llvm-6502/lib/Target

Target Independent Opportunities:

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FreeBench/mason contains code like this:

static p_type m0u(p_type p) {
  int m[]={0, 8, 1, 2, 16, 5, 13, 7, 14, 9, 3, 4, 11, 12, 15, 10, 17, 6};
  p_type pu;
  pu.a = m[p.a];
  pu.b = m[p.b];
  pu.c = m[p.c];
  return pu;
}

We currently compile this into a memcpy from a static array into 'm', then
a bunch of loads from m.  It would be better to avoid the memcpy and just do
loads from the static array.

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Get the C front-end to expand hypot(x,y) -> llvm.sqrt(x*x+y*y) when errno and
precision don't matter (ffastmath).  Misc/mandel will like this. :)

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For all targets, not just X86:
When llvm.memcpy, llvm.memset, or llvm.memmove are lowered, they should be 
optimized to a few store instructions if the source is constant and the length
is smallish (< 8). This will greatly help some tests like Shootout/strcat.c
and fldry.

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Solve this DAG isel folding deficiency:

int X, Y;

void fn1(void)
{
  X = X | (Y << 3);
}

compiles to

fn1:
	movl Y, %eax
	shll $3, %eax
	orl X, %eax
	movl %eax, X
	ret

The problem is the store's chain operand is not the load X but rather
a TokenFactor of the load X and load Y, which prevents the folding.

There are two ways to fix this:

1. The dag combiner can start using alias analysis to realize that y/x
   don't alias, making the store to X not dependent on the load from Y.
2. The generated isel could be made smarter in the case it can't
   disambiguate the pointers.

Number 1 is the preferred solution.

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For dag combiner and instcombine:
Fold: "A / (B << N)" where B is a power of 2, to "A >> (N + log2(B))".
Fold: "A % (B << N)" where B is a power of 2, to "A & ((B << N) - 1)".

int t(int X, int Y) { return 1 << (X+4); }  --> 16 << X


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