libruntime: Add fiber context switch code for LoongArch.

libphobos/ChangeLog:

	* libdruntime/config/loongarch/switchcontext.S: New file.

libphobos/libdruntime/config/loongarch/switchcontext.S

@@ -0,0 +1,133 @@
+/* LoongArch support code for fibers and multithreading.
+   Copyright (C) 2023 Free Software Foundation, Inc.
+
+This file is part of GCC.
+
+GCC is free software; you can redistribute it and/or modify it under
+the terms of the GNU General Public License as published by the Free
+Software Foundation; either version 3, or (at your option) any later
+version.
+
+GCC is distributed in the hope that it will be useful, but WITHOUT ANY
+WARRANTY; without even the implied warranty of MERCHANTABILITY or
+FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+for more details.
+
+Under Section 7 of GPL version 3, you are granted additional
+permissions described in the GCC Runtime Library Exception, version
+3.1, as published by the Free Software Foundation.
+
+You should have received a copy of the GNU General Public License and
+a copy of the GCC Runtime Library Exception along with this program;
+see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
+<http://www.gnu.org/licenses/>.  */
+
+#include "../common/threadasm.S"
+
+/**
+ * Performs a context switch.
+ *
+ * $a0 - void** - ptr to old stack pointer
+ * $a1 - void*  - new stack pointer
+ *
+ */
+
+#if defined(__loongarch_lp64)
+#  define GPR_L ld.d
+#  define GPR_S st.d
+#  define SZ_GPR 8
+#  define ADDSP(si)   addi.d  $sp, $sp, si
+#elif defined(__loongarch64_ilp32)
+#  define GPR_L ld.w
+#  define GPR_S st.w
+#  define SZ_GPR 4
+#  define ADDSP(si)   addi.w  $sp, $sp, si
+#else
+#  error Unsupported GPR size (must be 64-bit or 32-bit).
+#endif
+
+#if defined(__loongarch_double_float)
+#  define FPR_L fld.d
+#  define FPR_S fst.d
+#  define SZ_FPR 8
+#elif defined(__loongarch_single_float)
+#  define FPR_L fld.s
+#  define FPR_S fst.s
+#  define SZ_FPR 4
+#else
+#  define SZ_FPR 0
+#endif
+
+    .text
+    .align 2
+    .global fiber_switchContext
+    .type   fiber_switchContext, @function
+fiber_switchContext:
+    .cfi_startproc
+    ADDSP(-11 * SZ_GPR)
+
+    // fp regs and return address are stored below the stack
+    // because we don't want the GC to scan them.
+
+    // return address (r1)
+    GPR_S  $r1, $sp, -SZ_GPR
+
+#if SZ_FPR != 0
+    // callee-saved scratch FPRs (f24-f31)
+    FPR_S  $f24, $sp, -SZ_GPR-1*SZ_FPR
+    FPR_S  $f25, $sp, -SZ_GPR-2*SZ_FPR
+    FPR_S  $f26, $sp, -SZ_GPR-3*SZ_FPR
+    FPR_S  $f27, $sp, -SZ_GPR-4*SZ_FPR
+    FPR_S  $f28, $sp, -SZ_GPR-5*SZ_FPR
+    FPR_S  $f29, $sp, -SZ_GPR-6*SZ_FPR
+    FPR_S  $f30, $sp, -SZ_GPR-7*SZ_FPR
+    FPR_S  $f31, $sp, -SZ_GPR-8*SZ_FPR
+#endif
+
+    // callee-saved GPRs (r21, fp (r22), r23-r31)
+    GPR_S $r21, $sp, 0*SZ_GPR
+    GPR_S  $fp, $sp, 1*SZ_GPR
+    GPR_S  $s0, $sp, 2*SZ_GPR
+    GPR_S  $s1, $sp, 3*SZ_GPR
+    GPR_S  $s2, $sp, 4*SZ_GPR
+    GPR_S  $s3, $sp, 5*SZ_GPR
+    GPR_S  $s4, $sp, 6*SZ_GPR
+    GPR_S  $s5, $sp, 7*SZ_GPR
+    GPR_S  $s6, $sp, 8*SZ_GPR
+    GPR_S  $s7, $sp, 9*SZ_GPR
+    GPR_S  $s8, $sp, 10*SZ_GPR
+
+    // swap stack pointer
+    GPR_S $sp, $a0, 0
+    move $sp, $a1
+
+    GPR_L  $r1, $sp, -SZ_GPR
+
+#if SZ_FPR != 0
+    FPR_L  $f24, $sp, -SZ_GPR-1*SZ_FPR
+    FPR_L  $f25, $sp, -SZ_GPR-2*SZ_FPR
+    FPR_L  $f26, $sp, -SZ_GPR-3*SZ_FPR
+    FPR_L  $f27, $sp, -SZ_GPR-4*SZ_FPR
+    FPR_L  $f28, $sp, -SZ_GPR-5*SZ_FPR
+    FPR_L  $f29, $sp, -SZ_GPR-6*SZ_FPR
+    FPR_L  $f30, $sp, -SZ_GPR-7*SZ_FPR
+    FPR_L  $f31, $sp, -SZ_GPR-8*SZ_FPR
+#endif
+
+    GPR_L $r21, $sp, 0*SZ_GPR
+    GPR_L  $fp, $sp, 1*SZ_GPR
+    GPR_L  $s0, $sp, 2*SZ_GPR
+    GPR_L  $s1, $sp, 3*SZ_GPR
+    GPR_L  $s2, $sp, 4*SZ_GPR
+    GPR_L  $s3, $sp, 5*SZ_GPR
+    GPR_L  $s4, $sp, 6*SZ_GPR
+    GPR_L  $s5, $sp, 7*SZ_GPR
+    GPR_L  $s6, $sp, 8*SZ_GPR
+    GPR_L  $s7, $sp, 9*SZ_GPR
+    GPR_L  $s8, $sp, 10*SZ_GPR
+
+    ADDSP(11 * SZ_GPR)
+
+    jr     $r1 // return
+    .cfi_endproc
+    .size fiber_switchContext,.-fiber_switchContext