Use ILP to extract more ILP

Article: Escape From the Planet of x86
By: Paul DeMone (pdemone.delete@this.igs.net), June 20, 2003 10:48 pm
Room: Moderated Discussions
Bill Todd (billtodd@metrocast.net) on 6/20/03 wrote:
---------------------------
[...]
>Not in the short term that we're talking about. Multiple well-funded and highly-qualified
>compiler teams have been working on EPIC compilers for half a decade or more now,
>so while it's not unreasonable to speculate that there may still be a good deal
>more compiler potential to mine all the low-hanging fruit was picked quite some
>time ago and that potential is likely to be realized only gradually (if at all)
>over time.

Winkel's 2002 paper on Optimal global scheduling for IPF processors suggests
that integer linear programming (ILP) techniques can be used to obtain provably
optimal code scheduling. He used Vtune to identify hot code sections in various
SPECint2k programs and found that even with preliminary algorithms that take
advantage of only basic IPF features he could reduce code schedule length of
fully optimized code from the Intel IPF compiler by an average of 30% for an I2.
For an in-order machine that basically means 30% faster code.

This intense form of optimization is currently too computationally intensive
for use over an entire program but the technique can be used selectively as
directed by feedback driven optimization. Or perhaps slightly relaxed versions
of the technique based on heuristics may be developed that don't quite give
perfect code scheduling all the time but are many times faster and can be
employed over the entire program.

> By contrast, my impression is that the AMD64 compiler efforts started
>considerably more recently and have been far less well funded: AFAIK we have only
>a single 64-bit SPECint result to look at so far (from the gcc compiler; nothing
>from either the Portland or Microsoft compilers), not a couple of generations' worth
>on multiple compilers as Itanic has.

Compiling for both x86 and RISC has been studied for a quarter century.
Adding 8 more GPRs to x86 is not going to open too many new doors.
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