使用GDB等驗證Golang的「編譯器會爲某些場合進行專門優化，避免字符串轉換時的額外分配和複製操做」

編譯器會爲某些場合進行專門優化，避免字符串轉換時的額外分配和複製操做:

• 將[]byte轉換爲string key，去map[string]查詢的時候。
• 將string轉換爲[]byte，進行for range迭代時，直接取字節賦值給局部變量。

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0xc420047ea8

LLDB 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 沒法查看數組地址，沒法驗證

➜  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

• 不知道爲何，切換版本先後delve都驗證失敗了
• 切換版本先後，LLDB都驗證成功了
• GDB沒法查看GO1.10的數組地址