cmd/compile: optimization of constant pool on arm

[benshi001]

For the following code

func ass12345678(a int) int {
return a + 0xffff
}

Currently go will store 0xffff to the constant pool and load it in runtime.
a.go:6 0x95a04 e59fb06c MOVW 0x6c(R15), R11
a.go:6 0x95a08 e080000b ADD R11, R0, R0
.................................
a.go:9 0x95a78 0000ffff STRD.EQ [R0], -PC, R15, R15

But gcc optimized it to
a = a + 0x10000
a = a - 1

Both 1 and 0x10000 can be directly encoded to $immediate-12 into the instructions without any access to memory.

[benshi001]

Also，

a | 0xffff ->
orr $0xff, a, a
orr $0xff00, a, a

a & 0xffff0 ->
bic $0xf000000f, a, a
bic $0x0ff00000, a, a

[gopherbot]

CL https://golang.org/cl/39552 mentions this issue.

[benshi001]

test.zip

[benshi001]

The above attachment is a rough test, the output log on my raspberry pi 2 is
test0(constant pool) cost 29 seconds, test1(imm12) cost 20 seconds

it means,
total 17179869120 pairs of (ldr/add) cost 29 seconds, while 17179869120 pairs(add/add) cost 20 seconds, about 50% improvement.

However, the constant pool is in the data cache. If not, the cache miss will cause much more inefficiency.

[bradfitz]

@josharian, can you point @benshi001 to directions on how to run compiler benchmark tests?

@benshi001, you want to make pretty commit messages using https://godoc.org/golang.org/x/perf/cmd/benchstat like this one 50688fc

[josharian]

The time has come for me to clean up compiler benchmarking a bit. Let me do that first.

[josharian]

Not done, but cleaned up enough for the moment. Do:

go get -u golang.org/x/tools/cmd/compilebench github.com/josharian/compilecmp golang.org/x/perf/cmd/benchstat

Make sure all resulting binaries are in your $PATH.

Commit your work. For memory benchmarking:

compilecmp -n 10

For exection time benchmarking (with everything else closed):

compilecmp -n 50 -cpu

These will compare master to HEAD. compilecmp supports lots of other variations, run with -h for more. Ask if you have questions or feature requests.

[benshi001]

I have troubles with my benchmark test that @josharian suggested. The reason is due to the network security policy.

I ssh connected to a remote host locates in USA, and did

git clone https://go.googlesource.com/go
git fetch https://go.googlesource.com/go refs/changes/52/39552/2 && git checkout FETCH_HEAD
GOPATH=/root/gopath go get -u golang.org/x/tools/cmd/compilebench
GOPATH=/root/gopath go get -u github.com/josharian/compilecmp
GOPATH=/root/gopath go get -u golang.org/x/perf/cmd/benchstat
PATH=/root/gopath/bin:$PATH
compilecmp -n 10

Then I got the expected result on the remote host via ssh terminal.

I copy the /root/go and the /root/gopath from the remote host to my local raspberry pi 2.
/root/go -> /home/pi/go
/root/gopath -> /home/pi/gopath

And did
GOPATH=/home/pi/gopath go build golang.org/x/tools/cmd/compilebench
GOPATH=/home/pi/gopath go build github.com/josharian/compilecmp
GOPATH=/home/pi/gopath go build golang.org/x/perf/cmd/benchstat

then the tools are built into ARM ELF.

But when I did
compilecmp -n 10, I only got part of the results,
compilecmp master HEAD
06:48:59 copy tree at master ( 19bd145 ) to /home/pi/.compilecmp/19bd145d0721a28658b15deb548f22a3405d83bd
06:49:03 /home/pi/.compilecmp/19bd145d0721a28658b15deb548f22a3405d83bd/src/make.bash
07:00:39 copy tree at HEAD ( 0f679e481a7cbcb0fbb930670581fa57cd027eee ) to /home/pi/.compilecmp/0f679e481a7cbcb0fbb930670581fa57cd027eee
07:00:57 /home/pi/.compilecmp/0f679e481a7cbcb0fbb930670581fa57cd027eee/src/make.bash
before: /home/pi/.compilecmp/19bd145d0721a28658b15deb548f22a3405d83bd
after: /home/pi/.compilecmp/0f679e481a7cbcb0fbb930670581fa57cd027eee
benchstat /tmp/590206338 /tmp/787780345
completed 10 of 10, estimated time remaining 0s (eta 7:13AM)
name old text-bytes new text-bytes delta
HelloSize 595k ± 0% 594k ± 0% -0.07% (p=0.000 n=10+10)

name old data-bytes new data-bytes delta
HelloSize 3.59k ± 0% 3.59k ± 0% ~ (all equal)

name old bss-bytes new bss-bytes delta
HelloSize 75.4k ± 0% 75.4k ± 0% ~ (all equal)

name old exe-bytes new exe-bytes delta
HelloSize 1.03M ± 0% 1.03M ± 0% ~ (all equal)

The others like
name old user-ns/op new user-ns/op delta
Template 488M ±11% 488M ± 6% ~ (p=0.920 n=10+9)
Unicode 249M ±10% 242M ±12% ~ (p=0.304 n=10+10)
GoTypes 1.45G ± 5% 1.48G ± 6% ~ (p=0.342 n=10+10)
Flate 216M ±15% 223M ±10% ~ (p=0.201 n=9+10)
GoParser 380M ± 9% 381M ± 8% ~ (p=0.762 n=10+9)
Reflect 603M ± 5% 599M ± 7% ~ (p=0.919 n=9+10)
Tar 258M ±16% 263M ± 6% ~ (p=0.752 n=10+10)
XML 502M ± 8% 522M ± 7% ~ (p=0.106 n=10+9)

are missing. what are wrong with my operations?