[Qemu-commits] [qemu/qemu] 00c8fa: tcg: optimise memory layout of TCGTemp

[GitHub]

  Branch: refs/heads/master
  Home:   https://github.com/qemu/qemu
  Commit: 00c8fa9ffeee7458e5ed62c962faf638156c18da
      
https://github.com/qemu/qemu/commit/00c8fa9ffeee7458e5ed62c962faf638156c18da
  Author: Emilio G. Cota <address@hidden>
  Date:   2015-05-05 (Tue, 05 May 2015)

  Changed paths:
    M tcg/tcg.h

  Log Message:
  -----------
  tcg: optimise memory layout of TCGTemp

This brings down the size of the struct from 56 to 32 bytes on 64-bit,
and to 20 bytes on 32-bit. This leads to memory savings:

Before:
$ find . -name 'tcg.o' | xargs size
   text    data     bss     dec     hex filename
  41131   29800      88   71019   1156b ./aarch64-softmmu/tcg/tcg.o
  37969   29416      96   67481   10799 ./x86_64-linux-user/tcg/tcg.o
  39354   28816      96   68266   10aaa ./arm-linux-user/tcg/tcg.o
  40802   29096      88   69986   11162 ./arm-softmmu/tcg/tcg.o
  39417   29672      88   69177   10e39 ./x86_64-softmmu/tcg/tcg.o

After:
$ find . -name 'tcg.o' | xargs size
   text    data     bss     dec     hex filename
  40883   29800      88   70771   11473 ./aarch64-softmmu/tcg/tcg.o
  37473   29416      96   66985   105a9 ./x86_64-linux-user/tcg/tcg.o
  38858   28816      96   67770   108ba ./arm-linux-user/tcg/tcg.o
  40554   29096      88   69738   1106a ./arm-softmmu/tcg/tcg.o
  39169   29672      88   68929   10d41 ./x86_64-softmmu/tcg/tcg.o

Note that using an entire byte for some enums that need less than
that wastes a few bits (noticeable in 32 bits, where we use
20 bytes instead of 16) but avoids extraction code, which overall
is a win--I've tested several variations of the patch, and the appended
is the best performer for OpenSSL's bntest by a very small margin:

Before:
$ taskset -c 0 perf stat -r 15 -- x86_64-linux-user/qemu-x86_64 
img/bntest-x86_64 >/dev/null
[...]
 Performance counter stats for 'x86_64-linux-user/qemu-x86_64 
img/bntest-x86_64' (15 runs):

      10538.479833 task-clock (msec)  # 0.999 CPUs utilized  ( +-  0.38% )
         772 context-switches   # 0.073 K/sec          ( +-  2.03% )
           0 cpu-migrations     # 0.000 K/sec          ( +-100.00% )
       2,207 page-faults        # 0.209 K/sec          ( +-  0.08% )
      10.552871687 seconds time elapsed                      ( +-  0.39% )

After:
$ taskset -c 0 perf stat -r 15 -- x86_64-linux-user/qemu-x86_64 
img/bntest-x86_64 >/dev/null
 Performance counter stats for 'x86_64-linux-user/qemu-x86_64 
img/bntest-x86_64' (15 runs):

      10459.968847 task-clock (msec)  # 0.999 CPUs utilized  ( +-  0.30% )
         739 context-switches   # 0.071 K/sec          ( +-  1.71% )
           0 cpu-migrations     # 0.000 K/sec          ( +- 68.14% )
       2,204 page-faults        # 0.211 K/sec          ( +-  0.10% )
      10.473900411 seconds time elapsed                      ( +-  0.30% )

Suggested-by: Stefan Weil <address@hidden>
Suggested-by: Richard Henderson <address@hidden>
Reviewed-by: Alex Bennée <address@hidden>
Signed-off-by: Emilio G. Cota <address@hidden>
Signed-off-by: Richard Henderson <address@hidden>


  Commit: 38003aee196a96edccd4d64471beb1b67e9b2b17
      
https://github.com/qemu/qemu/commit/38003aee196a96edccd4d64471beb1b67e9b2b17
  Author: Peter Maydell <address@hidden>
  Date:   2015-05-06 (Wed, 06 May 2015)

  Changed paths:
    M tcg/tcg.h

  Log Message:
  -----------
  Merge remote-tracking branch 'remotes/rth/tags/tcg-next-20150505' into staging

size reduction merge

# gpg: Signature made Wed May  6 00:21:43 2015 BST using RSA key ID 4DD0279B
# gpg: Good signature from "Richard Henderson <address@hidden>"
# gpg:                 aka "Richard Henderson <address@hidden>"
# gpg:                 aka "Richard Henderson <address@hidden>"

* remotes/rth/tags/tcg-next-20150505:
  tcg: optimise memory layout of TCGTemp

Signed-off-by: Peter Maydell <address@hidden>


Compare: https://github.com/qemu/qemu/compare/233353ec93e4...38003aee196a