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, @functionmain:.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