我认为您缺少链接器脚本和一些启动代码 - 免责声明:我是 riscv 的新手。
您会在 Internet 上找到有关这两个主题的大量信息,但您基本上需要指定程序在 RAM 中的位置,以建立堆栈并初始化帧指针:
如果您希望能够在程序中调用函数并声明自动 C 变量(例如 a、b、c),则这是必需的。
我用的是Windows工具链出于本示例的目的,来自 Kendryte(Linux 版本可用here),并检索到 Windows 版本的 qemuhere.
1) 链接描述文件:该示例使用 riscv64-unknown-elf-ld 使用的默认链接描述文件的稍微修改的示例:
riscv64-unknown-elf-ld --verbose > riscv64-virt.ld
编辑 riscv64-virt.ld,并仅保留以下分隔的行:
==================================================
添加qemu-system-riscv64虚拟机内存布局的描述:
OUTPUT_ARCH(riscv)
MEMORY
{
/* qemu-system-risc64 virt machine */
RAM (rwx) : ORIGIN = 0x80000000, LENGTH = 128M
}
ENTRY(_start)
Use ORIGIN(RAM)
and LENGTH(RAM)
而不是硬编码值,并提供__stack_top
symbol:
PROVIDE (__executable_start = SEGMENT_START("text-segment", ORIGIN(RAM))); . = SEGMENT_START("text-segment", ORIGIN(RAM)) + SIZEOF_HEADERS;
PROVIDE(__stack_top = ORIGIN(RAM) + LENGTH(RAM));
顺便说一句,有多种方法可以了解 qemu 系统目标机的内存布局,但我通常查看其设备树文件:
qemu-system-riscv64 -machine virt -machine dumpdtb=riscv64-virt.dtb
dtc -I dtb -O dts -o riscv-virt.dts riscv-virt.dtb
描述内存的部分告诉我们它从 0x80000000 开始:
memory@80000000 {
device_type = "memory";
reg = <0x0 0x80000000 0x0 0x8000000>;
};
riscv64-virt.ld:
/* Script for -z combreloc: combine and sort reloc sections */
/* Copyright (C) 2014-2018 Free Software Foundation, Inc.
Copying and distribution of this script, with or without modification,
are permitted in any medium without royalty provided the copyright
notice and this notice are preserved. */
OUTPUT_FORMAT("elf64-littleriscv", "elf64-littleriscv",
"elf64-littleriscv")
OUTPUT_ARCH(riscv)
MEMORY
{
/* qemu-system-risc64 virt machine */
RAM (rwx) : ORIGIN = 0x80000000, LENGTH = 128M
}
ENTRY(_start)
SECTIONS
{
/* Read-only sections, merged into text segment: */
PROVIDE (__executable_start = SEGMENT_START("text-segment", ORIGIN(RAM))); . = SEGMENT_START("text-segment", ORIGIN(RAM)) + SIZEOF_HEADERS;
PROVIDE(__stack_top = ORIGIN(RAM) + LENGTH(RAM));
.interp : { *(.interp) }
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{
*(.rela.init)
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*(.rela.fini)
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*(.rela.sbss2 .rela.sbss2.* .rela.gnu.linkonce.sb2.*)
*(.rela.bss .rela.bss.* .rela.gnu.linkonce.b.*)
PROVIDE_HIDDEN (__rela_iplt_start = .);
*(.rela.iplt)
PROVIDE_HIDDEN (__rela_iplt_end = .);
}
.rela.plt :
{
*(.rela.plt)
}
.init :
{
KEEP (*(SORT_NONE(.init)))
}
.plt : { *(.plt) }
.iplt : { *(.iplt) }
.text :
{
*(.text.unlikely .text.*_unlikely .text.unlikely.*)
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KEEP (*(SORT_NONE(.fini)))
}
PROVIDE (__etext = .);
PROVIDE (_etext = .);
PROVIDE (etext = .);
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.rodata1 : { *(.rodata1) }
.sdata2 :
{
*(.sdata2 .sdata2.* .gnu.linkonce.s2.*)
}
.sbss2 : { *(.sbss2 .sbss2.* .gnu.linkonce.sb2.*) }
.eh_frame_hdr : { *(.eh_frame_hdr) *(.eh_frame_entry .eh_frame_entry.*) }
.eh_frame : ONLY_IF_RO { KEEP (*(.eh_frame)) *(.eh_frame.*) }
.gcc_except_table : ONLY_IF_RO { *(.gcc_except_table
.gcc_except_table.*) }
.gnu_extab : ONLY_IF_RO { *(.gnu_extab*) }
/* These sections are generated by the Sun/Oracle C++ compiler. */
.exception_ranges : ONLY_IF_RO { *(.exception_ranges
.exception_ranges*) }
/* Adjust the address for the data segment. We want to adjust up to
the same address within the page on the next page up. */
. = DATA_SEGMENT_ALIGN (CONSTANT (MAXPAGESIZE), CONSTANT (COMMONPAGESIZE));
/* Exception handling */
.eh_frame : ONLY_IF_RW { KEEP (*(.eh_frame)) *(.eh_frame.*) }
.gnu_extab : ONLY_IF_RW { *(.gnu_extab) }
.gcc_except_table : ONLY_IF_RW { *(.gcc_except_table .gcc_except_table.*) }
.exception_ranges : ONLY_IF_RW { *(.exception_ranges .exception_ranges*) }
/* Thread Local Storage sections */
.tdata :
{
PROVIDE_HIDDEN (__tdata_start = .);
*(.tdata .tdata.* .gnu.linkonce.td.*)
}
.tbss : { *(.tbss .tbss.* .gnu.linkonce.tb.*) *(.tcommon) }
.preinit_array :
{
PROVIDE_HIDDEN (__preinit_array_start = .);
KEEP (*(.preinit_array))
PROVIDE_HIDDEN (__preinit_array_end = .);
}
.init_array :
{
PROVIDE_HIDDEN (__init_array_start = .);
KEEP (*(SORT_BY_INIT_PRIORITY(.init_array.*) SORT_BY_INIT_PRIORITY(.ctors.*)))
KEEP (*(.init_array EXCLUDE_FILE (*crtbegin.o *crtbegin?.o *crtend.o *crtend?.o ) .ctors))
PROVIDE_HIDDEN (__init_array_end = .);
}
.fini_array :
{
PROVIDE_HIDDEN (__fini_array_start = .);
KEEP (*(SORT_BY_INIT_PRIORITY(.fini_array.*) SORT_BY_INIT_PRIORITY(.dtors.*)))
KEEP (*(.fini_array EXCLUDE_FILE (*crtbegin.o *crtbegin?.o *crtend.o *crtend?.o ) .dtors))
PROVIDE_HIDDEN (__fini_array_end = .);
}
.ctors :
{
/* gcc uses crtbegin.o to find the start of
the constructors, so we make sure it is
first. Because this is a wildcard, it
doesn't matter if the user does not
actually link against crtbegin.o; the
linker won't look for a file to match a
wildcard. The wildcard also means that it
doesn't matter which directory crtbegin.o
is in. */
KEEP (*crtbegin.o(.ctors))
KEEP (*crtbegin?.o(.ctors))
/* We don't want to include the .ctor section from
the crtend.o file until after the sorted ctors.
The .ctor section from the crtend file contains the
end of ctors marker and it must be last */
KEEP (*(EXCLUDE_FILE (*crtend.o *crtend?.o ) .ctors))
KEEP (*(SORT(.ctors.*)))
KEEP (*(.ctors))
}
.dtors :
{
KEEP (*crtbegin.o(.dtors))
KEEP (*crtbegin?.o(.dtors))
KEEP (*(EXCLUDE_FILE (*crtend.o *crtend?.o ) .dtors))
KEEP (*(SORT(.dtors.*)))
KEEP (*(.dtors))
}
.jcr : { KEEP (*(.jcr)) }
.data.rel.ro : { *(.data.rel.ro.local* .gnu.linkonce.d.rel.ro.local.*) *(.data.rel.ro .data.rel.ro.* .gnu.linkonce.d.rel.ro.*) }
.dynamic : { *(.dynamic) }
. = DATA_SEGMENT_RELRO_END (0, .);
.data :
{
*(.data .data.* .gnu.linkonce.d.*)
SORT(CONSTRUCTORS)
}
.data1 : { *(.data1) }
.got : { *(.got.plt) *(.igot.plt) *(.got) *(.igot) }
/* We want the small data sections together, so single-instruction offsets
can access them all, and initialized data all before uninitialized, so
we can shorten the on-disk segment size. */
.sdata :
{
__global_pointer$ = . + 0x800;
*(.srodata.cst16) *(.srodata.cst8) *(.srodata.cst4) *(.srodata.cst2) *(.srodata .srodata.*)
*(.sdata .sdata.* .gnu.linkonce.s.*)
}
_edata = .; PROVIDE (edata = .);
. = .;
__bss_start = .;
.sbss :
{
*(.dynsbss)
*(.sbss .sbss.* .gnu.linkonce.sb.*)
*(.scommon)
}
.bss :
{
*(.dynbss)
*(.bss .bss.* .gnu.linkonce.b.*)
*(COMMON)
/* Align here to ensure that the .bss section occupies space up to
_end. Align after .bss to ensure correct alignment even if the
.bss section disappears because there are no input sections.
FIXME: Why do we need it? When there is no .bss section, we don't
pad the .data section. */
. = ALIGN(. != 0 ? 64 / 8 : 1);
}
. = ALIGN(64 / 8);
. = SEGMENT_START("ldata-segment", .);
. = ALIGN(64 / 8);
_end = .; PROVIDE (end = .);
. = DATA_SEGMENT_END (.);
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.stab 0 : { *(.stab) }
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Symbols in the DWARF debugging sections are relative to the beginning
of the section so we begin them at 0. */
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/* GNU DWARF 1 extensions */
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/* DWARF 2 */
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/* SGI/MIPS DWARF 2 extensions */
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.gnu.attributes 0 : { KEEP (*(.gnu.attributes)) }
/DISCARD/ : { *(.note.GNU-stack) *(.gnu_debuglink) *(.gnu.lto_*) }
}
2)startup.s:(学分:here and here).
.section .init, "ax"
.global _start
_start:
.cfi_startproc
.cfi_undefined ra
.option push
.option norelax
la gp, __global_pointer$
.option pop
la sp, __stack_top
add s0, sp, zero
jal zero, main
.cfi_endproc
.end
add.c:(您的代码)
int main() {
int a = 4;
int b = 12;
while (1) {
int c = a + b;
}
return 0;
}
3) 编译/链接,并创建列表:
riscv64-unknown-elf-gcc -g -ffreestanding -O0 -Wl,--gc-sections -nostartfiles -nostdlib -nodefaultlibs -Wl,-T,riscv64-virt.ld -o add.elf startup.s add.c
riscv64-unknown-elf-objdump -D add.elf > add.objdump
4)在控制台中启动qemu:
qemu-system-riscv64 -machine virt -m 128M -gdb tcp::1234,ipv4 -kernel add.elf
我不确定您使用的 qemu 选项:-drive file=a.out,format=raw
是正确的,我认为它们不是,但我没有花时间检查,并使用了我通常使用的选项:-kernel add.elf
4)在另一个控制台中启动gdb(为了方便起见,我在这里使用了我用TUI支持mingw64编译的GDB)。
riscv64-elf-gdb --tui add.elf
(gdb) target remote localhost:1234
Remote debugging using localhost:1234
main () at add.c:5
(gdb) p a
$1 = 4
(gdb) p b
$2 = 12
(gdb) p c
$3 = 16
(gdb)
这可能有点长,但我希望这会有所帮助。
请注意,启动代码对于您的代码来说已经足够好了,但是缺少一些重要的初始化,例如将数据部分从闪存复制到 RAM(此处不相关),以及清除 .bss 部分。