sufficient to implement
-   the architecturally mandated semantics, and avoids having to monitor
-   regular stores.
-
-   In system emulation mode only one CPU will be running at once, so
-   this sequence is effectively atomic.  In user emulation mode we
-   throw an exception and handle the atomic operation elsewhere.  */
 static void gen_load_exclusive(DisasContext *s, int rt, int rt2,
                                TCGv_i32 addr, int size)
 {
     TCGv_i32 tmp = tcg_temp_new_i32();
+    TCGMemOp opc = size | MO_ALIGN | s->be_data;
 
     s->is_ldex = true;
 
-    switch (size) {
-    case 0:
-        gen_aa32_ld8u(s, tmp, addr, get_mem_index(s));
-        break;
-    case 1:
-        gen_aa32_ld16ua(s, tmp, addr, get_mem_index(s));
-        break;
-    case 2:
-    case 3:
-        gen_aa32_ld32ua(s, tmp, addr, get_mem_index(s));
-        break;
-    default:
-        abort();
-    }
-
     if (size == 3) {
         TCGv_i32 tmp2 = tcg_temp_new_i32();
-        TCGv_i32 tmp3 = tcg_temp_new_i32();
+        TCGv_i64 t64 = tcg_temp_new_i64();
+
+        gen_aa32_ld_i64(s, t64, addr, get_mem_index(s), opc);
+        tcg_gen_mov_i64(cpu_exclusive_val, t64);
+        tcg_gen_extr_i64_i32(tmp, tmp2, t64);
+        tcg_temp_free_i64(t64);
 
-        tcg_gen_addi_i32(tmp2, addr, 4);
-        gen_aa32_ld32u(s, tmp3, tmp2, get_mem_index(s));
+        store_reg(s, rt2, tmp2);
         tcg_temp_free_i32(tmp2);
-        tcg_gen_concat_i32_i64(cpu_exclusive_val, tmp, tmp3);
-        store_reg(s, rt2, tmp3);
     } else {
+        gen_aa32_ld_i32(s, tmp, addr, get_mem_index(s), opc);
         tcg_gen_extu_i32_i64(cpu_exclusive_val, tmp);
     }
 
@@ -7729,23 +7709,15 @@ static void gen_clrex(DisasContext *s)
     tcg_gen_movi_i64(cpu_exclusive_addr, -1);
 }
 
-#ifdef CONFIG_USER_ONLY
-static void gen_store_exclusive(DisasContext *s, int rd, int rt, int rt2,
-                                TCGv_i32 addr, int size)
-{
-    tcg_gen_extu_i32_i64(cpu_exclusive_test, addr);
-    tcg_gen_movi_i32(cpu_exclusive_info,
-                     size | (rd << 4) | (rt << 8) | (rt2 << 12));
-    gen_exception_internal_insn(s, 4, EXCP_STREX);
-}
-#else
 static void gen_store_exclusive(DisasContext *s, int rd, int rt, int rt2,
                                 TCGv_i32 addr, int size)
 {
-    TCGv_i32 tmp;
-    TCGv_i64 val64, extaddr;
+    TCGv_i32 t0, t1, t2;
+    TCGv_i64 extaddr;
+    TCGv taddr;
     TCGLabel *done_label;
     TCGLabel *fail_label;
+    TCGMemOp opc = size | MO_ALIGN | s->be_data;
 
     /* if (env->exclusive_addr == addr && env->exclusive_val == [addr]) {
          [addr] = {Rt};
@@ -7760,69 +7732,45 @@ static void gen_store_exclusive(DisasContext *s, int 
rd, int rt, int rt2,
     tcg_gen_brcond_i64(TCG_COND_NE, extaddr, cpu_exclusive_addr, fail_label);
     tcg_temp_free_i64(extaddr);
 
-    tmp = tcg_temp_new_i32();
-    switch (size) {
-    case 0:
-        gen_aa32_ld8u(s, tmp, addr, get_mem_index(s));
-        break;
-    case 1:
-        gen_aa32_ld16u(s, tmp, addr, get_mem_index(s));
-        break;
-    case 2:
-    case 3:
-        gen_aa32_ld32u(s, tmp, addr, get_mem_index(s));
-        break;
-    default:
-        abort();
-    }
-
-    val64 = tcg_temp_new_i64();
+    taddr = gen_aa32_addr(s, addr, opc);
+    t0 = tcg_temp_new_i32();
+    t1 = load_reg(s, rt);
     if (size == 3) {
-        TCGv_i32 tmp2 = tcg_temp_new_i32();
-        TCGv_i32 tmp3 = tcg_temp_new_i32();
-        tcg_gen_addi_i32(tmp2, addr, 4);
-        gen_aa32_ld32u(s, tmp3, tmp2, get_mem_index(s));
-        tcg_temp_free_i32(tmp2);
-        tcg_gen_concat_i32_i64(val64, tmp, tmp3);
-        tcg_temp_free_i32(tmp3);
-    } else {
-        tcg_gen_extu_i32_i64(val64, tmp);
-    }
-    tcg_temp_free_i32(tmp);
+        TCGv_i64 o64 = tcg_temp_new_i64();
+        TCGv_i64 n64 = tcg_temp_new_i64();
 
-    tcg_gen_brcond_i64(TCG_COND_NE, val64, cpu_exclusive_val, fail_label);
-    tcg_temp_free_i64(val64);
+        t2 = load_reg(s, rt2);
+        tcg_gen_concat_i32_i64(n64, t1, t2);
+        tcg_temp_free_i32(t2);
+        gen_aa32_frob64(s, n64);
 
-    tmp = load_reg(s, rt);
-    switch (size) {
-    case 0:
-        gen_aa32_st8(s, tmp, addr, get_mem_index(s));
-        break;
-    case 1:
-        gen_aa32_st16(s, tmp, addr, get_mem_index(s));
-        break;
-    case 2:
-    case 3:
-        gen_aa32_st32(s, tmp, addr, get_mem_index(s));
-        break;
-    default:
-        abort();
-    }
-    tcg_temp_free_i32(tmp);
-    if (size == 3) {
-        tcg_gen_addi_i32(addr, addr, 4);
-        tmp = load_reg(s, rt2);
-        gen_aa32_st32(s, tmp, addr, get_mem_index(s));
-        tcg_temp_free_i32(tmp);
+        tcg_gen_atomic_cmpxchg_i64(o64, taddr, cpu_exclusive_val, n64,
+                                   get_mem_index(s), opc);
+        tcg_temp_free_i64(n64);
+
+        gen_aa32_frob64(s, o64);
+        tcg_gen_setcond_i64(TCG_COND_NE, o64, o64, cpu_exclusive_val);
+        tcg_gen_extrl_i64_i32(t0, o64);
+
+        tcg_temp_free_i64(o64);
+    } else {
+        t2 = tcg_temp_new_i32();
+        tcg_gen_extrl_i64_i32(t2, cpu_exclusive_val);
+        tcg_gen_atomic_cmpxchg_i32(t0, taddr, t2, t1, get_mem_index(s), opc);
+        tcg_gen_setcond_i32(TCG_COND_NE, t0, t0, t2);
+        tcg_temp_free_i32(t2);
     }
-    tcg_gen_movi_i32(cpu_R[rd], 0);
+    tcg_temp_free_i32(t1);
+    tcg_temp_free(taddr);
+    tcg_gen_mov_i32(cpu_R[rd], t0);
+    tcg_temp_free_i32(t0);
     tcg_gen_br(done_label);
+
     gen_set_label(fail_label);
     tcg_gen_movi_i32(cpu_R[rd], 1);
     gen_set_label(done_label);
     tcg_gen_movi_i64(cpu_exclusive_addr, -1);
 }
-#endif
 
 /* gen_srs:
  * @env: CPUARMState
-- 
2.5.5


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