https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86952
Martin Liška <marxin at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|REOPENED |ASSIGNED
--- Comment #15 from Martin Liška <marxin at gcc dot gnu.org> ---
(In reply to Daniel Borkmann from comment #12)
> I've been looking into this issue quite recently and improved the benchmark
> tool a bit along the way. There need to be multiple considerations wrt to
> traversing the switch cases, the case is here is doing round robin, but
> additional distributions / tests could be added. Pushed here just in case:
> https://github.com/borkmann/microbenchmark
Thanks a lot for the benchmark.
>
> Numbers I'm getting are stable:
>
> * Xeon E3-1240, packet.net c1.small.x86 instance:
>
> # make prep
> [...]
> # make
> gcc -g -I. -O2 -c -o test.o test.c
> gcc -g -I. -O2 -mindirect-branch=thunk --param=case-values-threshold=20
> -c -o switch-no-table.o switch-no-table.c
> gcc -g -I. -O2 -mindirect-branch=thunk -c -o switch.o switch.c
> gcc -g -I. -O2 -c -o switch-no-retpol.o switch-no-retpol.c
> gcc -o test test.o switch-no-table.o switch.o switch-no-retpol.o
> taskset 1 ./test
> no retpoline : 6098325270
> no jump table: 6298192058 (no retpoline: 103.28%)
> jump table : 22081802856 (no retpoline: 362.10%, no jump table:
> 350.61%)
> # make
> taskset 1 ./test
> no retpoline : 6098439816
> no jump table: 6298242270 (no retpoline: 103.28%)
> jump table : 22107872854 (no retpoline: 362.52%, no jump table:
> 351.02%)
> # make
> taskset 1 ./test
> no retpoline : 6098187038
> no jump table: 6298308128 (no retpoline: 103.28%)
> jump table : 22071053524 (no retpoline: 361.93%, no jump table:
> 350.43%)
>
> * Xeon Gold 5120, packet.net m2.xlarge.x86 instance:
>
> # make prep
> [...]
> # make
> gcc -g -I. -O2 -c -o test.o test.c
> gcc -g -I. -O2 -mindirect-branch=thunk --param=case-values-threshold=20
> -c -o switch-no-table.o switch-no-table.c
> gcc -g -I. -O2 -mindirect-branch=thunk -c -o switch.o switch.c
> gcc -g -I. -O2 -c -o switch-no-retpol.o switch-no-retpol.c
> gcc -o test test.o switch-no-table.o switch.o switch-no-retpol.o
> taskset 1 ./test
> no retpoline : 5450356814
> no jump table: 5620673036 (no retpoline: 103.12%)
> jump table : 21448285314 (no retpoline: 393.52%, no jump table:
> 381.60%)
> # make
> taskset 1 ./test
> no retpoline : 5450356100
> no jump table: 5620678302 (no retpoline: 103.12%)
> jump table : 21448119720 (no retpoline: 393.52%, no jump table:
> 381.59%)
> # make
> taskset 1 ./test
> no retpoline : 5450331258
> no jump table: 5620839740 (no retpoline: 103.13%)
> jump table : 21446922902 (no retpoline: 393.50%, no jump table:
> 381.56%)
I can confirm the numbers. I've got:
model name : Intel(R) Core(TM) i7-4770 CPU @ 3.40GHz
taskset 1 ./test
no retpoline : 4311969467
no jump table: 5146081372 (no retpoline: 119.34%)
jump table : 18845846887 (no retpoline: 437.06%, no jump table: 366.22%)
>
> I've also looked into clang for their -mretpoline flag, and they generally
> turn off jump table generation in this case. For gcc, the s390 folks
> implemented a target override for the default case-values-threshold to raise
> it to 20.
Note that GCC has similar parameter:
--param case-values-threshold
The smallest number of different values for which it is best to
use a jump-table instead of a tree of conditional branches. If the value is 0,
use the default for the machine. The default is 0.
For 20 branches, I've got even worse numbers:
https://github.com/marxin/microbenchmark-1/tree/retpoline-table
taskset 1 ./test
no retpoline : 5096377521
no jump table: 5169400990 (no retpoline: 101.43%)
jump table : 28830137876 (no retpoline: 565.70%, no jump table: 557.71%)
So are you suggesting to disable jump tables with retpolines at all?
For x86 something similar could be done. Anyway, H.J. Lu asked me
> to reopen this issue (but seems like I cannot make this change from my
> account).
Yep, I would need an account ending with @gcc.org to change a bug.
