nvc0/ir: replace cvt instructions with add to improve shader performance

gives me an performance boost of 0.2% in pixmark_piano on my gk106, gm204 and
gp107.

reduces the amount of generated convert instructions by roughly 30% in
shader-db.

v2: only for 32 bit operations
    move some common code out of the switch
    handle OP_SAT with modifiers
v3: only for registers and const memory
    rework if clauses
    merge isCvt into this patch
v4: merge isCvt into its use

Signed-off-by: Karol Herbst <kherbst@redhat.com>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>

src/gallium/drivers/nouveau/codegen/nv50_ir_lowering_nvc0.cpp

@@ -719,6 +719,66 @@ NVC0LegalizePostRA::propagateJoin(BasicBlock *bb)
    bb->remove(bb->getEntry());
 }
 
+// replaces instructions which would end up as f2f or i2i with faster
+// alternatives:
+//  - fabs(a)     -> fadd(0, abs a)
+//  - fneg(a)     -> fadd(neg 0, neg a)
+//  - ineg(a)     -> iadd(0, neg a)
+//  - fneg(abs a) -> fadd(neg 0, neg abs a)
+//  - sat(a)      -> sat add(0, a)
+void
+NVC0LegalizePostRA::replaceCvt(Instruction *cvt)
+{
+   if (!isFloatType(cvt->sType) && typeSizeof(cvt->sType) != 4)
+      return;
+   if (cvt->sType != cvt->dType)
+      return;
+   // we could make it work, but in this case we have optimizations disabled
+   // and we don't really care either way.
+   if (cvt->src(0).getFile() != FILE_GPR &&
+       cvt->src(0).getFile() != FILE_MEMORY_CONST)
+      return;
+
+   Modifier mod0, mod1;
+
+   switch (cvt->op) {
+   case OP_ABS:
+      if (cvt->src(0).mod)
+         return;
+      if (!isFloatType(cvt->sType))
+         return;
+      mod0 = 0;
+      mod1 = NV50_IR_MOD_ABS;
+      break;
+   case OP_NEG:
+      if (!isFloatType(cvt->sType) && cvt->src(0).mod)
+         return;
+      if (isFloatType(cvt->sType) &&
+          (cvt->src(0).mod && cvt->src(0).mod != Modifier(NV50_IR_MOD_ABS)))
+         return;
+
+      mod0 = isFloatType(cvt->sType) ? NV50_IR_MOD_NEG : 0;
+      mod1 = cvt->src(0).mod == Modifier(NV50_IR_MOD_ABS) ?
+         NV50_IR_MOD_NEG_ABS : NV50_IR_MOD_NEG;
+      break;
+   case OP_SAT:
+      if (!isFloatType(cvt->sType) && cvt->src(0).mod.abs())
+         return;
+      mod0 = 0;
+      mod1 = cvt->src(0).mod;
+      cvt->saturate = true;
+      break;
+   default:
+      return;
+   }
+
+   cvt->op = OP_ADD;
+   cvt->moveSources(0, 1);
+   cvt->setSrc(0, rZero);
+   cvt->src(0).mod = mod0;
+   cvt->src(1).mod = mod1;
+}
+
 bool
 NVC0LegalizePostRA::visit(BasicBlock *bb)
 {
@@ -758,6 +818,9 @@ NVC0LegalizePostRA::visit(BasicBlock *bb)
                next = hi;
          }
 
+         if (i->op == OP_SAT || i->op == OP_NEG || i->op == OP_ABS)
+            replaceCvt(i);
+
          if (i->op != OP_MOV && i->op != OP_PFETCH)
             replaceZero(i);
       }

src/gallium/drivers/nouveau/codegen/nv50_ir_lowering_nvc0.h

@@ -81,6 +81,7 @@ private:
    virtual bool visit(Function *);
    virtual bool visit(BasicBlock *);
 
+   void replaceCvt(Instruction *);
    void replaceZero(Instruction *);
    bool tryReplaceContWithBra(BasicBlock *);
    void propagateJoin(BasicBlock *);