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authorRichard Henderson <rth@twiddle.net>2015-09-22 13:01:15 -0700
committerRichard Henderson <rth@twiddle.net>2015-10-07 20:36:53 +1100
commitb125f9dc7bd68cd4c57189db4da83b0620b28a72 (patch)
tree8d2b7a06e64121738343a3abebb5070b64f4ff21 /tcg
parentf293709c6af7a65a9bcec09cdba7a60183657a3e (diff)
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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>
Diffstat (limited to 'tcg')
-rw-r--r--tcg/tcg.c14
-rw-r--r--tcg/tcg.h5
2 files changed, 14 insertions, 5 deletions
diff --git a/tcg/tcg.c b/tcg/tcg.c
index 5609108..682af8a 100644
--- a/tcg/tcg.c
+++ b/tcg/tcg.c
@@ -385,9 +385,10 @@ void tcg_prologue_init(TCGContext *s)
total_size = s->code_gen_buffer_size - prologue_size;
s->code_gen_buffer_size = total_size;
- /* Compute a high-water mark, at which we voluntarily flush the
- buffer and start over. */
- s->code_gen_buffer_max_size = total_size - TCG_MAX_OP_SIZE * OPC_BUF_SIZE;
+ /* Compute a high-water mark, at which we voluntarily flush the buffer
+ and start over. The size here is arbitrary, significantly larger
+ than we expect the code generation for any one opcode to require. */
+ s->code_gen_highwater = s->code_gen_buffer + (total_size - 1024);
tcg_register_jit(s->code_gen_buffer, total_size);
@@ -2438,6 +2439,13 @@ int tcg_gen_code(TCGContext *s, tcg_insn_unit *gen_code_buf)
#ifndef NDEBUG
check_regs(s);
#endif
+ /* Test for (pending) buffer overflow. The assumption is that any
+ one operation beginning below the high water mark cannot overrun
+ the buffer completely. Thus we can test for overflow after
+ generating code without having to check during generation. */
+ if (unlikely(s->code_gen_ptr > s->code_gen_highwater)) {
+ return -1;
+ }
}
tcg_debug_assert(num_insns >= 0);
s->gen_insn_end_off[num_insns] = tcg_current_code_size(s);
diff --git a/tcg/tcg.h b/tcg/tcg.h
index 5fbbd15..a696922 100644
--- a/tcg/tcg.h
+++ b/tcg/tcg.h
@@ -559,10 +559,11 @@ struct TCGContext {
void *code_gen_prologue;
void *code_gen_buffer;
size_t code_gen_buffer_size;
- /* threshold to flush the translated code buffer */
- size_t code_gen_buffer_max_size;
void *code_gen_ptr;
+ /* Threshold to flush the translated code buffer. */
+ void *code_gen_highwater;
+
TBContext tb_ctx;
/* The TCGBackendData structure is private to tcg-target.c. */