Backward jump by -2048 does not generate c.j

[davidharrishmc]

I have a backward jump of -2048 = 0x800. I expect RISC-V GCC would produce c.j -2048 (0xB001). Instead, it gives an uncompressed jump by -2048 (0x801ff06f).

Is this the intended behavior, or is there a code generation bug?

This can be reproduced with the following code. The unexpected uncompressed instruction is at 11804:

0000000000011800 <forward2048>:
   11800:	003100b3          	add	ra,sp,gp
   11804:	801ff06f          	j	11004 <back2048>


------ code

# cj.S
# Test case to illustrate GCC not generating compresed backward branches by -2048
# compile with
# riscv64-unknown-elf-gcc -march=rv64gc cj.S -o cj.elf
# riscv64-unknown-elf-objdump -D cj.elf > cj.elf.objdump

.global main

main: 
    j forward2048
.align 2
back2048:   
    j done

.align 11
forward2048: # address 80000800
    add x1, x2, x3 # dummy 4-byte instruction
    # j back2048 is a backward branch by a distance -2048 
    # I expect this should assemble into c.j -2048 (0xB001)
    # Instead, GCC gives  11804:	801ff06f          	j	11004 <back2048>
    j back2048

done:
    add x4, x5, x6
    
self_loop:
    j self_loop

[davidharrishmc]

c.jal has the same behavior.

I would have expected this backward jal by 0x800 to produce machine language code 3001 rather than 801ff0ef.

80003800: 801ff0ef jal 80003000 <b11_c.jal>