You always need something from libc, unless you're using the embedded mode. Swift runtime is built on top of libc++ or libstdc++, which are built on top of a libc.
If we're making binary size measurements for "Hello, World!" printers on Linux, one could produce an executable taking 1096 bytes on arm64 with Embedded Swift.
For posterity here's the source code for that, showing how using syscalls without any libc would look like. No allocator is included though, to reduce the amount of dependencies.
import LinuxSyscall
@_cdecl("_start")
func start() {
let hello: StaticString = "Hello, World!\n"
__syscall3(64, 1, Int(bitPattern: hello.utf8Start), hello.utf8CodeUnitCount)
__syscall1(93, 0)
}
where LinuxSyscall directory contains this module.modulemap:
module LinuxSyscall {
header "aarch64/syscall_arch.h"
export *
}
and LinuxSyscall/aarch64/syscall_arch.h looks like this:
// ----------------------------------------------------------------------
// Copyright © 2005-2020 Rich Felker, et al.
// Permission is hereby granted, free of charge, to any person obtaining
// a copy of this software and associated documentation files (the
// "Software"), to deal in the Software without restriction, including
// without limitation the rights to use, copy, modify, merge, publish,
// distribute, sublicense, and/or sell copies of the Software, and to
// permit persons to whom the Software is furnished to do so, subject to
// the following conditions:
// The above copyright notice and this permission notice shall be
// included in all copies or substantial portions of the Software.
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
// EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
// MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
// IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
// CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,
// TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
// SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
// ----------------------------------------------------------------------
#define __asm_syscall(...) do { \
__asm__ __volatile__ ( "svc 0" \
: "=r"(x0) : __VA_ARGS__ : "memory", "cc"); \
return x0; \
} while (0)
static inline long __syscall0(long n)
{
register long x8 __asm__("x8") = n;
register long x0 __asm__("x0");
__asm_syscall("r"(x8));
}
static inline long __syscall1(long n, long a)
{
register long x8 __asm__("x8") = n;
register long x0 __asm__("x0") = a;
__asm_syscall("r"(x8), "0"(x0));
}
static inline long __syscall2(long n, long a, long b)
{
register long x8 __asm__("x8") = n;
register long x0 __asm__("x0") = a;
register long x1 __asm__("x1") = b;
__asm_syscall("r"(x8), "0"(x0), "r"(x1));
}
static inline long __syscall3(long n, long a, long b, long c)
{
register long x8 __asm__("x8") = n;
register long x0 __asm__("x0") = a;
register long x1 __asm__("x1") = b;
register long x2 __asm__("x2") = c;
__asm_syscall("r"(x8), "0"(x0), "r"(x1), "r"(x2));
}
static inline long __syscall4(long n, long a, long b, long c, long d)
{
register long x8 __asm__("x8") = n;
register long x0 __asm__("x0") = a;
register long x1 __asm__("x1") = b;
register long x2 __asm__("x2") = c;
register long x3 __asm__("x3") = d;
__asm_syscall("r"(x8), "0"(x0), "r"(x1), "r"(x2), "r"(x3));
}
static inline long __syscall5(long n, long a, long b, long c, long d, long e)
{
register long x8 __asm__("x8") = n;
register long x0 __asm__("x0") = a;
register long x1 __asm__("x1") = b;
register long x2 __asm__("x2") = c;
register long x3 __asm__("x3") = d;
register long x4 __asm__("x4") = e;
__asm_syscall("r"(x8), "0"(x0), "r"(x1), "r"(x2), "r"(x3), "r"(x4));
}
static inline long __syscall6(long n, long a, long b, long c, long d, long e, long f)
{
register long x8 __asm__("x8") = n;
register long x0 __asm__("x0") = a;
register long x1 __asm__("x1") = b;
register long x2 __asm__("x2") = c;
register long x3 __asm__("x3") = d;
register long x4 __asm__("x4") = e;
register long x5 __asm__("x5") = f;
__asm_syscall("r"(x8), "0"(x0), "r"(x1), "r"(x2), "r"(x3), "r"(x4), "r"(x5));
}
You can build it with a recent development snapshot using this command:
swiftc -Osize -enable-experimental-feature Embedded \
-wmo \
--target=aarch64-none-none-elf \
-I . \
-nostartfiles \
-Xfrontend -function-sections \
-Xlinker --gc-sections \
-c -o hello.o \
hello.swift
and link with lld:
ld.lld hello.o --gc-sections -Bstatic -EL -o hello
