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>

include/exec/tb-context.h

@@ -31,8 +31,9 @@ typedef struct TBContext TBContext;
 
 struct TBContext {
 
-    TranslationBlock *tbs;
+    TranslationBlock **tbs;
     struct qht htable;
+    size_t tbs_size;
     int nb_tbs;
     /* any access to the tbs or the page table must use this lock */
     QemuMutex tb_lock;

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;

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);

translate-all.c

@@ -781,12 +781,13 @@ static inline void code_gen_alloc(size_t tb_size)
         exit(1);
     }
 
-    /* Estimate a good size for the number of TBs we can support.  We
+    /* size this conservatively -- realloc later if needed */
-       still haven't deducted the prologue from the buffer size here,
+    tcg_ctx.tb_ctx.tbs_size =
-       but that's minimal and won't affect the estimate much.  */
+        tcg_ctx.code_gen_buffer_size / CODE_GEN_AVG_BLOCK_SIZE / 8;
-    tcg_ctx.code_gen_max_blocks
+    if (unlikely(!tcg_ctx.tb_ctx.tbs_size)) {
-        = tcg_ctx.code_gen_buffer_size / CODE_GEN_AVG_BLOCK_SIZE;
+        tcg_ctx.tb_ctx.tbs_size = 64 * 1024;
-    tcg_ctx.tb_ctx.tbs = g_new(TranslationBlock, tcg_ctx.code_gen_max_blocks);
+    }
+    tcg_ctx.tb_ctx.tbs = g_new(TranslationBlock *, tcg_ctx.tb_ctx.tbs_size);
 
     qemu_mutex_init(&tcg_ctx.tb_ctx.tb_lock);
 }
@@ -828,13 +829,20 @@ bool tcg_enabled(void)
 static TranslationBlock *tb_alloc(target_ulong pc)
 {
     TranslationBlock *tb;
+    TBContext *ctx;
 
     assert_tb_locked();
 
-    if (tcg_ctx.tb_ctx.nb_tbs >= tcg_ctx.code_gen_max_blocks) {
+    tb = tcg_tb_alloc(&tcg_ctx);
+    if (unlikely(tb == NULL)) {
         return NULL;
     }
-    tb = &tcg_ctx.tb_ctx.tbs[tcg_ctx.tb_ctx.nb_tbs++];
+    ctx = &tcg_ctx.tb_ctx;
+    if (unlikely(ctx->nb_tbs == ctx->tbs_size)) {
+        ctx->tbs_size *= 2;
+        ctx->tbs = g_renew(TranslationBlock *, ctx->tbs, ctx->tbs_size);
+    }
+    ctx->tbs[ctx->nb_tbs++] = tb;
     tb->pc = pc;
     tb->cflags = 0;
     tb->invalid = false;
@@ -850,8 +858,10 @@ void tb_free(TranslationBlock *tb)
        Ignore the hard cases and just back up if this TB happens to
        be the last one generated.  */
     if (tcg_ctx.tb_ctx.nb_tbs > 0 &&
-            tb == &tcg_ctx.tb_ctx.tbs[tcg_ctx.tb_ctx.nb_tbs - 1]) {
+            tb == tcg_ctx.tb_ctx.tbs[tcg_ctx.tb_ctx.nb_tbs - 1]) {
-        tcg_ctx.code_gen_ptr = tb->tc_ptr;
+        size_t struct_size = ROUND_UP(sizeof(*tb), qemu_icache_linesize);
+
+        tcg_ctx.code_gen_ptr = tb->tc_ptr - struct_size;
         tcg_ctx.tb_ctx.nb_tbs--;
     }
 }
@@ -1666,7 +1676,7 @@ static TranslationBlock *tb_find_pc(uintptr_t tc_ptr)
     m_max = tcg_ctx.tb_ctx.nb_tbs - 1;
     while (m_min <= m_max) {
         m = (m_min + m_max) >> 1;
-        tb = &tcg_ctx.tb_ctx.tbs[m];
+        tb = tcg_ctx.tb_ctx.tbs[m];
         v = (uintptr_t)tb->tc_ptr;
         if (v == tc_ptr) {
             return tb;
@@ -1676,7 +1686,7 @@ static TranslationBlock *tb_find_pc(uintptr_t tc_ptr)
             m_min = m + 1;
         }
     }
-    return &tcg_ctx.tb_ctx.tbs[m_max];
+    return tcg_ctx.tb_ctx.tbs[m_max];
 }
 
 #if !defined(CONFIG_USER_ONLY)
@@ -1874,7 +1884,7 @@ void dump_exec_info(FILE *f, fprintf_function cpu_fprintf)
     direct_jmp_count = 0;
     direct_jmp2_count = 0;
     for (i = 0; i < tcg_ctx.tb_ctx.nb_tbs; i++) {
-        tb = &tcg_ctx.tb_ctx.tbs[i];
+        tb = tcg_ctx.tb_ctx.tbs[i];
         target_code_size += tb->size;
         if (tb->size > max_target_code_size) {
             max_target_code_size = tb->size;
@@ -1894,8 +1904,7 @@ void dump_exec_info(FILE *f, fprintf_function cpu_fprintf)
     cpu_fprintf(f, "gen code size       %td/%zd\n",
                 tcg_ctx.code_gen_ptr - tcg_ctx.code_gen_buffer,
                 tcg_ctx.code_gen_highwater - tcg_ctx.code_gen_buffer);
-    cpu_fprintf(f, "TB count            %d/%d\n",
+    cpu_fprintf(f, "TB count            %d\n", tcg_ctx.tb_ctx.nb_tbs);
-            tcg_ctx.tb_ctx.nb_tbs, tcg_ctx.code_gen_max_blocks);
     cpu_fprintf(f, "TB avg target size  %d max=%d bytes\n",
             tcg_ctx.tb_ctx.nb_tbs ? target_code_size /
                     tcg_ctx.tb_ctx.nb_tbs : 0,