tcg: Check for overflow via highwater mark

We currently pre-compute an worst case code size for any TB, which
works out to be 122kB.  Since the average TB size is near 1kB, this
wastes quite a lot of storage.

Instead, check for overflow in between generating code for each opcode.
The overhead of the check isn't measurable and wastage is minimized.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <rth@twiddle.net>

1 files changed, 26 insertions, 5 deletions

diff --git a/translate-all.c b/translate-all.c
index b43bd03..333eba4 100644
--- a/translate-all.c
+++ b/translate-all.c
@@ -223,6 +223,7 @@ static target_long decode_sleb128(uint8_t **pp)
 
 static int encode_search(TranslationBlock *tb, uint8_t *block)
 {
+    uint8_t *highwater = tcg_ctx.code_gen_highwater;
     uint8_t *p = block;
     int i, j, n;
 
@@ -241,6 +242,14 @@ static int encode_search(TranslationBlock *tb, uint8_t *block)
         }
         prev = (i == 0 ? 0 : tcg_ctx.gen_insn_end_off[i - 1]);
         p = encode_sleb128(p, tcg_ctx.gen_insn_end_off[i] - prev);
+
+        /* Test for (pending) buffer overflow.  The assumption is that any
+           one row beginning below the high water mark cannot overrun
+           the buffer completely.  Thus we can test for overflow after
+           encoding a row without having to check during encoding.  */
+        if (unlikely(p > highwater)) {
+            return -1;
+        }
     }
 
     return p - block;
@@ -756,9 +765,7 @@ static TranslationBlock *tb_alloc(target_ulong pc)
 {
     TranslationBlock *tb;
 
-    if (tcg_ctx.tb_ctx.nb_tbs >= tcg_ctx.code_gen_max_blocks ||
-        (tcg_ctx.code_gen_ptr - tcg_ctx.code_gen_buffer) >=
-         tcg_ctx.code_gen_buffer_max_size) {
+    if (tcg_ctx.tb_ctx.nb_tbs >= tcg_ctx.code_gen_max_blocks) {
         return NULL;
     }
     tb = &tcg_ctx.tb_ctx.tbs[tcg_ctx.tb_ctx.nb_tbs++];
@@ -1063,12 +1070,15 @@ TranslationBlock *tb_gen_code(CPUState *cpu,
     if (use_icount) {
         cflags |= CF_USE_ICOUNT;
     }
+
     tb = tb_alloc(pc);
-    if (!tb) {
+    if (unlikely(!tb)) {
+ buffer_overflow:
         /* flush must be done */
         tb_flush(cpu);
         /* cannot fail at this point */
         tb = tb_alloc(pc);
+        assert(tb != NULL);
         /* Don't forget to invalidate previous TB info.  */
         tcg_ctx.tb_ctx.tb_invalidated_flag = 1;
     }
@@ -1109,8 +1119,19 @@ TranslationBlock *tb_gen_code(CPUState *cpu,
     tcg_ctx.code_time -= profile_getclock();
 #endif
 
+    /* ??? Overflow could be handled better here.  In particular, we
+       don't need to re-do gen_intermediate_code, nor should we re-do
+       the tcg optimization currently hidden inside tcg_gen_code.  All
+       that should be required is to flush the TBs, allocate a new TB,
+       re-initialize it per above, and re-do the actual code generation.  */
     gen_code_size = tcg_gen_code(&tcg_ctx, gen_code_buf);
+    if (unlikely(gen_code_size < 0)) {
+        goto buffer_overflow;
+    }
     search_size = encode_search(tb, (void *)gen_code_buf + gen_code_size);
+    if (unlikely(search_size < 0)) {
+        goto buffer_overflow;
+    }
 
 #ifdef CONFIG_PROFILER
     tcg_ctx.code_time += profile_getclock();
@@ -1681,7 +1702,7 @@ void dump_exec_info(FILE *f, fprintf_function cpu_fprintf)
     cpu_fprintf(f, "Translation buffer state:\n");
     cpu_fprintf(f, "gen code size       %td/%zd\n",
                 tcg_ctx.code_gen_ptr - tcg_ctx.code_gen_buffer,
-                tcg_ctx.code_gen_buffer_max_size);
+                tcg_ctx.code_gen_highwater - tcg_ctx.code_gen_buffer);
     cpu_fprintf(f, "TB count            %d/%d\n",
             tcg_ctx.tb_ctx.nb_tbs, tcg_ctx.code_gen_max_blocks);
     cpu_fprintf(f, "TB avg target size  %d max=%d bytes\n",