[Bug gas/22598] [RISCV] No way to disable two-instruction sequences for branch or relocation for jal instructions

[wilson at gcc dot gnu.org]

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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