llvm-6502/lib/Target/PowerPC/README_ALTIVEC.txt

//===- README_ALTIVEC.txt - Notes for improving Altivec code gen ----------===//

Implement PPCInstrInfo::isLoadFromStackSlot/isStoreToStackSlot for vector
registers, to generate better spill code.

//===----------------------------------------------------------------------===//

The first should be a single lvx from the constant pool, the second should be 
a xor/stvx:

void foo(void) {
  int x[8] __attribute__((aligned(128))) = { 1, 1, 1, 17, 1, 1, 1, 1 };
  bar (x);
}

#include <string.h>
void foo(void) {
  int x[8] __attribute__((aligned(128)));
  memset (x, 0, sizeof (x));
  bar (x);
}

//===----------------------------------------------------------------------===//

Altivec: Codegen'ing MUL with vector FMADD should add -0.0, not 0.0:
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=8763

When -ffast-math is on, we can use 0.0.

//===----------------------------------------------------------------------===//

  Consider this:
  v4f32 Vector;
  v4f32 Vector2 = { Vector.X, Vector.X, Vector.X, Vector.X };

Since we know that "Vector" is 16-byte aligned and we know the element offset 
of ".X", we should change the load into a lve*x instruction, instead of doing
a load/store/lve*x sequence.

//===----------------------------------------------------------------------===//

For functions that use altivec AND have calls, we are VRSAVE'ing all call
clobbered regs.

//===----------------------------------------------------------------------===//

Implement passing vectors by value into calls and receiving them as arguments.

//===----------------------------------------------------------------------===//

GCC apparently tries to codegen { C1, C2, Variable, C3 } as a constant pool load
of C1/C2/C3, then a load and vperm of Variable.

//===----------------------------------------------------------------------===//

We need a way to teach tblgen that some operands of an intrinsic are required to
be constants.  The verifier should enforce this constraint.

//===----------------------------------------------------------------------===//

We currently codegen SCALAR_TO_VECTOR as a store of the scalar to a 16-byte
aligned stack slot, followed by a load/vperm.  We should probably just store it
to a scalar stack slot, then use lvsl/vperm to load it.  If the value is already
in memory this is a big win.

//===----------------------------------------------------------------------===//

extract_vector_elt of an arbitrary constant vector can be done with the 
following instructions:

vTemp = vec_splat(v0,2);    // 2 is the element the src is in.
vec_ste(&destloc,0,vTemp);

We can do an arbitrary non-constant value by using lvsr/perm/ste.

//===----------------------------------------------------------------------===//

If we want to tie instruction selection into the scheduler, we can do some
constant formation with different instructions.  For example, we can generate
"vsplti -1" with "vcmpequw R,R" and 1,1,1,1 with "vsubcuw R,R", and 0,0,0,0 with
"vsplti 0" or "vxor", each of which use different execution units, thus could
help scheduling.

This is probably only reasonable for a post-pass scheduler.

//===----------------------------------------------------------------------===//