xtensa: support call0 ABI

Most changes are mechanical replacement of 'retw' instruction with
'abi_ret' macro, defined to 'retw' or 'ret' according to ABI.
Assembly code that makes calls is duplicated for call0 ABI with changed
register numbers for parameters/return value and call instruction.
'entry' instructions are replaced with 'abi_entry' macro.

More interesting changes:
- non-leaf assembly functions (e.g. _dl_tlsdesc_dynamic,
  _dl_linux_resolve, SYSCALL_ERROR_HANDLER, PSEUDO) now need to preserve
  registers around intermediate calls they make, use temporary stack
  frame for that;
- setjmp/longjmp only need to save and restore return address, stack
  pointer and callee-saved registers in the jmpbuf;
- __clone and syscall functions had hardcoded offsets to parameter
  passed on stack, on call0 ABI they don't need stack frame, so the
  offset is different. Replace these offsets with FRAMESIZE macro.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>

1 files changed, 30 insertions, 0 deletions

diff --git a/libc/sysdeps/linux/xtensa/crt1.S b/libc/sysdeps/linux/xtensa/crt1.S
index 025ebd070..efbe264c0 100644
--- a/libc/sysdeps/linux/xtensa/crt1.S
+++ b/libc/sysdeps/linux/xtensa/crt1.S
@@ -76,6 +76,7 @@
 	.global	_start
 	.type	_start, @function
 _start:
+#if defined(__XTENSA_WINDOWED_ABI__)
 	/* Clear a0 to obviously mark the outermost frame.  */
 	movi	a0, 0
 
@@ -104,6 +105,35 @@ _start:
 	   But let the libc call main.    */
 	movi	a4, __uClibc_main
 	callx4	a4
+#elif defined(__XTENSA_CALL0_ABI__)
+	/* Setup the shared library termination function.  */
+	mov	a7, a2
+
+	/* Load up the user's main function.  */
+	movi	a2, main
+
+	/* Extract the arguments as encoded on the stack and set up
+	   the arguments for `main': argc, argv.  envp will be determined
+	   later in __uClibc_main.  */
+	l32i	a3, a1, 0	/* Load the argument count.  */
+	addi	a4, a1, 4	/* Compute the argv pointer.  */
+
+	/* Push address of our own entry points to .fini and .init.  */
+	movi	a5, _init
+	movi	a6, _fini
+
+	/* Provide the highest stack address to the user code (for stacks
+	   which grow downwards).  Note that we destroy the stack version
+	   of argc here.  */
+	s32i	a1, a1, 0
+
+	/* Call the user's main function, and exit with its value.
+	   But let the libc call main.    */
+	movi	a0, __uClibc_main
+	callx0	a0
+#else
+#error Unsupported Xtensa ABI
+#endif
 
 	/* Crash if somehow `exit' does return.  */
 	ill