How to remove "noise" from GCC/clang assembly output?

Question

I want to inspect the assembly output of applying boost::variant in my code in order to see which intermediate calls are optimized away.

When I compile the following example (with GCC 5.3 using g++ -O3 -std=c++14 -S), it seems as if the compiler optimizes away everything and directly returns 100:

(...)
main:
.LFB9320:
    .cfi_startproc
    movl    $100, %eax
    ret
    .cfi_endproc
(...)

---

#include <boost/variant.hpp>

struct Foo
{
    int get() { return 100; }
};

struct Bar
{
    int get() { return 999; }
};

using Variant = boost::variant<Foo, Bar>;


int run(Variant v)
{
    return boost::apply_visitor([](auto& x){return x.get();}, v);
}
int main()
{
    Foo f;
    return run(f);
}

However, the full assembly output contains much more than the above excerpt, which to me looks like it is never called. Is there a way to tell GCC/clang to remove all that "noise" and just output what is actually called when the program is ran?

---

full assembly output:

    .file   "main1.cpp"
    .section    .rodata.str1.8,"aMS",@progbits,1
    .align 8
.LC0:
    .string "/opt/boost/include/boost/variant/detail/forced_return.hpp"
    .section    .rodata.str1.1,"aMS",@progbits,1
.LC1:
    .string "false"
    .section    .text.unlikely._ZN5boost6detail7variant13forced_returnIvEET_v,"axG",@progbits,_ZN5boost6detail7variant13forced_returnIvEET_v,comdat
.LCOLDB2:
    .section    .text._ZN5boost6detail7variant13forced_returnIvEET_v,"axG",@progbits,_ZN5boost6detail7variant13forced_returnIvEET_v,comdat
.LHOTB2:
    .p2align 4,,15
    .weak   _ZN5boost6detail7variant13forced_returnIvEET_v
    .type   _ZN5boost6detail7variant13forced_returnIvEET_v, @function
_ZN5boost6detail7variant13forced_returnIvEET_v:
.LFB1197:
    .cfi_startproc
    subq    $8, %rsp
    .cfi_def_cfa_offset 16
    movl    $_ZZN5boost6detail7variant13forced_returnIvEET_vE19__PRETTY_FUNCTION__, %ecx
    movl    $49, %edx
    movl    $.LC0, %esi
    movl    $.LC1, %edi
    call    __assert_fail
    .cfi_endproc
.LFE1197:
    .size   _ZN5boost6detail7variant13forced_returnIvEET_v, .-_ZN5boost6detail7variant13forced_returnIvEET_v
    .section    .text.unlikely._ZN5boost6detail7variant13forced_returnIvEET_v,"axG",@progbits,_ZN5boost6detail7variant13forced_returnIvEET_v,comdat
.LCOLDE2:
    .section    .text._ZN5boost6detail7variant13forced_returnIvEET_v,"axG",@progbits,_ZN5boost6detail7variant13forced_returnIvEET_v,comdat
.LHOTE2:
    .section    .text.unlikely._ZN5boost6detail7variant13forced_returnIiEET_v,"axG",@progbits,_ZN5boost6detail7variant13forced_returnIiEET_v,comdat
.LCOLDB3:
    .section    .text._ZN5boost6detail7variant13forced_returnIiEET_v,"axG",@progbits,_ZN5boost6detail7variant13forced_returnIiEET_v,comdat
.LHOTB3:
    .p2align 4,,15
    .weak   _ZN5boost6detail7variant13forced_returnIiEET_v
    .type   _ZN5boost6detail7variant13forced_returnIiEET_v, @function
_ZN5boost6detail7variant13forced_returnIiEET_v:
.LFB9757:
    .cfi_startproc
    subq    $8, %rsp
    .cfi_def_cfa_offset 16
    movl    $_ZZN5boost6detail7variant13forced_returnIiEET_vE19__PRETTY_FUNCTION__, %ecx
    movl    $39, %edx
    movl    $.LC0, %esi
    movl    $.LC1, %edi
    call    __assert_fail
    .cfi_endproc
.LFE9757:
    .size   _ZN5boost6detail7variant13forced_returnIiEET_v, .-_ZN5boost6detail7variant13forced_returnIiEET_v
    .section    .text.unlikely._ZN5boost6detail7variant13forced_returnIiEET_v,"axG",@progbits,_ZN5boost6detail7variant13forced_returnIiEET_v,comdat
.LCOLDE3:
    .section    .text._ZN5boost6detail7variant13forced_returnIiEET_v,"axG",@progbits,_ZN5boost6detail7variant13forced_returnIiEET_v,comdat
.LHOTE3:
    .section    .text.unlikely,"ax",@progbits
.LCOLDB4:
    .text
.LHOTB4:
    .p2align 4,,15
    .globl  _Z3runN5boost7variantI3FooJ3BarEEE
    .type   _Z3runN5boost7variantI3FooJ3BarEEE, @function
_Z3runN5boost7variantI3FooJ3BarEEE:
.LFB9310:
    .cfi_startproc
    subq    $8, %rsp
    .cfi_def_cfa_offset 16
    movl    (%rdi), %eax
    cltd
    xorl    %edx, %eax
    cmpl    $19, %eax
    ja  .L7
    jmp *.L9(,%rax,8)
    .section    .rodata
    .align 8
    .align 4
.L9:
    .quad   .L30
    .quad   .L10
    .quad   .L7
    .quad   .L7
    .quad   .L7
    .quad   .L7
    .quad   .L7
    .quad   .L7
    .quad   .L7
    .quad   .L7
    .quad   .L7
    .quad   .L7
    .quad   .L7
    .quad   .L7
    .quad   .L7
    .quad   .L7
    .quad   .L7
    .quad   .L7
    .quad   .L7
    .quad   .L7
    .text
    .p2align 4,,10
    .p2align 3
.L7:
    call    _ZN5boost6detail7variant13forced_returnIiEET_v
    .p2align 4,,10
    .p2align 3
.L30:
    movl    $100, %eax
.L8:
    addq    $8, %rsp
    .cfi_remember_state
    .cfi_def_cfa_offset 8
    ret
    .p2align 4,,10
    .p2align 3
.L10:
    .cfi_restore_state
    movl    $999, %eax
    jmp .L8
    .cfi_endproc
.LFE9310:
    .size   _Z3runN5boost7variantI3FooJ3BarEEE, .-_Z3runN5boost7variantI3FooJ3BarEEE
    .section    .text.unlikely
.LCOLDE4:
    .text
.LHOTE4:
    .globl  _Z3runN5boost7variantI3FooI3BarEEE
    .set    _Z3runN5boost7variantI3FooI3BarEEE,_Z3runN5boost7variantI3FooJ3BarEEE
    .section    .text.unlikely
.LCOLDB5:
    .section    .text.startup,"ax",@progbits
.LHOTB5:
    .p2align 4,,15
    .globl  main
    .type   main, @function
main:
.LFB9320:
    .cfi_startproc
    movl    $100, %eax
    ret
    .cfi_endproc
.LFE9320:
    .size   main, .-main
    .section    .text.unlikely
.LCOLDE5:
    .section    .text.startup
.LHOTE5:
    .section    .rodata
    .align 32
    .type   _ZZN5boost6detail7variant13forced_returnIvEET_vE19__PRETTY_FUNCTION__, @object
    .size   _ZZN5boost6detail7variant13forced_returnIvEET_vE19__PRETTY_FUNCTION__, 58
_ZZN5boost6detail7variant13forced_returnIvEET_vE19__PRETTY_FUNCTION__:
    .string "T boost::detail::variant::forced_return() [with T = void]"
    .align 32
    .type   _ZZN5boost6detail7variant13forced_returnIiEET_vE19__PRETTY_FUNCTION__, @object
    .size   _ZZN5boost6detail7variant13forced_returnIiEET_vE19__PRETTY_FUNCTION__, 57
_ZZN5boost6detail7variant13forced_returnIiEET_vE19__PRETTY_FUNCTION__:
    .string "T boost::detail::variant::forced_return() [with T = int]"
    .ident  "GCC: (Ubuntu 5.3.0-3ubuntu1~14.04) 5.3.0 20151204"
    .section    .note.GNU-stack,"",@progbits

Answer 1

Stripping out the .cfi directives, unused labels, and comment lines is a solved problem: the scripts behind Matt Godbolt's compiler explorer are open source on its github project. It can even do colour highlighting to match source lines to asm lines (using the debug info).

You can set it up locally so you can feed it files that are part of your project with all the #include paths and so on (using -I/...). And so you can use it on private source code that you don't want to send out over the Internet.

Matt Godbolt's CppCon2017 talk “What Has My Compiler Done for Me Lately? Unbolting the Compiler's Lid”