clang emits code for constructors that causes debuggers to behave unexpectedly

[Quuxplusone]

Bugzilla Link	PR30620
Status	NEW
Importance	P normal
Reported by	edmunoz@microsoft.com
Reported on	2016-10-05 19:08:04 -0700
Last modified on	2016-10-05 20:16:47 -0700
Version	unspecified
Hardware	PC MacOS X
CC	llvm-bugs@lists.llvm.org, paul_robinson@playstation.sony.com
Fixed by commit(s)
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This is reported by jacdavis@microsoft.com (he's waiting for his account on
lldvm.org/bugs to be created):

clang emits a wrapper constructor whose purpose is to invoke child
constructors, copy constructors, and ultimately, the real constructor of a
class.
This wrapper constructor is mapped to the same source code as the real
constructor and is not marked with a trampoline symbol (which would tell the
debugger to continue stepping through that code) or the target no-stmt flag in
the line info.

Unfortunately, this means that both gdb and lldb (when debugging this clang
compiled binary) stop in the wrapper constructor rather than the targets.  Now,
when a bp is set in the source of the real constructor, the debugger will stop
when that bp is hit. The logical callstack (not displayed in the window) then
looks like this:

real_ctor

wrapper_ctor

main

Stepping off the end of the real constructor stops in the wrapper constructor.

Interesting symbols:

wrapper constructor disassem

a.out`CSquid::CSquid:
    0x100000f60 <+0>:  pushq  %rbp
    0x100000f61 <+1>:  movq   %rsp, %rbp
    0x100000f64 <+4>:  subq   $0x10, %rsp
    0x100000f68 <+8>:  movq   %rdi, -0x8(%rbp)
    0x100000f6c <+12>: movq   -0x8(%rbp), %rdi
    0x100000f70 <+16>: callq  0x100000fa0               ; call to real constructor CSquid::CSquid at constructorbug.cpp:8
    0x100000f79 <+25>: popq   %rbp
    0x100000f7a <+26>: retq
    0x100000f7b <+27>: nopl   (%rax,%rax)

real constructor disassem:

.out`CSquid::CSquid:
    0x100000fa0 <+0>:  pushq  %rbp
    0x100000fa1 <+1>:  movq   %rsp, %rbp
    0x100000fa4 <+4>:  movq   %rdi, -0x8(%rbp)
    0x100000fa8 <+8>:  movq   -0x8(%rbp), %rdi
    0x100000fb2 <+18>: popq   %rbp
    0x100000fb3 <+19>: req

Wrapper constructor subprogram symbol:

0x000000b8:     TAG_subprogram [11] *
                 AT_low_pc( 0x0000000100000f60 )
                 AT_high_pc( 0x0000000100000f7b )
                 AT_frame_base( rbp )
                 AT_object_pointer( {0x000000db} )
                 AT_MIPS_linkage_name( "_ZN6CSquidC1Ev" )
                 AT_specification( {0x0000000000000044}"CSquid" )

0x000000db:         TAG_formal_parameter [12]
                     AT_location( fbreg -8 )
                     AT_name( "this" )
                     AT_type( {0x0000000000000165} ( CSquid* ) )
                     AT_artificial( 0x01 )

0x000000ec:         NULL

Real constructor subprogram symbol:

0x00000130:     TAG_subprogram [11] *
                 AT_low_pc( 0x0000000100000fa0 )
                 AT_high_pc( 0x0000000100000fb4 )
                 AT_frame_base( rbp )
                 AT_object_pointer( {0x00000153} )
                 AT_MIPS_linkage_name( "_ZN6CSquidC2Ev" )
                 AT_specification( {0x0000000000000044}"CSquid" )

0x00000153:         TAG_formal_parameter [12]
                     AT_location( fbreg -8 )
                     AT_name( "this" )
                     AT_type( {0x0000000000000165} ( CSquid* ) )
                     AT_artificial( 0x01 )

0x00000164:         NULL

Line table for both:

0x0000004f: DW_LNE_set_address( 0x0000000100000f60 )
0x0000005a: address += 0,  line += 7
            0x0000000100000f60      1      8      0 is_stmt

0x0000005b: DW_LNS_set_column( 5 )
0x0000005d: DW_LNS_set_prologue_end

0x0000005e: address += 10,  line += 0
            0x0000000100000f70      1      8      5 is_stmt prologue_end

0x0000005f: address += 5,  line += 2
            0x0000000100000f75      1     10      5 is_stmt

0x00000060: DW_LNS_advance_pc( 6 )
0x00000062: DW_LNE_end_sequence
            0x0000000100000f7b      1     10      5 is_stmt

0x00000082: DW_LNE_set_address( 0x0000000100000fa0 )
0x0000008d: address += 0,  line += 7
            0x0000000100000fa0      1      8      0 is_stmt

0x0000008e: DW_LNS_set_column( 13 )
0x00000090: DW_LNS_set_prologue_end
0x00000091: address += c,  line += 1
            0x0000000100000fac      1      9     13 is_stmt prologue_end

0x00000092: DW_LNS_set_column( 5 )
0x00000094: address += 6,  line += 1
            0x0000000100000fb2      1     10      5 is_stmt

0x00000095: DW_LNS_advance_pc( 2 )
0x00000097: DW_LNE_end_sequence
            0x0000000100000fb4      1     10      5 is_stmt

[Quuxplusone]

Well, the is_stmt flag is generated pretty naively (and late) so
it's unlikely we can get that turned off for these wrappers.
I assume marking them artificial isn't sufficient? (although it
should not be hard.)

Inventing a trampoline link might not be too hard.