The library code
const
unsigned char __clz_tab[] =
{
0,1,2,2,3,3,3,3,4,4,4,4,4,4,4,4,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,
6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,
7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,
7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,
8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,
8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,
8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,
8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,
};
long foo(long x)
{
return __clz_tab[x % (sizeof(__clz_tab)/sizeof(__clz_tab[0]))];
}
Build the shared(.so) and static(.o) library.
slava@slava-P34V2:/work/risc-v/musl-riscv/tmp$ riscv64-unknown-linux-gnu-gcc -shared -ffreestanding -nostdlib -fPIC -o relocation.so relocation.c
slava@slava-P34V2:/work/risc-v/musl-riscv/tmp$ riscv64-unknown-linux-gnu-gcc -c -ffreestanding -nostdlib -fPIC -o relocation.o relocation.c
The shared library has a
.got (Global Offset Table) section to reference the __clz_tab array. The .got section is adjusted by the loader with the relocation data from the .rela.dyn section.slava@slava-P34V2:/work/risc-v/musl-riscv/tmp$ riscv64-unknown-linux-gnu-objdump -hr relocation.so
relocation.so: file format elf64-littleriscv
Sections:
Idx Name Size VMA LMA File off Algn
0 .hash 00000030 00000000000000e8 00000000000000e8 000000e8 2**3
CONTENTS, ALLOC, LOAD, READONLY, DATA
1 .dynsym 000000a8 0000000000000118 0000000000000118 00000118 2**3
CONTENTS, ALLOC, LOAD, READONLY, DATA
2 .dynstr 00000027 00000000000001c0 00000000000001c0 000001c0 2**0
CONTENTS, ALLOC, LOAD, READONLY, DATA
3 .rela.dyn 00000018 00000000000001e8 00000000000001e8 000001e8 2**3
CONTENTS, ALLOC, LOAD, READONLY, DATA
4 .text 00000038 0000000000000200 0000000000000200 00000200 2**2
CONTENTS, ALLOC, LOAD, READONLY, CODE
5 .rodata 00000100 0000000000000238 0000000000000238 00000238 2**3
CONTENTS, ALLOC, LOAD, READONLY, DATA
6 .dynamic 000000e0 0000000000001338 0000000000001338 00000338 2**3
CONTENTS, ALLOC, LOAD, DATA
7 .got 00000020 0000000000001418 0000000000001418 00000418 2**3
CONTENTS, ALLOC, LOAD, DATA
8 .comment 00000011 0000000000000000 0000000000000000 00000438 2**0
CONTENTS, READONLY
slava@slava-P34V2:/work/risc-v/musl-riscv/tmp$ riscv64-unknown-linux-gnu-objdump --disassemble relocation.so
relocation.so: file format elf64-littleriscv
Disassembly of section .text:
0000000000000200 <foo>:
200: fe010113 addi sp,sp,-32
204: 00813c23 sd s0,24(sp)
208: 02010413 addi s0,sp,32
20c: fea43423 sd a0,-24(s0)
210: fe843783 ld a5,-24(s0)
214: 0ff7f793 andi a5,a5,255
218: 00001717 auipc a4,0x1
21c: 21873703 ld a4,536(a4) # 1430 <_GLOBAL_OFFSET_TABLE_+0x8>
220: 00f707b3 add a5,a4,a5
224: 0007c783 lbu a5,0(a5)
228: 00078513 mv a0,a5
22c: 01813403 ld s0,24(sp)
230: 02010113 addi sp,sp,32
234: 00008067 ret
slava@slava-P34V2:/work/risc-v/musl-riscv/tmp$ riscv64-unknown-linux-gnu-readelf -r relocation.so
Relocation section '.rela.dyn' at offset 0x1e8 contains 1 entries:
Offset Info Type Sym. Value Sym. Name + Addend
000000001430 000300000002 R_RISCV_64 0000000000000238 __clz_tab + 0
slava@slava-P34V2:/work/risc-v/musl-riscv/tmp$ riscv64-unknown-linux-gnu-objdump -hr relocation.o
relocation.o: file format elf64-littleriscv
Sections:
Idx Name Size VMA LMA File off Algn
0 .text 00000038 0000000000000000 0000000000000000 00000040 2**2
CONTENTS, ALLOC, LOAD, RELOC, READONLY, CODE
1 .data 00000000 0000000000000000 0000000000000000 00000078 2**0
CONTENTS, ALLOC, LOAD, DATA
2 .bss 00000000 0000000000000000 0000000000000000 00000078 2**0
ALLOC
3 .rodata 00000100 0000000000000000 0000000000000000 00000078 2**3
CONTENTS, ALLOC, LOAD, READONLY, DATA
4 .comment 00000012 0000000000000000 0000000000000000 00000178 2**0
CONTENTS, READONLY
RELOCATION RECORDS FOR [.text]:
OFFSET TYPE VALUE
0000000000000018 R_RISCV_GOT_HI20 __clz_tab
000000000000001c R_RISCV_PCREL_LO12_I .L0
The static library also has a
.got (Global Offset Table) section to reference the __clz_tab array. The .got section is adjusted by the loader with the relocation data from the .rela.dyn section.slava@slava-P34V2:/work/risc-v/musl-riscv/tmp$ riscv64-unknown-linux-gnu-objdump -hr relocation.o
relocation.o: file format elf64-littleriscv
Sections:
Idx Name Size VMA LMA File off Algn
0 .text 00000038 0000000000000000 0000000000000000 00000040 2**2
CONTENTS, ALLOC, LOAD, RELOC, READONLY, CODE
1 .data 00000000 0000000000000000 0000000000000000 00000078 2**0
CONTENTS, ALLOC, LOAD, DATA
2 .bss 00000000 0000000000000000 0000000000000000 00000078 2**0
ALLOC
3 .rodata 00000100 0000000000000000 0000000000000000 00000078 2**3
CONTENTS, ALLOC, LOAD, READONLY, DATA
4 .comment 00000012 0000000000000000 0000000000000000 00000178 2**0
CONTENTS, READONLY
RELOCATION RECORDS FOR [.text]:
OFFSET TYPE VALUE
0000000000000018 R_RISCV_GOT_HI20 __clz_tab
000000000000001c R_RISCV_PCREL_LO12_I .L0
slava@slava-P34V2:/work/risc-v/musl-riscv/tmp$ riscv64-unknown-linux-gnu-objdump --disassemble relocation.o
relocation.o: file format elf64-littleriscv
Disassembly of section .text:
0000000000000000 <foo>:
0: fe010113 addi sp,sp,-32
4: 00813c23 sd s0,24(sp)
8: 02010413 addi s0,sp,32
c: fea43423 sd a0,-24(s0)
10: fe843783 ld a5,-24(s0)
14: 0ff7f793 andi a5,a5,255
0000000000000018 <.L0 >:
18: 00000717 auipc a4,0x0
1c: 00073703 ld a4,0(a4) # 18 <.L0 >
20: 00f707b3 add a5,a4,a5
24: 0007c783 lbu a5,0(a5)
28: 00078513 mv a0,a5
2c: 01813403 ld s0,24(sp)
30: 02010113 addi sp,sp,32
34: 00008067 ret
slava@slava-P34V2:/work/risc-v/musl-riscv/tmp$ riscv64-unknown-linux-gnu-readelf -r relocation.o
Relocation section '.rela.text' at offset 0x2a8 contains 2 entries:
Offset Info Type Sym. Value Sym. Name + Addend
000000000018 000800000014 R_RISCV_GOT_HI20 0000000000000000 __clz_tab + 0
00000000001c 000600000018 R_RISCV_PCREL_LO1 0000000000000018 .L0 + 0
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