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tcg: Check for overflow via highwater mark

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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>
This commit is contained in:
Richard Henderson 2015-09-22 13:01:15 -07:00
parent f293709c6a
commit b125f9dc7b
4 changed files with 40 additions and 16 deletions
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31
translate-all.c
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@ -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",