Weight variables by their number of uses for register allocation.

src/IceInst.cpp

Index: src/IceInst.cpp
diff --git a/src/IceInst.cpp b/src/IceInst.cpp
index 4b89eb77b0fa88f5f692f637a5ef892305b1a62b..252cd51fd6bfb746eea2a0959ff6f5d6023d1848 100644
--- a/src/IceInst.cpp
+++ b/src/IceInst.cpp
@@ -179,6 +179,8 @@ bool Inst::liveness(InstNumberT InstNumber, LivenessBV &Live,
 
   Dead = false;
   if (Dest) {
+    // TODO(ascull): get the loop nest depth
+    Dest->addUse(1);

[jvoung (off chromium), 2015/08/26 21:45:46]

Hmm, not sure we should count definitions as a Use (would that inflate the
weight in a way that is undesirable, e.g., with a sequence of two-address
instructions?)

[ascull, 2015/08/26 22:06:38]

If there are two uses of a variable in a function doesn't that mean the variable
should have higher register priority? Or is it no possible to have two memory
arguments?

[jvoung (off chromium), 2015/08/26 22:35:16]

Sorry, by two-address I meant, for example, add instructions that can only do:

a += b

vs an add instruction that can do:

a = b + c

I didn't mean address as something related to a memory operand.

Yes, to "If there are two uses of a variable in a function doesn't that mean the
variable
should have higher register priority?"

What I meant was here you're counting a use whenever a variable defined (Dest),
not just when it's used as a (Src).  In any case, this might be okay.  Every
instance of a "Dest" is a potential "Spill", and a every instance of a "Src" may
be a potential "Fill", and you can check the stats before and after this change
with "--szstats".

[ascull, 2015/08/26 22:53:49]

This is what I meant it to do. But thinking again SSA means this has little/no
impact. There might be some custom code from a TargetLowering that would
benefit.

I've removed this for now.

     SizeT VarNum = Liveness->getLiveIndex(Dest->getIndex());
     if (Live[VarNum]) {
       if (!isDestNonKillable()) {
@@ -203,7 +205,9 @@ bool Inst::liveness(InstNumberT InstNumber, LivenessBV &Live,
     Operand *Src = getSrc(I);
     SizeT NumVars = Src->getNumVars();
     for (SizeT J = 0; J < NumVars; ++J, ++VarIndex) {
-      const Variable *Var = Src->getVar(J);
+      Variable *Var = Src->getVar(J);
+      // TODO(ascull): get the loop nest depth
+      Var->addUse(1);
       SizeT VarNum = Liveness->getLiveIndex(Var->getIndex());
       if (!Live[VarNum]) {
         setLastUse(VarIndex);