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Re: [PATCH, PR 10474] Shedule pass_cprop_hardreg before pass_thread_prologue_and_epilogue


Hi,

On Fri, Apr 19, 2013 at 12:37:58AM +0200, Steven Bosscher wrote:
> On Fri, Apr 19, 2013 at 12:09 AM, Martin Jambor wrote:
> > I also have not tried scheduling the hard register copy propagation
> > pass twice and measuring the impact on compile times.  Any suggestion
> > what might be a good testcase for that?
> 
> I think a better question is when this would be useful in the first
> place, and why. In other words: If you propagate hardregs before
> shrink wrapping, what could be a source of new opportunities after
> shrink wrapping?

Yes, we also did that and neither I nor Honza could think of any
potential problems there.  And of course, I'd also measure how many
statements the second run of the pass changed.  I'll probably do that
tomorrow anyway.

> 
> 
> The only things I can think of, given the current pass order, are:
> 
> * different basic block order due to shrink wrapping
>  regcprop's effectiveness depends on the order of the basic blocks
> (unfortunately)
> 
> * different basic block contents due to head/tail merging (pass_jump2)
>  Head/tail merging extends some basic blocks and shortens others. The
> elongated basic blocks may present new opportunities (regcprop is a
> local pass).
> 
> * different basic block contents due to dead store elimination (pass_dse2)
>  A removed dead store may also make an address calculation redundant,
> changing the regcprop value chains.
> 
> * different basic block contents due to peephole2
>  A peephole2 may present new regcprop opportunities, peephole2 misses
> the context to avoid trivial copies.
> 
> 
> On the other hand, running regcprop earlier also helps some other
> passes. For example, I think regcprop before jump2 may result in more
> successful head/tail merging attempts by making more input operands
> match, but it could hurt if_after_reload by extending live times of
> registers.
> 
> 
> But wouldn't it be better to avoid these argument-register pseudos
> being assigned to callee-saved registers? Perhaps splitting the live
> range of the pseudos before the first call on each path will do the
> trick, and let IRA pick the right registers for you instead.

First, where can I have a look how a live range is split?  ;-)

But second, if such a call is in a loop (or accessible by more than
one path), I wouldn't it be wrong to do that?  To avoid that, I
suppose I might end up doing another shrink-wrapping-like search for
the right edge for prologue and actually coming up with a very similar
result to the propagation and shrink-wrapping preparation.  But I'm
willing to try.

Thanks a lot,

Martin


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