rv64: bug in encoding of bge?

[Jim Wilson]

On 1/9/19 6:10 AM, Paulo Matos wrote:
> 0000000000000000 <foo>:
>     0:   00c5d063                bge     a1,a2,0 <foo>
>     4:   feb65ee3                bge     a2,a1,0 <foo>
>     8:   fec5fce3                bgeu    a1,a2,0 <foo>
>     c:   feb67ae3                bgeu    a2,a1,0 <foo>
> 
> 
> The first encoding looks fine but the first byte of the others (0xfe)
> just don't look correct. For example, for the second instruction I would
> have expected:
>     4:   00b65063                bge a2,a1,0

The upper bits hold part of the offset.  The first instruction branches 
to itself so the offset is 0.  The following branches go backwards to 
address 0, so the offset is negative.  That causes the upper 7 bits to 
all be 1.

If you want a different view of disassembly, spike-dasm which is part of 
riscv-isa-sim does it a little differently, and can be used to confirm.

gamma05:2030$ cat tmp.file
DASM(feb65ee3)
gamma05:2031$ build-install/bin/spike-dasm < tmp.file
bge     a2, a1, pc - 4
gamma05:2032$

Please see the ISA manual for info about instruction encodings.

Jim