cmd/compile: assembly listing (-S flag) reports "<unknown line number>"

[ALTree]

Code:

package foo

func Mod32(x uint32) uint32 {
    return x % 3
}

func Mod64(x uint64) uint64 {
    return x % 3
}

build -gcflags -S, on tip, outputs:

"".Mod32 t=1 size=27 args=0x10 locals=0x0
    [cut]
    0x0000 00000 (/home/alberto/Desktop/rand.go:3)  FUNCDATA    $1, gclocals·33cdeccccebe80329f1fdbee7f5874cb(SB)
    0x0000 00000 (/home/alberto/Desktop/rand.go:4)  MOVL    "".x+8(FP), AX
    0x0004 00004 (<unknown line number>)    MOVL    $-1431655765, CX
    0x0009 00009 (/home/alberto/Desktop/rand.go:4)  MULL    CX
    0x000b 00011 (/home/alberto/Desktop/rand.go:4)  SHRL    $1, DX
    [cut]

"".Mod64 t=1 size=34 args=0x10 locals=0x0
    [cut]
    0x0000 00000 (/home/alberto/Desktop/rand.go:7)  FUNCDATA    $0, gclocals·23e8278e2b69a3a75fa59b23c49ed6ad(SB)
    0x0000 00000 (/home/alberto/Desktop/rand.go:7)  FUNCDATA    $1, gclocals·33cdeccccebe80329f1fdbee7f5874cb(SB)
    0x0000 00000 (/home/alberto/Desktop/rand.go:8)  MOVQ    $-6148914691236517205, AX
    0x000a 00010 (/home/alberto/Desktop/rand.go:8)  MOVQ    "".x+8(FP), CX
    0x000f 00015 (/home/alberto/Desktop/rand.go:8)  MULQ    CX
    [cut]

note the <unknown line number> in the Mod32 disassembly. Note that Mod64 is fine.

This is an SSA regression: on go1.6.2, and on tip with -ssa=0, every instruction gets its own line number.

[ALTree]

gotip build -a -gcflags -S std

and then grepping for "<unknown line number>" returns 1927 matches on tip, and 0 matches on tip with SSA disabled.

[bradfitz]

/cc @randall77 @cherrymui @josharian @ianlancetaylor

[bradfitz]

Go1.7Maybe if the fix is easy and non-invasive?

[cherrymui]

The large constant in Mod32 is generated by the frontend, the node of which has line number 0. The constant in Mod64 is generated by SSA optimization, where the line number propagates through. In the legacy backend, if we set the line number to 0, it does not change lineno (cmd/compile/internal/gc/subr.go:/setlineno). Maybe we can do the same in pushLine when building SSA?

[cherrymui]

Mailed http://golang.org/cl/24641. Not really sure about it though.

[gopherbot]

CL https://golang.org/cl/24641 mentions this issue.