cmd/link: the last file name is missing in gopclntab

[cherrymui]

What version of Go are you using (go version)?

tip

What did you do?

$ go build hello.go 
$ go tool objdump hello

Look at the end of the output, the last two functions has file name missing:

[... ...]

TEXT runtime.kevent(SB) /Users/cherryyz/src/go-tip/src/runtime/sys_darwin_amd64.s
        sys_darwin_amd64.s:491  0x4b150 8b7c2408        MOVL 0x8(SP), DI        
        sys_darwin_amd64.s:492  0x4b154 488b742410      MOVQ 0x10(SP), SI       
        sys_darwin_amd64.s:493  0x4b159 8b542418        MOVL 0x18(SP), DX       
        sys_darwin_amd64.s:494  0x4b15d 4c8b542420      MOVQ 0x20(SP), R10      
        sys_darwin_amd64.s:495  0x4b162 448b442428      MOVL 0x28(SP), R8       
        sys_darwin_amd64.s:496  0x4b167 4c8b4c2430      MOVQ 0x30(SP), R9       
        sys_darwin_amd64.s:497  0x4b16c b86b010002      MOVL $0x200016b, AX     
        sys_darwin_amd64.s:498  0x4b171 0f05            SYSCALL                 
        sys_darwin_amd64.s:499  0x4b173 7303            JAE 0x4b178             
        sys_darwin_amd64.s:500  0x4b175 48f7d8          NEGQ AX                 
        sys_darwin_amd64.s:501  0x4b178 89442438        MOVL AX, 0x38(SP)       
        sys_darwin_amd64.s:502  0x4b17c c3              RET                     
        :-1                     0x4b17d cc              INT $0x3                
        :-1                     0x4b17e cc              INT $0x3                
        :-1                     0x4b17f cc              INT $0x3                

TEXT runtime/internal/atomic.Xaddint64(SB) 
        :59     0x4b180 e90b000000      JMP runtime/internal/atomic.Xadd64(SB)  
        :-1     0x4b185 cc              INT $0x3                                
        :-1     0x4b186 cc              INT $0x3                                
        :-1     0x4b187 cc              INT $0x3                                
        :-1     0x4b188 cc              INT $0x3                                
        :-1     0x4b189 cc              INT $0x3                                
        :-1     0x4b18a cc              INT $0x3                                
        :-1     0x4b18b cc              INT $0x3                                
        :-1     0x4b18c cc              INT $0x3                                
        :-1     0x4b18d cc              INT $0x3                                
        :-1     0x4b18e cc              INT $0x3                                
        :-1     0x4b18f cc              INT $0x3                                

TEXT runtime/internal/atomic.Xadd64(SB) 
        :92     0x4b190 488b5c2408      MOVQ 0x8(SP), BX        
        :93     0x4b195 488b442410      MOVQ 0x10(SP), AX       
        :94     0x4b19a 4889c1          MOVQ AX, CX             
        :95     0x4b19d f0480fc103      LOCK XADDQ AX, 0(BX)    
        :97     0x4b1a2 4801c8          ADDQ CX, AX             
        :98     0x4b1a5 4889442418      MOVQ AX, 0x18(SP)       
        :99     0x4b1aa c3              RET 

According to https://golang.org/s/go12symtab,

The pcfile pc-value table produces indexes into a table of source file names. That table has the format:
N+1 name1 name2 name3 … nameN

Parsing the binary manually, it has N at the place where it should be N+1. objdump does what the doc says, and so ignores the last entry.

This problem has existed since at least Go 1.6 (did not check even earlier versions.)

(runtime/internal/atomic.Xaddint64 should be intrinsified, which is another issue. I have sent a CL.)

[gopherbot]

CL https://golang.org/cl/29275 mentions this issue.