Branch instructions that depend on target distance

[Andrew Stubbs]

On 24/02/2020 11:05, Petr Tesarik wrote:
> Hi all,
> 
> I'm looking into reviving the efforts to port gcc to VideoCore IV [1].
> One issue I've run into is the need to find out target branch distance
> at compile time. I looked around, and it's not the first one
> architecture with such requirement, but AFAICS it has never been solved
> properly.
> 
> For example, AVR tracks instruction length. Later, ret_cond_branch()
> selects between a branch instruction and an inverted branch followed by
> an unconditional jump based on these calculated lengths.
> 
> This works great ... until there's some inline asm() statement, for
> which gcc cannot keep track of the length attribute, so it is probably
> taken as zero. Linker then fails with a cryptic message:
> 
>> relocation truncated to fit: R_AVR_7_PCREL against `no symbol'

You can probably fix this by implementing the ADJUST_INSN_LENGTH macro 
and recognising the inline assembler. See the internals manual.

We encountered similar issues with the recent GCN port, and the correct 
solution was to add the length attribute everywhere. The attributes are 
often conservative estimates (rather than having extra alternatives for 
every possible encoding), so the asm problem is mitigated somewhat, at 
the cost of a few "far" branches where they're not strictly necessary.

There were also addition problems because "far" branches clobber the 
condition register, and "near" branches do not, but that's another story.

Andrew