--- /dev/null
+/* Copyright (c) 2017-2018 SiFive Inc. All rights reserved.
+
+ This copyrighted material is made available to anyone wishing to use,
+ modify, copy, or redistribute it subject to the terms and conditions
+ of the FreeBSD License. This program is distributed in the hope that
+ it will be useful, but WITHOUT ANY WARRANTY expressed or implied,
+ including the implied warranties of MERCHANTABILITY or FITNESS FOR
+ A PARTICULAR PURPOSE. A copy of this license is available at
+ http://www.opensource.org/licenses.
+*/
+
+/* crt0.S: Entry point for RISC-V METAL programs. */
+
+.section .text.libgloss.start
+.global _start
+.type _start, @function
+
+ /* _start is defined by the METAL to have been called with the following
+ * arguments:
+ * a0: the hart ID of the currently executing hart. Harts can start at
+ * any arbitrary point, it's the C library's job to ensure the code is
+ * safe.
+ * a1: a pointer to a description of the machine on which this code is
+ * currently executing. This is probably 0 on an embedded system
+ * because they tend to not be dynamically portable. As such, newlib
+ * ignores this argument.
+ * a2: a pointer to a function that must be run after the envirnoment has
+ * been initialized, but before user code can be expected to be run.
+ * If this is 0 then there is no function to be run. */
+_start:
+.cfi_startproc
+.cfi_undefined ra
+
+ /* This is a bit funky: it's not usually sane for _start to return, but in
+ * this case we actually want to in order to signal an error to the METAL. */
+ mv s0, ra
+
+ /* Before doing anything we must initialize the global pointer, as we cannot
+ * safely perform any access that may be relaxed without GP being set. This
+ * is done with relaxation disabled to avoid relaxing the address calculation
+ * to just "addi gp, gp, 0". */
+.option push
+.option norelax
+ la gp, __global_pointer$
+.option pop
+
+ /* The METAL is designed for a bare-metal environment and therefor is expected
+ * to define its own stack pointer. We also align the stack pointer here
+ * because the only RISC-V ABI that's currently defined mandates 16-byte
+ * stack alignment. */
+ la sp, _sp
+
+ /* Increment by hartid number of stack sizes */
+ li t0, 0
+ la t1, __stack_size
+1:
+ beq t0, a0, 1f
+ add sp, sp, t1
+ addi t0, t0, 1
+ j 1b
+1:
+ andi sp, sp, -16
+
+ /* If we're not hart 0, skip the initialization work */
+ la t0, __metal_boot_hart
+ bne a0, t0, _skip_init
+
+ /* Embedded systems frequently require relocating the data segment before C
+ * code can be run -- for example, the data segment may exist in flash upon
+ * boot and then need to get relocated into a non-persistant writable memory
+ * before C code can execute. If this is the case we do so here. This step
+ * is optional: if the METAL provides an environment in which this relocation
+ * is not necessary then it must simply set metal_segment_data_source_start to
+ * be equal to metal_segment_data_target_start. */
+ la t0, metal_segment_data_source_start
+ la t1, metal_segment_data_target_start
+ la t2, metal_segment_data_target_end
+
+ beq t0, t1, 2f
+ bge t1, t2, 2f
+
+1:
+ lw a0, 0(t0)
+ addi t0, t0, 4
+ sw a0, 0(t1)
+ addi t1, t1, 4
+ blt t1, t2, 1b
+2:
+
+ /* Copy the ITIM section */
+ la t0, metal_segment_itim_source_start
+ la t1, metal_segment_itim_target_start
+ la t2, metal_segment_itim_target_end
+
+ beq t0, t1, 2f
+ bge t1, t2, 2f
+
+1:
+ lw a0, 0(t0)
+ addi t0, t0, 4
+ sw a0, 0(t1)
+ addi t1, t1, 4
+ blt t1, t2, 1b
+2:
+
+ /* Zero the BSS segment. */
+ la t1, metal_segment_bss_target_start
+ la t2, metal_segment_bss_target_end
+
+ bge t1, t2, 2f
+
+1:
+ sw x0, 0(t1)
+ addi t1, t1, 4
+ blt t1, t2, 1b
+2:
+
+ /* At this point we're in an environment that can execute C code. The first
+ * thing to do is to make the callback to the parent environment if it's been
+ * requested to do so. */
+ beqz a2, 1f
+ jalr a2
+1:
+
+ /* The RISC-V port only uses new-style constructors and destructors. */
+ la a0, __libc_fini_array
+ call atexit
+ call __libc_init_array
+
+_skip_init:
+
+ /* Synchronize harts so that secondary harts wait until hart 0 finishes
+ initializing */
+ call _synchronize_harts
+
+ /* Check RISC-V isa and enable FS bits if Floating Point architecture. */
+ csrr a5, misa
+ li a4, 0x10028
+ and a5, a5, a4
+ beqz a5, 1f
+ csrr a5, mstatus
+ lui a4, 0x2
+ or a5, a5, a4
+ csrw mstatus, a5
+ csrwi fcsr, 0
+1:
+
+ /* This is a C runtime, so main() is defined to have some arguments. Since
+ * there's nothing sane the METAL can pass we don't bother with that but
+ * instead just setup as close to a NOP as we can. */
+ li a0, 1 /* argc=1 */
+ la a1, argv /* argv = {"libgloss", NULL} */
+ la a2, envp /* envp = {NULL} */
+ call secondary_main
+
+ /* Call exit to handle libc's cleanup routines. Under normal contains this
+ * shouldn't even get called, but I'm still not using a tail call here
+ * because returning to the METAL is the right thing to do in pathological
+ * situations. */
+ call exit
+
+ /* And here's where we return. Again, it's a bit odd but the METAL defines
+ * this as a bad idea (ie, as opposed to leaving it undefined) and at this
+ * point it's really the only thing left to do. */
+ mv ra, s0
+ ret
+
+.cfi_endproc
+
+/* RISC-V systems always use __libc_{init,fini}_array, but for compatibility we
+ * define _{init,fini} to do nothing. */
+.global _init
+.type _init, @function
+.global _fini
+.type _fini, @function
+_init:
+_fini:
+ ret
+.size _init, .-_init
+.size _fini, .-_fini
+
+/* By default, secondary_main will cause secondary harts to spin forever.
+ * Users can redefine secondary_main themselves to run code on secondary harts */
+.weak secondary_main
+.global secondary_main
+.type secondary_main, @function
+
+secondary_main:
+ addi sp, sp, -16
+#if __riscv_xlen == 32
+ sw ra, 4(sp)
+#else
+ sd ra, 8(sp)
+#endif
+ csrr t0, mhartid
+ la t1, __metal_boot_hart
+ beq t0, t1, 2f
+1:
+ wfi
+ j 1b
+2:
+ call main
+#if __riscv_xlen == 32
+ lw ra, 4(sp)
+#else
+ ld ra, 8(sp)
+#endif
+ addi sp, sp, 16
+ ret
+
+/* This shim allows main() to be passed a set of arguments that can satisfy the
+ * requirements of the C API. */
+.section .rodata.libgloss.start
+argv:
+.dc.a name
+envp:
+.dc.a 0
+name:
+.asciz "libgloss"
+
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