Feat: prototype of self-hosted library

This is very much a work in progress. Still ruminating on the structure
of the library. It feels right but I want a more "social" presence -
namely the ability to link to a libc seemlessly. The solution is most
likely weak aliasing that musl uses - but musl itself weak aliases
global symbols, e.g malloc. We would have to define weak internal
symbols that musl defines as strong links but this knows too much about
the internals of musl...

3 files changed, 47 insertions, 0 deletions

diff --git a/sys/rt/amd64/crt1.s b/sys/rt/amd64/crt1.s
new file mode 100644
index 0000000..aaad0ba
--- /dev/null
+++ b/sys/rt/amd64/crt1.s
@@ -0,0 +1,21 @@
+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
new file mode 100644
index 0000000..eeba8b1
--- /dev/null
+++ b/sys/rt/amd64/crti.s
@@ -0,0 +1,17 @@
+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
new file mode 100644
index 0000000..45d1a29
--- /dev/null
+++ b/sys/rt/amd64/crtn.s
@@ -0,0 +1,9 @@
+section .init
+    ;; compiler inserts here
+    pop rbp ;; undo our push
+    ret
+
+section .fini
+    ;; compiler inserts here
+    pop rbp ;; undo our push
+    ret