tcg: Increase hit rate of lookup_tb_ptr

We can call tb_htable_lookup even when the tb_jmp_cache is completely
empty.  Therefore, un-nest most of the code dependent on tb != NULL
from the read from the cache.

This improves the hit rate of lookup_tb_ptr; for instance, when booting
and immediately shutting down debian-arm, the hit rate improves from
93.2% to 99.4%.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Emilio G. Cota <cota@braap.org>
Signed-off-by: Richard Henderson <rth@twiddle.net>

diff --git a/tcg-runtime.c b/tcg-runtime.c
index 7fa90ce50852ad553ef6dfe899a0d85517bcdfc5..ec3a34e4615524c2d733b645c3245959afdbbfcd 100644 (file)
--- a/tcg-runtime.c
+++ b/tcg-runtime.c
@@ -149,23 +149,23 @@ void *HELPER(lookup_tb_ptr)(CPUArchState *env, target_ulong addr)
     CPUState *cpu = ENV_GET_CPU(env);
     TranslationBlock *tb;
     target_ulong cs_base, pc;
-    uint32_t flags;
-
-    tb = atomic_rcu_read(&cpu->tb_jmp_cache[tb_jmp_cache_hash_func(addr)]);
-    if (likely(tb)) {
-        cpu_get_tb_cpu_state(env, &pc, &cs_base, &flags);
-        if (likely(tb->pc == addr && tb->cs_base == cs_base &&
-                   tb->flags == flags)) {
-            goto found;
-        }
+    uint32_t flags, addr_hash;
+
+    addr_hash = tb_jmp_cache_hash_func(addr);
+    tb = atomic_rcu_read(&cpu->tb_jmp_cache[addr_hash]);
+    cpu_get_tb_cpu_state(env, &pc, &cs_base, &flags);
+
+    if (unlikely(!(tb
+                   && tb->pc == addr
+                   && tb->cs_base == cs_base
+                   && tb->flags == flags))) {
         tb = tb_htable_lookup(cpu, addr, cs_base, flags);
-        if (likely(tb)) {
-            atomic_set(&cpu->tb_jmp_cache[tb_jmp_cache_hash_func(addr)], tb);
-            goto found;
+        if (!tb) {
+            return tcg_ctx.code_gen_epilogue;
         }
+        atomic_set(&cpu->tb_jmp_cache[addr_hash], tb);
     }
-    return tcg_ctx.code_gen_epilogue;
- found:
+
     qemu_log_mask_and_addr(CPU_LOG_EXEC, addr,
                            "Chain %p [%d: " TARGET_FMT_lx "] %s\n",
                            tb->tc_ptr, cpu->cpu_index, addr,