[Bug go/83992] New: wrong debug info: wrong NOTE_INSN_BLOCK_BEG placement causes

[ubizjak at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83992

            Bug ID: 83992
           Summary: wrong debug info: wrong NOTE_INSN_BLOCK_BEG placement
                    causes
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: go
          Assignee: ian at airs dot com
          Reporter: ubizjak at gmail dot com
                CC: cmang at google dot com
  Target Milestone: ---

The following problem causes TestCrashDumpsAllThreads gotools testcase failure,
as explained in [1]. The core of the problem is in wrong placement of $LBB
label in the for loop [2].

Following testcase illustrates the problem:

--cut here--
package main

func loop(i int, c chan bool) {
        close(c)
        for {
                for j := 0; j < 0x7fffffff; j++ {
                }
        }
}
--cut here--

the testcase can be compiled with a crosscompiler to alpha-linux-gnu:

~/gcc-build-alpha/gcc/go1 -O2 -fkeep-static-functions l.go

to generate assembly, where $LBB2 is placed after "lda" insn.

main.loop:
...
$L2:
        lda $1,-1($1)
$LBB2:
$LM6:
        bne $1,$L2
        br $31,$L3
$LBE2:
...

In ._final rtl dump, we can see that (note 52) is in wrong place. It should
be placed in front of (insn 13).

(code_label 14 6 12 2 (nil) [1 uses])
(note 12 14 13 [bb 4] NOTE_INSN_BASIC_BLOCK)
(insn:TI 13 12 52 (set (reg:DI 1 $1 [orig:70 ivtmp_1 ] [70])
        (plus:DI (reg:DI 1 $1 [orig:70 ivtmp_1 ] [70])
            (const_int -1 [0xffffffffffffffff]))) 7 {*adddi_internal}
     (nil))
(note 52 13 15 0x7f6b5f2791e0 NOTE_INSN_BLOCK_BEG)
(jump_insn:TI 15 52 45 (set (pc)
        (if_then_else (ne (reg:DI 1 $1 [orig:70 ivtmp_1 ] [70])
                (const_int 0 [0]))
            (label_ref:DI 14)
            (pc))) "l.go":6 169 {*bcc_normal}
     (int_list:REG_BR_PROB 1062895956 (nil))
 -> 14)
(note 45 15 46 [bb 5] NOTE_INSN_BASIC_BLOCK)
(jump_insn:TI 46 45 47 (set (pc)
        (label_ref 16)) 200 {jump}
     (nil)
 -> 16)
(barrier 47 46 32)
(note 32 47 54 NOTE_INSN_DELETED)
(note 54 32 0 0x7f6b5f2791e0 NOTE_INSN_BLOCK_END)

The wrong label placement results in invalid debug_ranges, which are essential
for libbacktrace to determine name of inlined funcion.

[1] https://gcc.gnu.org/ml/gcc-patches/2018-01/msg01842.html
[2] https://gcc.gnu.org/ml/gcc-patches/2018-01/msg01932.html