glsl: Make is_16bit_constant from i965 an ir_constant method.

The i965 MUL instruction doesn't natively support 32-bit by 32-bit
integer multiplication; additional instructions (MACH/MOV) are required.
However, we can avoid those if we know one of the operands can be
represented in 16 bits or less.  The vector backend's is_16bit_constant
static helper function checks for this.

We want to be able to use it in the scalar backend as well, which means
moving the function to a more generally-usable location.  Since it isn't
i965 specific, I decided to make it an ir_constant method, in case it
ends up being useful to other people as well.

v2: Rename from is_16bit_integer_constant to is_uint16_constant, as
    suggested by Ilia Mirkin.  Update comments to clarify that it does
    apply to both int and uint types, as long as the value is
    non-negative and fits in 16-bits.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>

src/glsl/ir.cpp

@@ -1223,6 +1223,15 @@ ir_constant::is_basis() const
    return ones == 1;
 }
 
+bool
+ir_constant::is_uint16_constant() const
+{
+   if (!type->is_integer())
+      return false;
+
+   return value.u[0] < (1 << 16);
+}
+
 ir_loop::ir_loop()
 {
    this->ir_type = ir_type_loop;

src/glsl/ir.h

@@ -265,6 +265,13 @@ public:
     */
    virtual bool is_basis() const;
 
+   /**
+    * Determine if an r-value is an unsigned integer constant which can be
+    * stored in 16 bits.
+    *
+    * \sa ir_constant::is_uint16_constant.
+    */
+   virtual bool is_uint16_constant() const { return false; }
 
    /**
     * Return a generic value of error_type.
@@ -2164,6 +2171,14 @@ public:
    virtual bool is_negative_one() const;
    virtual bool is_basis() const;
 
+   /**
+    * Return true for constants that could be stored as 16-bit unsigned values.
+    *
+    * Note that this will return true even for signed integer ir_constants, as
+    * long as the value is non-negative and fits in 16-bits.
+    */
+   virtual bool is_uint16_constant() const;
+
    /**
     * Value of the constant.
     *

src/mesa/drivers/dri/i965/brw_vec4_visitor.cpp

@@ -1174,20 +1174,6 @@ vec4_visitor::emit_lrp(const dst_reg &dst,
    }
 }
 
-static bool
-is_16bit_constant(ir_rvalue *rvalue)
-{
-   ir_constant *constant = rvalue->as_constant();
-   if (!constant)
-      return false;
-
-   if (constant->type != glsl_type::int_type &&
-       constant->type != glsl_type::uint_type)
-      return false;
-
-   return constant->value.u[0] < (1 << 16);
-}
-
 void
 vec4_visitor::visit(ir_expression *ir)
 {
@@ -1371,12 +1357,12 @@ vec4_visitor::visit(ir_expression *ir)
 	  * operand.  If we can determine that one of the args is in the low
 	  * 16 bits, though, we can just emit a single MUL.
           */
-         if (is_16bit_constant(ir->operands[0])) {
+         if (ir->operands[0]->is_uint16_constant()) {
             if (brw->gen < 7)
                emit(MUL(result_dst, op[0], op[1]));
             else
                emit(MUL(result_dst, op[1], op[0]));
-         } else if (is_16bit_constant(ir->operands[1])) {
+         } else if (ir->operands[1]->is_uint16_constant()) {
             if (brw->gen < 7)
                emit(MUL(result_dst, op[1], op[0]));
             else