[llvm-dev] Re: Problem with LSR

[Anton Nadolskiy via llvm-dev]

Anyone?

2015-09-17 20:59 GMT+03:00 Anton Nadolskiy <anton.nadolskiy at gmail.com>:

> Hello, folks!
>
> I need help with LSR optimization.
>
> I've spent pretty much trying to understand what's going on here, but
> still don’t understand some things.
>
> *First*, what is Scale in Formula? Consider *for(i=0; i < n; i++)*. The
> initial formula for ICMP use will be:
>
> (1)    reg({%n,+,-1}<nw><%for.body>)
>
> Next we build other forms:
>
>     (2)     reg(%n) + 1*reg({0,+,-1}
>     (3)     reg({(-1 * %n),+,1}
>     (4)     reg((-1 * %n)) + 1*reg({0,+,1}
>     (5)     reg(%n) + -1*reg({0,+,1}
>     (6)     reg((-1 * %n)) + -1*reg({0,+,-1}
>
> I’m wondering about (4) and (6). What’s negative scale? It affects
> formula’s cost, so, for example, (4) loses to (5).
>
> *Second*, we change ICMP to compare against zero, so we get *for(i=n; i >
> 0; i--)* and (1) for initial formula.
>
> But we’re not considering
>     (7)     reg((-1 * %n)) + reg({0,+,1}
>
> which corresponds to our initial *for(i=0; i < n; i++)*.
>
> Again, I don’t know what’s the difference between this and (4) and (6),
> bit it will win (5) at least.
>
>  Now, I also want (7) to win (1), in cases like this
>
>
>
>
>
>
>
>
>
>
> *void foo( int n,  int red[], int green[],  int blue[], int alpha[],
>           int rdest[],  int gdest[], int bdest[],  int adest[] ){
>  for (int i=0;i<n;i++) {               red[i]   = rdest[i];
>  green[i] = gdest[i];               blue[i]  = bdest[i];
>  alpha[i] = adest[i];                   }}*
>
> Just compile it with GCC and CLANG and see how bad the things are. (.text
> section is 64 bytes for GCC and 184 for CLANG). I'm talking about x86, not
> sure for other architectures.
>
> The problem is that we shouldn’t change loop direction, i.e. apply (1). If
> you replace *n* in example above with constant, everything will be
> fine. (because
> *-1024 + reg({0,+,1}* will win *reg({1024,+,-1}*)
>
>  Even with (7) it will always lose to (1), because (1) has less registers.
>
> So, any ideas on this?
>
> P.S. I was thinking about a hack, when in certain conditions we just
> explicitly lower formula’s cost (NumRegs -= 1), but this is probably not
> the best way.
>
>
> Thanks,
> > Anton
>
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