Feature: self hosting prototype implemented

This is a large change. In order to remove myself from libc's arcane
interface, I implemented an independent runtime layer. It is based on
musl's wonderful implementation mostly. Critically, if libc is linked to
the program, then we cooperate. Namely, we call start main and let libc
do all initialization. If not, then we have a noop defined in rt3.a.

The general structure of the file is:
1. sys/$os/$arch contains all architecture dependent code
2. sys/$os/port contains all code that depends on the os, but is
   portable
3. rt/$arch contains all the runtime architecture dependent code
4. rt/* contains the portable runtime code.

Obviously testing is needed. Specifically, while code is checked in for
the most popular architectures, it only has been tested on one computer!
Overall this is exciting and as been educational.

6 files changed, 25 insertions, 47 deletions

diff --git a/sys/rt/amd64/crt1.s b/sys/rt/amd64/crt1.s
deleted file mode 100644
index aaad0ba..0000000
--- a/sys/rt/amd64/crt1.s
+++ /dev/null
@@ -1,21 +0,0 @@
-global  _start
-
-; NOTE: assumes program loader has put argc, argv, envc, envp on stack
-section .text
-_start:
-    xor rbp,rbp    ; base pointer undefined: set to 0
-    mov r9,rdx     ; 6th arg: function to register with atexit()
-    pop rsi        ; 2nd arg: argc
-    mov rdx,rsp    ; 3rd arg: argv
-    and rsp,$-16   ; align stack pointer to 16 bytes
-    mov $_fini,r8  ; 5th arg: fini
-    mov $_init,rcx ; 4th arg: init
-    mov $main,rdi  ; 1st arg: main
-
-    call rt·boot   ; int boot(
-                   ;       int(*main)(int,char*[],char*[]),
-                   ;       int argc,
-                   ;       char *argv[],
-                   ;       init, fini, atexit);
-
-.loop: jmp .loop  ; should never reach...
diff --git a/sys/rt/amd64/crti.s b/sys/rt/amd64/crti.s
deleted file mode 100644
index eeba8b1..0000000
--- a/sys/rt/amd64/crti.s
+++ /dev/null
@@ -1,17 +0,0 @@
-global _init
-global _fini
-
-section .init
-_init:
-    ; call pushes an 8 byte return address on the stack
-    ; 64 bit ABI requires stack to be aligned to 16 bytes
-    ; we realign with another 8 byte address
-    push rbp
-    mov  rbp, rsp
-    ;; compiler inserts the rest here
-
-section .fini
-_fini:
-    push rbp ; see above
-    mov  rbp, rsp
-    ;; compiler inserts the rest here
diff --git a/sys/rt/amd64/crtn.s b/sys/rt/amd64/crtn.s
deleted file mode 100644
index 45d1a29..0000000
--- a/sys/rt/amd64/crtn.s
+++ /dev/null
@@ -1,9 +0,0 @@
-section .init
-    ;; compiler inserts here
-    pop rbp ;; undo our push
-    ret
-
-section .fini
-    ;; compiler inserts here
-    pop rbp ;; undo our push
-    ret
diff --git a/sys/rt/amd64/rt1.s b/sys/rt/amd64/rt1.s
new file mode 100644
index 0000000..afb5c32
--- /dev/null
+++ b/sys/rt/amd64/rt1.s
@@ -0,0 +1,9 @@
+.extern rt·thunk
+
+.text
+.global _start
+_start:
+	xor %rbp,%rbp
+	mov %rsp,%rdi
+	andq $-16,%rsp
+	call rt·thunk
diff --git a/sys/rt/amd64/rti.s b/sys/rt/amd64/rti.s
new file mode 100644
index 0000000..4788968
--- /dev/null
+++ b/sys/rt/amd64/rti.s
@@ -0,0 +1,9 @@
+.section .init
+.global _init
+_init:
+	push %rax
+
+.section .fini
+.global _fini
+_fini:
+	push %rax
diff --git a/sys/rt/amd64/rtn.s b/sys/rt/amd64/rtn.s
new file mode 100644
index 0000000..29198b7
--- /dev/null
+++ b/sys/rt/amd64/rtn.s
@@ -0,0 +1,7 @@
+.section .init
+	pop %rax
+	ret
+
+.section .fini
+	pop %rax
+	ret