Re: negative latencies

[Vladimir Makarov <vmakarov at redhat dot com>]

On 2014-05-23, 3:49 AM, shmeel gutl wrote:
> On 21-May-14 06:30 PM, Vladimir Makarov wrote:
> > I am just curious what happens when you put
> >
> > insn2, insn1.
> >
> > and insn2 uses a result of insn1 in 6 cycles and insn1 producing the
> > result in 3 cycles, but there are not ready functional units (e.g.
> > arithmentic units) necessary for insn1 for 4 or more cycles.  It is
> > quite not trivial to guarantee that everything will be okay in general
> > case if you put insn2 before insn1.
> This is not a problem for this architecture. The units are fully
> pipelined and the only conflicts are in the first stage, during
> instruction issue. That is, the vliw must be legal. The gcc dfa handles
> this case fine.

Another problem is that besides insn-scheduler there are a lot of 
optimizations which can insert some insns between the two insns after 
scheduling.  In this case the result might be not ready for insn2.

So you at least should exclude 1st insn scheduling (before RA) and make 
2nd insn scheduling as the very last pass.

In general, a traditional approach is to do such things on assembler 
level (e.g. as for older MIPS processor without hardware interlocks).