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Language: C
Zero-overhead destructors in C
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- 09/18/2018
gem-hunterLanguage: C
Zero-overhead destructors in C
An implementation of zero-runtime-overhead destructors in C, relying on constant-folding optimisations.
For an in-depth account see article on CNX Software.
| compiler | status, under O3 equivalent |
|---|---|
| gcc | works since ver. 4.6.4 |
| clang | works since ver. 3.0.0 |
| icc | does not work as of ver. 18 |
| msvc | does not work as of ver. 2017 |
A constant-folded destructor invocation should appear like a direct funciton call when the destructor is not inlined. For example:
int main(int argc, char** argv) { begin_scope(0); Foo f; construct(0, f, Foo_ctor); // empty ctor, corresponding dtor named Foo_dtor, non-inlined end_scope(0); return EXIT_SUCCESS; }
Target amd64:
main:
subq $24, %rsp
movq %rsp, %rdi
call Foo_dtor
xorl %eax, %eax
addq $24, %rsp
ret
Target arm7l:
main:
str lr, [sp, #-4]!
sub sp, sp, #12
add r0, sp, #4
bl Foo_dtor
mov r0, #0
add sp, sp, #12
ldmfd sp!, {pc}
Target mips32:
main:
addiu $sp,$sp,-40
sw $31,36($sp)
jal Foo_dtor
addiu $4,$sp,24
lw $31,36($sp)
move $2,$0
j $31
addiu $sp,$sp,40
Conversely, when the compiler did not constant-fold the destructor invocation, the code should have an indirect function call -- via a register that was previosuly loaded with the address of the destructor, normally from a source on stack.
int main(int argc, char** argv) { begin_scope(0); Foo f; construct(0, f, Foo_ctor); __asm volatile ("": : : "memory"); // flush compiler's knowledge of constants that went to memory end_scope(0); // we might get an assert check as well if we haven't disabled those return EXIT_SUCCESS; }
Target amd64:
main:
subq $1048, %rsp
leaq 1024(%rsp), %rax
movq $Foo_dtor, 8(%rsp)
movq %rax, (%rsp)
movq 8(%rsp), %rax
testq %rax, %rax
je .L27
movq (%rsp), %rdi
call *%rax
xorl %eax, %eax
addq $1048, %rsp
ret
.L27:
movl $__PRETTY_FUNCTION__.1992, %ecx
movl $16, %edx
movl $.LC0, %esi
movl $.LC3, %edi
call __assert_fail