Implement longjmp

Author: Blackhex

winsup/cygwin/scripts/gendef

@@ -687,7 +687,63 @@ siglongjmp:
 	.globl  longjmp
 	.seh_proc longjmp
 longjmp:
+	// prologue
+	stp	fp, lr, [sp, #-0x20]!		// save FP and LR registers, allocate additional 16 bytes for function arguments
+	stp	x0, x1, [sp, #0x10]		// save function arguments (jump buffer and return value)
+	mov	fp, sp				// establishing frame chain
 	.seh_endprologue
+1:
+	bl	stabilize_sig_stack		// call stabilize_sig_stack which returns TLS pointer in x0
+	ldr	x2, [sp, #0x10]			// get jump buffer pointer from stack
+	ldr	x10, [x2]			// get old signal stack from jump buffer
+
+	// restore stack pointer in TLS
+	ldr	x11, =_cygtls.stackptr
+	add	x11, x0, x11
+	str	x10, [x11]
+
+	// release lock by decrementing counter
+	ldr	x11, =_cygtls.stacklock
+	add	x11, x0, x11
+	ldr	w12, [x11]
+	sub	w12, w12, #1
+	str	w12, [x11]
+
+	// we're not in cygwin anymore, clear "in cygwin" flag
+	ldr	x11, =_cygtls.incyg
+	add	x11, x0, x11
+	mov	w12, #0
+	str	w12, [x11]
+
+	// get saved return value before SP is restored
+	ldr	x0, [sp, #0x10]
+
+	// restore callee-saved registers from jump buffer
+	ldp	x19, x20, [x2, #0x08]		// restore x19, x20
+	ldp	x21, x22, [x2, #0x18]		// restore x21, x22
+	ldp	x23, x24, [x2, #0x28]		// restore x23, x24
+	ldp	x25, x26, [x2, #0x38]		// restore x25, x26
+	ldp	x27, x28, [x2, #0x48]		// restore x27, x28
+	ldp	fp, lr, [x2, #0x58]		// restore x29 (frame pointer) and x30 (link register)
+	ldr	x10, [x2, #0x68]		// get saved stack pointer
+	mov	sp, x10				// restore stack pointer
+	ldr	x10, [x2, #0x70]		// load floating-point control register
+	msr	fpcr, x10			// restore FPCR
+	ldr	x10, [x2, #0x78]		// load floating-point status register
+	msr	fpsr, x10			// restore FPSR
+
+	// restore floating-point registers (d8-d15)
+	ldp	d8, d9, [x2, #0x80]
+	ldp	d10, d11, [x2, #0x90]
+	ldp	d12, d13, [x2, #0xA0]
+	ldp	d14, d15, [x2, #0xB0]
+
+	// ensure return value is non-zero (C standard requirement)
+	cbnz	x0, 0f
+	mov	x0, #1
+0:
+	// epilogue
+	add	sp, sp, #0x10			// FP and LR are already restored, just restore SP as it would be popped
 	ret
 	.seh_endproc
 EOF