編譯器會為某些場合進行專門優化，避免字串轉換時的額外分配和複製操作:
- 將[]byte轉換為string key，去map[string]查詢的時候。
- 將string轉換為[]byte，進行for range迭代時，直接取位元組賦值給區域性變數。
Example Code
package main
func main() {
m := map[string]int {
"abc" : 123,
}
key := []byte("abc")
x, ok := m[string(key)]
println(x, ok)
}➜ go build -gcflags "-N -l" example.go GDB go1.7.6 驗證成功
- 地址都是0xc420047ee0
➜ gdb example
GNU gdb (GDB) 8.0.1
Copyright (C) 2017 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law. Type "show copying"
and "show warranty" for details.
This GDB was configured as "x86_64-apple-darwin17.0.0".
Type "show configuration" for configuration details.
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>.
Find the GDB manual and other documentation resources online at:
<http://www.gnu.org/software/gdb/documentation/>.
For help, type "help".
Type "apropos word" to search for commands related to "word"...
Reading symbols from example...done.
(gdb) b 9
Breakpoint 1 at 0x2164: file /Users/anderson/Code/lang-experiments/golang/src/compiler-avoid-copy-when-transform-string/example.go, line 9.
(gdb) c
The program is not being run.
(gdb) r
Starting program: /Users/anderson/Code/lang-experiments/golang/src/compiler-avoid-copy-when-transform-string/example
[New Thread 0x2703 of process 90436]
warning: unhandled dyld version (15)
[New Thread 0x1907 of process 90436]
[New Thread 0x1a03 of process 90436]
[New Thread 0x2503 of process 90436]
[New Thread 0x2603 of process 90436]
Thread 2 hit Breakpoint 1, main.main ()
at /Users/anderson/Code/lang-experiments/golang/src/compiler-avoid-copy-when-transform-string/example.go:9
9 x, ok := m[string(key)]
(gdb) p key
$1 = {array = 0xc420047ee0 "abc", len = 3, cap = 32}
(gdb) b runtime.mapaccess2_faststr
Breakpoint 2 at 0x94b0: file /Users/anderson/.gvm/gos/go1.7.6/src/runtime/hashmap_fast.go, line 297.
(gdb) c
Continuing.
Thread 2 hit Breakpoint 2, runtime.mapaccess2_faststr (t=0x5a2c0 <type.*+50144>, h=0xc42007c000, ky=..., ~r3=0x3, ~r4=32)
at /Users/anderson/.gvm/gos/go1.7.6/src/runtime/hashmap_fast.go:297
297 func mapaccess2_faststr(t *maptype, h *hmap, ky string) (unsafe.Pointer, bool) {
(gdb) p ky
$2 = 0xc420047ee0 "abc"delve go1.7.6 驗證失敗
-
地址分別是：
- 0xc420047f20
- 0xc420047ea8
➜ dlv exec example
Type 'help' for list of commands.
(dlv) b example.go:9
Breakpoint 1 set at 0x2164 for main.main() ./example.go:9
(dlv) c
> main.main() ./example.go:9 (hits goroutine(1):1 total:1) (PC: 0x2164)
4: m := map[string]int {
5: "abc" : 123,
6: }
7:
8: key := []byte("abc")
=> 9: x, ok := m[string(key)]
10:
11: println(x, ok)
12: }
(dlv) p key
[]uint8 len: 3, cap: 32, [97,98,99]
(dlv) p &key
(*[]uint8)(0xc420047f20) # 地址是0xc420047f20
(dlv) b runtime.mapaccess2_faststr
Breakpoint 2 set at 0x94c3 for runtime.mapaccess2_faststr() /Users/anderson/.gvm/gos/go1.7.6/src/runtime/hashmap_fast.go:297
(dlv) c
> runtime.mapaccess2_faststr() /Users/anderson/.gvm/gos/go1.7.6/src/runtime/hashmap_fast.go:297 (hits goroutine(1):1 total:1) (PC: 0x94c3)
292: return unsafe.Pointer(&zeroVal[0])
293: }
294: }
295: }
296:
=> 297: func mapaccess2_faststr(t *maptype, h *hmap, ky string) (unsafe.Pointer, bool) {
298: if raceenabled && h != nil {
299: callerpc := getcallerpc(unsafe.Pointer(&t))
300: racereadpc(unsafe.Pointer(h), callerpc, funcPC(mapaccess2_faststr))
301: }
302: if h == nil || h.count == 0 {
(dlv) p &ky
(*string)(0xc420047ea8) # 地址是0xc420047ea8LLDB go1.7.6 驗證成功
- 地址都是： 0x000000c420041ee0
➜ lldb example
(lldb) target create "example"
Current executable set to 'example' (x86_64).
(lldb) b example.go:9
Breakpoint 1: where = example`main.main + 292 at example.go:9, address = 0x0000000000002164
(lldb) r
Process 91197 launched: '/Users/anderson/Code/lang-experiments/golang/src/compiler-avoid-copy-when-transform-string/example' (x86_64)
Process 91197 stopped
* thread #1, stop reason = breakpoint 1.1
frame #0: 0x0000000000002164 example`main.main at example.go:9
6 }
7
8 key := []byte("abc")
-> 9 x, ok := m[string(key)]
10
11 println(x, ok)
12 }
Target 0: (example) stopped.
(lldb) p key
([]uint8) key = (len 3, cap 32) {
[0] = 97
[1] = 98
[2] = 99
}
(lldb) p &key
(*[]uint8) = 0x000000c420041ee0 (len 0, cap 0)
(lldb) b runtime.mapaccess2_faststr
Breakpoint 2: where = example`runtime.mapaccess2_faststr + 33 at hashmap_fast.go:302, address = 0x00000000000094d1
(lldb) p key
([]uint8) key = (len 3, cap 32) {
[0] = 97
[1] = 98
[2] = 99
}
(lldb) c
Process 91197 resuming
Process 91197 stopped
* thread #1, stop reason = breakpoint 2.1
frame #0: 0x00000000000094d1 example`runtime.mapaccess2_faststr(t=0x000000000005a2c0, h=0x000000c420076000, ky="abc", ~r3=0x0000000000000003, ~r4=true) at hashmap_fast.go:302
299 callerpc := getcallerpc(unsafe.Pointer(&t))
300 racereadpc(unsafe.Pointer(h), callerpc, funcPC(mapaccess2_faststr))
301 }
-> 302 if h == nil || h.count == 0 {
303 return unsafe.Pointer(&zeroVal[0]), false
304 }
305 if h.flags&hashWriting != 0 {
Target 0: (example) stopped.
(lldb) p ky
(string) ky = "abc"
(lldb) p &ky
(*string) = 0x000000c420041ee0 ""GDB go1.10 無法檢視陣列地址，無法驗證
➜ gdb example_1_10
GNU gdb (GDB) 8.0.1
Copyright (C) 2017 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law. Type "show copying"
and "show warranty" for details.
This GDB was configured as "x86_64-apple-darwin17.0.0".
Type "show configuration" for configuration details.
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>.
Find the GDB manual and other documentation resources online at:
<http://www.gnu.org/software/gdb/documentation/>.
For help, type "help".
Type "apropos word" to search for commands related to "word"...
Reading symbols from example_1_10...done.
Loading Go Runtime support.
(gdb) b 9
Breakpoint 1 at 0x104d4b4: file /Users/anderson/Code/lang-experiments/golang/src/compiler-avoid-copy-when-transform-string/example.go, line 9.
(gdb) r
Starting program: /Users/anderson/Code/lang-experiments/golang/src/compiler-avoid-copy-when-transform-string/example_1_10
[New Thread 0x1903 of process 92053]
warning: unhandled dyld version (15)
[New Thread 0x1807 of process 92053]
[New Thread 0x1a03 of process 92053]
[New Thread 0x2503 of process 92053]
[New Thread 0x2603 of process 92053]
Thread 2 hit Breakpoint 1, main.main () at /Users/anderson/Code/lang-experiments/golang/src/compiler-avoid-copy-when-transform-string/example.go:9
9 x, ok := m[string(key)]
(gdb) p key
$1 = []uint8 = {97 'a', 98 'b', 99 'c'}
(gdb) p &key
$2 = []uint8 * = {97 'a', 98 'b', 99 'c'}
(gdb) b runtime.mapaccess2_faststr
Breakpoint 2 at 0x1007ee0: file /usr/local/Cellar/go/1.10/libexec/src/runtime/hashmap_fast.go, line 261.
(gdb) c
Continuing.
Thread 2 hit Breakpoint 2, runtime.mapaccess2_faststr (h=0xc420057e78, ky="abc", t=0x105ad60 <type.*+55168>, ~r3=0x3, ~r4=32)
at /usr/local/Cellar/go/1.10/libexec/src/runtime/hashmap_fast.go:261
261 func mapaccess2_faststr(t *maptype, h *hmap, ky string) (unsafe.Pointer, bool) {
(gdb) p ky
$3 = "abc"
(gdb) p &ky
$4 = "abc"delve go1.10 驗證失敗
-
地址分別是：
- 0xc420057e60
- 0xc420057dd0
➜ dlv exec example_1_10
Type 'help' for list of commands.
(dlv) b example.go:9
Breakpoint 1 set at 0x104d4b4 for main.main() ./example.go:9
(dlv) c
> main.main() ./example.go:9 (hits goroutine(1):1 total:1) (PC: 0x104d4b4)
4: m := map[string]int {
5: "abc" : 123,
6: }
7:
8: key := []byte("abc")
=> 9: x, ok := m[string(key)]
10:
11: println(x, ok)
12: }
(dlv) p key
[]uint8 len: 3, cap: 32, [97,98,99]
(dlv) p &key
(*[]uint8)(0xc420057e60)
(dlv) b runtime.mapaccess2_faststr
Breakpoint 2 set at 0x1007ef3 for runtime.mapaccess2_faststr() /usr/local/Cellar/go/1.10/libexec/src/runtime/hashmap_fast.go:261
(dlv) c
> runtime.mapaccess2_faststr() /usr/local/Cellar/go/1.10/libexec/src/runtime/hashmap_fast.go:261 (hits goroutine(1):1 total:1) (PC: 0x1007ef3)
Warning: debugging optimized function
256: }
257: }
258: return unsafe.Pointer(&zeroVal[0])
259: }
260:
=> 261: func mapaccess2_faststr(t *maptype, h *hmap, ky string) (unsafe.Pointer, bool) {
262: if raceenabled && h != nil {
263: callerpc := getcallerpc()
264: racereadpc(unsafe.Pointer(h), callerpc, funcPC(mapaccess2_faststr))
265: }
266: if h == nil || h.count == 0 {
(dlv) p ky
"abc"
(dlv) p &ky
(*string)(0xc420057dd0)LLDB go1.10 驗證成功
- 地址都是：0x000000c420057e10
➜ lldb example_1_10
(lldb) target create "example_1_10"
Current executable set to 'example_1_10' (x86_64).
(lldb) b example.go:9
Breakpoint 1: where = example_1_10`main.main + 372 at example.go:9, address = 0x000000000104d4b4
(lldb) r
Process 91907 launched: '/Users/anderson/Code/lang-experiments/golang/src/compiler-avoid-copy-when-transform-string/example_1_10' (x86_64)
Process 91907 stopped
* thread #1, stop reason = breakpoint 1.1
frame #0: 0x000000000104d4b4 example_1_10`main.main at example.go:9
6 }
7
8 key := []byte("abc")
-> 9 x, ok := m[string(key)]
10
11 println(x, ok)
12 }
Target 0: (example_1_10) stopped.
(lldb) p key
([]uint8) key = (len 3, cap 32) {
[0] = 97
[1] = 98
[2] = 99
}
(lldb) p &key
(*[]uint8) = 0x000000c420057e10 (len 0, cap 0)
(lldb) b runtime.mapaccess2_faststr
Breakpoint 2: where = example_1_10`runtime.mapaccess2_faststr + 38 at hashmap_fast.go:266, address = 0x0000000001007f06
(lldb) c
Process 91907 resuming
Process 91907 stopped
* thread #1, stop reason = breakpoint 2.1
frame #0: 0x0000000001007f06 example_1_10`runtime.mapaccess2_faststr(h=0x000000c420057e78, ky="abc", t=0x000000000105ad60, ~r3=0x0000000000000003, ~r4=true) at hashmap_fast.go:266
263 callerpc := getcallerpc()
264 racereadpc(unsafe.Pointer(h), callerpc, funcPC(mapaccess2_faststr))
265 }
-> 266 if h == nil || h.count == 0 {
267 return unsafe.Pointer(&zeroVal[0]), false
268 }
269 if h.flags&hashWriting != 0 {
Target 0: (example_1_10) stopped.
(lldb) p ky
(string) ky = "abc"
(lldb) p &ky
(*string) = 0x000000c420057e10 ""
(lldb) 結論
- 不知道為什麼，切換版本前後delve都驗證失敗了
- 切換版本前後，LLDB都驗證成功了
- GDB無法檢視GO1.10的陣列地址