cmd/link: no dwarf when using -linkmode=external on Windows

[lukehoban]

Similar to #8973, it appears that dwarf data is missing when using default -linkmode=external on Windows.

c:\dd\go\src\github.com\lukehoban\cgotest>go version
go version go1.5.1 windows/amd64

c:\dd\go\src\github.com\lukehoban\cgotest>where gcc
C:\Program Files\mingw-w64\x86_64-5.2.0-posix-seh-rt_v4-rev0\mingw64\bin\gcc.exe

c:\dd\go\src\github.com\lukehoban\cgotest>cat cgotest.go
package main

/*
#include <stdio.h>
char* foo(void) { return "hello, world!"; }
*/
import "C"

import "fmt"
import "runtime"

func main() {
        runtime.GOMAXPROCS(runtime.NumCPU())
        fmt.Println(C.GoString(C.foo()))
}



c:\dd\go\src\github.com\lukehoban\cgotest>go build -ldflags="-v -linkmode external"
# github.com/lukehoban/cgotest
HEADER = -H11 -T0x401000 -D0x0 -R0x1000
searching for runtime.a in $WORK/runtime.a
searching for runtime.a in C:\Go/pkg/windows_amd64/runtime.a
 0.00 deadcode
 0.02 pclntab=346795 bytes, funcdata total 70844 bytes
 0.02 dodata
 0.02 reloc
 0.04 reloc
 0.05 asmb
 0.05 codeblk
 0.07 datblk
 0.07 sym
 0.07 dwarf
 0.07 headr
 0.07 symsize = 0
 0.08 symsize = 0
host link: "gcc" "-m64" "-gdwarf-2" "-mconsole" "-o" "C:\\Users\\lukeh\\AppData\\Local\\Temp\\go-build960901386\\github.com\\lukehoban\\cgotest\\_obj\\exe\\a.out.exe" "C:\\Users\\lukeh\\AppData\\Local\\Temp\\go-link-442716235/000000.o" "C:\\Users\\lukeh\\AppData\\Local\\Temp\\go-link-442716235/000001.o" "C:\\Users\\lukeh\\AppData\\Local\\Temp\\go-link-442716235/go.o" "-g" "-O2" "-g" "-O2" "-lm" "-mthreads" "-ladvapi32" "-lntdll" "-lws2_32" "-lkernel32" "-lwinmm"
 0.27 cpu time
45702 symbols
30924 liveness data

c:\dd\go\src\github.com\lukehoban\cgotest>objdump  --dwarf cgotest.exe | wc -l
3223

c:\dd\go\src\github.com\lukehoban\cgotest>objdump  --dwarf cgotest.exe | grep "runtime.buildVersion"



c:\dd\go\src\github.com\lukehoban\cgotest>go build -ldflags="-v -linkmode internal"
# github.com/lukehoban/cgotest
HEADER = -H11 -T0x401000 -D0x0 -R0x1000
searching for runtime.a in $WORK/runtime.a
searching for runtime.a in C:\Go/pkg/windows_amd64/runtime.a
 0.00 ldpe $WORK\github.com\lukehoban\cgotest.a(_all.o)
 0.00 ldpe C:\Go/pkg/windows_amd64/runtime/cgo.a(_all.o)
 0.00 deadcode
 0.02 pclntab=347739 bytes, funcdata total 70844 bytes
 0.02 dodata
 0.02 reloc
 0.04 reloc
 0.05 asmb
 0.05 codeblk
 0.08 datblk
 0.08 sym
 0.08 dwarf
 0.09 symsize = 0
 0.14 dwarf pass 2.
 0.16 headr
 0.16 symsize = 0
 0.16 symsize = 0
 0.17 cpu time
45695 symbols
30924 liveness data

c:\dd\go\src\github.com\lukehoban\cgotest>objdump  --dwarf cgotest.exe | wc -l
145046

c:\dd\go\src\github.com\lukehoban\cgotest>objdump  --dwarf cgotest.exe | grep "runtime.buildVersion"
    <4e0e0>   DW_AT_name        : runtime.buildVersion
    4e0df       runtime.buildVersion

[alexbrainman]

Sure. It is dup of #10776.

Alex