https://sourceware.org/bugzilla/show_bug.cgi?id=22598
--- Comment #8 from Jim Wilson <wilson at gcc dot gnu.org> ---
Branches work the same way on RISC-V as they do on MIPS. Here is a mips
example to show that.
rohan:2123$ cat tmp.s
_start:
.skip 4096
beq $6, $7, 100
bne $4, $5, 4096
.skip 92
_end_label:
rohan:2124$ ./as-new -o tmp.o tmp.s
rohan:2125$ ../binutils/objdump -dr tmp.o
tmp.o: file format elf32-ntradbigmips
Disassembly of section .text:
00000000 <_start>:
...
1000: 10c70019 beq a2,a3,1068 <_start+0x1068>
1000: R_MIPS_PC16 *ABS*+0x60
1004: 00000000 nop
1008: 14850400 bne a0,a1,200c <_end_label+0xfa0>
1008: R_MIPS_PC16 *ABS*+0xffc
100c: 00000000 nop
...
0000106c <_end_label>:
106c: 00000000 nop
rohan:2126$ ../ld-new/ld tmp.o
bash: ../ld-new/ld: No such file or directory
rohan:2127$ ../ld/ld-new tmp.o
../ld/ld-new: warning: cannot find entry symbol __start; defaulting to
00000000100000d0
tmp.o: In function `_start':
(.text+0x1000): relocation truncated to fit: R_MIPS_PC16 against `*UND*'
(.text+0x1008): relocation truncated to fit: R_MIPS_PC16 against `*UND*'
rohan:2128$
Note that a branch to 100 is a request to branch to the absolute address 100,
which gives a linker error because that ends up out-of-range.
If I use the . +/- immediate syntax, I get the result you want: branches to
offsets from the current address.
rohan:2129$ cat tmp.s
_start:
.skip 4096
beq $6, $7, .+100
bne $4, $5, .-4096
.skip 92
_end_label:
rohan:2130$ ./as-new -o tmp.o tmp.s
rohan:2131$ ../ld/ld-new tmp.o
../ld/ld-new: warning: cannot find entry symbol __start; defaulting to
00000000100000d0
rohan:2132$ ../binutils/objdump -dr a.out
a.out: file format elf32-ntradbigmips
Disassembly of section .text:
100000d0 <_ftext>:
...
100010d0: 10c70018 beq a2,a3,10001134 <_ftext+0x1064>
100010d4: 00000000 nop
100010d8: 1485fbff bne a0,a1,100000d8 <_ftext+0x8>
100010dc: 00000000 nop
...
1000113c <_end_label>:
1000113c: 00000000 nop
rohan:2133$
This handling of constant immediates is the same as how we handle labels. If
you say
bne a4, a5, _start+4
then _start+4 is an absolute address to branch to. It is not a value to add to
the current PC to generate an address. It is not an offset to be inserted
directly into the instruction. It is a value to be converted into an offset by
subtracting the current PC. So immediates are being handled exactly the same
as labels.
The only difference with RISC-V is that we relax branches by default, and hence
you get these branch sequences instead of a single branch instruction. If you
provide an address that the assembler can detect is safe at assembly time, then
you can get a single branch. You can do that by using the . +/- immediate
syntax, or by using a label close to the branch instruction. Otherwise, the
assembler must emit a branch sequence that may be relaxed at link time.
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