tcg: allocate TB structs before the corresponding translated code

Allocating an arbitrarily-sized array of tbs results in either
(a) a lot of memory wasted or (b) unnecessary flushes of the code
cache when we run out of TB structs in the array.

An obvious solution would be to just malloc a TB struct when needed,
and keep the TB array as an array of pointers (recall that tb_find_pc()
needs the TB array to run in O(log n)).

Perhaps a better solution, which is implemented in this patch, is to
allocate TB's right before the translated code they describe. This
results in some memory waste due to padding to have code and TBs in
separate cache lines--for instance, I measured 4.7% of padding in the
used portion of code_gen_buffer when booting aarch64 Linux on a
host with 64-byte cache lines. However, it can allow for optimizations
in some host architectures, since TCG backends could safely assume that
the TB and the corresponding translated code are very close to each
other in memory. See this message by rth for a detailed explanation:

  https://lists.gnu.org/archive/html/qemu-devel/2017-03/msg05172.html
  Subject: Re: GSoC 2017 Proposal: TCG performance enhancements
  Message-ID: <1e67644b-4b30-887e-d329-1848e94c9484@twiddle.net>

Suggested-by: Richard Henderson <rth@twiddle.net>
Reviewed-by: Pranith Kumar <bobby.prani@gmail.com>
Signed-off-by: Emilio G. Cota <cota@braap.org>
Message-Id: <1496790745-314-3-git-send-email-cota@braap.org>
[rth: Simplify the arithmetic in tcg_tb_alloc]
Signed-off-by: Richard Henderson <rth@twiddle.net>

2 files changed, 21 insertions, 1 deletions

diff --git a/tcg/tcg.c b/tcg/tcg.c
index 564292f..3559829 100644
--- a/tcg/tcg.c
+++ b/tcg/tcg.c
@@ -383,6 +383,26 @@ void tcg_context_init(TCGContext *s)
     }
 }
 
+/*
+ * Allocate TBs right before their corresponding translated code, making
+ * sure that TBs and code are on different cache lines.
+ */
+TranslationBlock *tcg_tb_alloc(TCGContext *s)
+{
+    uintptr_t align = qemu_icache_linesize;
+    TranslationBlock *tb;
+    void *next;
+
+    tb = (void *)ROUND_UP((uintptr_t)s->code_gen_ptr, align);
+    next = (void *)ROUND_UP((uintptr_t)(tb + 1), align);
+
+    if (unlikely(next > s->code_gen_highwater)) {
+        return NULL;
+    }
+    s->code_gen_ptr = next;
+    return tb;
+}
+
 void tcg_prologue_init(TCGContext *s)
 {
     size_t prologue_size, total_size;
diff --git a/tcg/tcg.h b/tcg/tcg.h
index 5ec48d1..9e37722 100644
--- a/tcg/tcg.h
+++ b/tcg/tcg.h
@@ -697,7 +697,6 @@ struct TCGContext {
        here, because there's too much arithmetic throughout that relies
        on addition and subtraction working on bytes.  Rely on the GCC
        extension that allows arithmetic on void*.  */
-    int code_gen_max_blocks;
     void *code_gen_prologue;
     void *code_gen_epilogue;
     void *code_gen_buffer;
@@ -756,6 +755,7 @@ static inline bool tcg_op_buf_full(void)
 /* tb_lock must be held for tcg_malloc_internal. */
 void *tcg_malloc_internal(TCGContext *s, int size);
 void tcg_pool_reset(TCGContext *s);
+TranslationBlock *tcg_tb_alloc(TCGContext *s);
 
 void tb_lock(void);
 void tb_unlock(void);