translate-all: use a binary search tree to track TBs in TBContext

This is a prerequisite for supporting multiple TCG contexts, since
we will have threads generating code in separate regions of
code_gen_buffer.

For this we need a new field (.size) in struct tb_tc to keep
track of the size of the translated code. This field uses a size_t
to avoid adding a hole to the struct, although really an unsigned
int would have been enough.

The comparison function we use is optimized for the common case:
insertions. Profiling shows that upon booting debian-arm, 98%
of comparisons are between existing tb's (i.e. a->size and b->size
are both !0), which happens during insertions (and removals, but
those are rare). The remaining cases are lookups. From reading the glib
sources we see that the first key is always the lookup key. However,
the code does not assume this to always be the case because this
behaviour is not guaranteed in the glib docs. However, we embed
this knowledge in the code as a branch hint for the compiler.

Note that tb_free does not free space in the code_gen_buffer anymore,
since we cannot easily know whether the tb is the last one inserted
in code_gen_buffer. The next patch in this series renames tb_free
to tb_remove to reflect this.

Performance-wise, lookups in tb_find_pc are the same as before:
O(log n). However, insertions are O(log n) instead of O(1), which
results in a small slowdown when booting debian-arm:

Performance counter stats for 'build/arm-softmmu/qemu-system-arm \
	-machine type=virt -nographic -smp 1 -m 4096 \
	-netdev user,id=unet,hostfwd=tcp::2222-:22 \
	-device virtio-net-device,netdev=unet \
	-drive file=img/arm/jessie-arm32.qcow2,id=myblock,index=0,if=none \
	-device virtio-blk-device,drive=myblock \
	-kernel img/arm/aarch32-current-linux-kernel-only.img \
	-append console=ttyAMA0 root=/dev/vda1 \
	-name arm,debug-threads=on -smp 1' (10 runs):

- Before:

       8048.598422      task-clock (msec)         #    0.931 CPUs utilized            ( +-  0.28% )
            16,974      context-switches          #    0.002 M/sec                    ( +-  0.12% )
                 0      cpu-migrations            #    0.000 K/sec
            10,125      page-faults               #    0.001 M/sec                    ( +-  1.23% )
    35,144,901,879      cycles                    #    4.367 GHz                      ( +-  0.14% )
   <not supported>      stalled-cycles-frontend
   <not supported>      stalled-cycles-backend
    65,758,252,643      instructions              #    1.87  insns per cycle          ( +-  0.33% )
    10,871,298,668      branches                  # 1350.707 M/sec                    ( +-  0.41% )
       192,322,212      branch-misses             #    1.77% of all branches          ( +-  0.32% )

       8.640869419 seconds time elapsed                                          ( +-  0.57% )

- After:
       8146.242027      task-clock (msec)         #    0.923 CPUs utilized            ( +-  1.23% )
            17,016      context-switches          #    0.002 M/sec                    ( +-  0.40% )
                 0      cpu-migrations            #    0.000 K/sec
            18,769      page-faults               #    0.002 M/sec                    ( +-  0.45% )
    35,660,956,120      cycles                    #    4.378 GHz                      ( +-  1.22% )
   <not supported>      stalled-cycles-frontend
   <not supported>      stalled-cycles-backend
    65,095,366,607      instructions              #    1.83  insns per cycle          ( +-  1.73% )
    10,803,480,261      branches                  # 1326.192 M/sec                    ( +-  1.95% )
       195,601,289      branch-misses             #    1.81% of all branches          ( +-  0.39% )

       8.828660235 seconds time elapsed                                          ( +-  0.38% )

Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Emilio G. Cota <cota@braap.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

1 files changed, 5 insertions, 1 deletions

diff --git a/include/exec/exec-all.h b/include/exec/exec-all.h
index f14c6a56eb..e2d598082e 100644
--- a/include/exec/exec-all.h
+++ b/include/exec/exec-all.h
@@ -306,10 +306,14 @@ static inline void tb_invalidate_phys_addr(AddressSpace *as, hwaddr addr)
 
 /*
  * Translation Cache-related fields of a TB.
+ * This struct exists just for convenience; we keep track of TB's in a binary
+ * search tree, and the only fields needed to compare TB's in the tree are
+ * @ptr and @size.
+ * Note: the address of search data can be obtained by adding @size to @ptr.
  */
 struct tb_tc {
     void *ptr;    /* pointer to the translated code */
-    uint8_t *search;  /* pointer to search data */
+    size_t size;
 };
 
 struct TranslationBlock {