glsl: Add lowering pass for GLSL ES 3.00 pack/unpack operations (v4)

Lower them to arithmetic and bit manipulation expressions.

v2: Rewrite using ir_builder [for idr].
v3: Comment typos. [for mattst88]
v4: Fix arithmetic error in comments.
    Factor out a shift instruction.
    Don't heap allocate factory.instructions.
    [for paul]

Reviewed-by: Ian Romanick <[email protected]> (v2)
Reviewed-by: Matt Tuner <[email protected]> (v3)
Reviewed-by: Paul Berry <[email protected]> (v4)
Signed-off-by: Chad Versace <[email protected]>

1 files changed, 1035 insertions, 0 deletions

diff --git a/src/glsl/lower_packing_builtins.cpp b/src/glsl/lower_packing_builtins.cpp
new file mode 100644
index 00000000000..136d4cdfb32
--- /dev/null
+++ b/src/glsl/lower_packing_builtins.cpp
@@ -0,0 +1,1035 @@
+/*
+ * Copyright © 2012 Intel Corporation
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a
+ * copy of this software and associated documentation files (the "Software"),
+ * to deal in the Software without restriction, including without limitation
+ * the rights to use, copy, modify, merge, publish, distribute, sublicense,
+ * and/or sell copies of the Software, and to permit persons to whom the
+ * Software is furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice (including the next
+ * paragraph) shall be included in all copies or substantial portions of the
+ * Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
+ * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+ * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+ * DEALINGS IN THE SOFTWARE.
+ */
+
+#include "ir.h"
+#include "ir_builder.h"
+#include "ir_optimization.h"
+#include "ir_rvalue_visitor.h"
+
+namespace {
+
+using namespace ir_builder;
+
+/**
+ * A visitor that lowers built-in floating-point pack/unpack expressions
+ * such packSnorm2x16.
+ */
+class lower_packing_builtins_visitor : public ir_rvalue_visitor {
+public:
+   /**
+    * \param op_mask is a bitmask of `enum lower_packing_builtins_op`
+    */
+   explicit lower_packing_builtins_visitor(int op_mask)
+      : op_mask(op_mask),
+        progress(false)
+   {
+      /* Mutually exclusive options. */
+      assert(!((op_mask & LOWER_PACK_HALF_2x16) &&
+               (op_mask & LOWER_PACK_HALF_2x16_TO_SPLIT)));
+
+      assert(!((op_mask & LOWER_UNPACK_HALF_2x16) &&
+               (op_mask & LOWER_UNPACK_HALF_2x16_TO_SPLIT)));
+
+      factory.instructions = &factory_instructions;
+   }
+
+   virtual ~lower_packing_builtins_visitor()
+   {
+      assert(factory_instructions.is_empty());
+   }
+
+   bool get_progress() { return progress; }
+
+   void handle_rvalue(ir_rvalue **rvalue)
+   {
+      if (!*rvalue)
+	 return;
+
+      ir_expression *expr = (*rvalue)->as_expression();
+      if (!expr)
+	 return;
+
+      enum lower_packing_builtins_op lowering_op =
+         choose_lowering_op(expr->operation);
+
+      if (lowering_op == LOWER_PACK_UNPACK_NONE)
+         return;
+
+      setup_factory(ralloc_parent(expr));
+
+      ir_rvalue *op0 = expr->operands[0];
+      ralloc_steal(factory.mem_ctx, op0);
+
+      switch (lowering_op) {
+      case LOWER_PACK_SNORM_2x16:
+         *rvalue = lower_pack_snorm_2x16(op0);
+         break;
+      case LOWER_PACK_UNORM_2x16:
+         *rvalue = lower_pack_unorm_2x16(op0);
+         break;
+      case LOWER_PACK_HALF_2x16:
+         *rvalue = lower_pack_half_2x16(op0);
+         break;
+      case LOWER_PACK_HALF_2x16_TO_SPLIT:
+         *rvalue = split_pack_half_2x16(op0);
+         break;
+      case LOWER_UNPACK_SNORM_2x16:
+         *rvalue = lower_unpack_snorm_2x16(op0);
+         break;
+      case LOWER_UNPACK_UNORM_2x16:
+         *rvalue = lower_unpack_unorm_2x16(op0);
+         break;
+      case LOWER_UNPACK_HALF_2x16:
+         *rvalue = lower_unpack_half_2x16(op0);
+         break;
+      case LOWER_UNPACK_HALF_2x16_TO_SPLIT:
+         *rvalue = split_unpack_half_2x16(op0);
+         break;
+      case LOWER_PACK_UNPACK_NONE:
+         assert(!"not reached");
+         break;
+      }
+
+      teardown_factory();
+      progress = true;
+   }
+
+private:
+   const int op_mask;
+   bool progress;
+   ir_factory factory;
+   exec_list factory_instructions;
+
+   /**
+    * Determine the needed lowering operation by filtering \a expr_op
+    * through \ref op_mask.
+    */
+   enum lower_packing_builtins_op
+   choose_lowering_op(ir_expression_operation expr_op)
+   {
+      /* C++ regards int and enum as fundamentally different types.
+       * So, we can't simply return from each case; we must cast the return
+       * value.
+       */
+      int result;
+
+      switch (expr_op) {
+      case ir_unop_pack_snorm_2x16:
+         result = op_mask & LOWER_PACK_SNORM_2x16;
+         break;
+      case ir_unop_pack_unorm_2x16:
+         result = op_mask & LOWER_PACK_UNORM_2x16;
+         break;
+      case ir_unop_pack_half_2x16:
+         result = op_mask & (LOWER_PACK_HALF_2x16 | LOWER_PACK_HALF_2x16_TO_SPLIT);
+         break;
+      case ir_unop_unpack_snorm_2x16:
+         result = op_mask & LOWER_UNPACK_SNORM_2x16;
+         break;
+      case ir_unop_unpack_unorm_2x16:
+         result = op_mask & LOWER_UNPACK_UNORM_2x16;
+         break;
+      case ir_unop_unpack_half_2x16:
+         result = op_mask & (LOWER_UNPACK_HALF_2x16 | LOWER_UNPACK_HALF_2x16_TO_SPLIT);
+         break;
+      default:
+         result = LOWER_PACK_UNPACK_NONE;
+         break;
+      }
+
+      return static_cast<enum lower_packing_builtins_op>(result);
+   }
+
+   void
+   setup_factory(void *mem_ctx)
+   {
+      assert(factory.mem_ctx == NULL);
+      assert(factory.instructions->is_empty());
+
+      factory.mem_ctx = mem_ctx;
+   }
+
+   void
+   teardown_factory()
+   {
+      base_ir->insert_before(factory.instructions);
+      assert(factory.instructions->is_empty());
+      factory.mem_ctx = NULL;
+   }
+
+   template <typename T>
+   ir_constant*
+   constant(T x)
+   {
+      return factory.constant(x);
+   }
+
+   /**
+    * \brief Pack two uint16's into a single uint32.
+    *
+    * Interpret the given uvec2 as a uint16 pair. Pack the pair into a uint32
+    * where the least significant bits specify the first element of the pair.
+    * Return the uint32.
+    */
+   ir_rvalue*
+   pack_uvec2_to_uint(ir_rvalue *uvec2_rval)
+   {
+      assert(uvec2_rval->type == glsl_type::uvec2_type);
+
+      /* uvec2 u = UVEC2_RVAL; */
+      ir_variable *u = factory.make_temp(glsl_type::uvec2_type,
+                                          "tmp_pack_uvec2_to_uint");
+      factory.emit(assign(u, uvec2_rval));
+
+      /* return (u.y << 16) | (u.x & 0xffff); */
+      return bit_or(lshift(swizzle_y(u), constant(16u)),
+                    bit_and(swizzle_x(u), constant(0xffffu)));
+   }
+
+   /**
+    * \brief Unpack a uint32 into two uint16's.
+    *
+    * Interpret the given uint32 as a uint16 pair where the uint32's least
+    * significant bits specify the pair's first element. Return the uint16
+    * pair as a uvec2.
+    */
+   ir_rvalue*
+   unpack_uint_to_uvec2(ir_rvalue *uint_rval)
+   {
+      assert(uint_rval->type == glsl_type::uint_type);
+
+      /* uint u = UINT_RVAL; */
+      ir_variable *u = factory.make_temp(glsl_type::uint_type,
+                                          "tmp_unpack_uint_to_uvec2_u");
+      factory.emit(assign(u, uint_rval));
+
+      /* uvec2 u2; */
+      ir_variable *u2 = factory.make_temp(glsl_type::uvec2_type,
+                                           "tmp_unpack_uint_to_uvec2_u2");
+
+      /* u2.x = u & 0xffffu; */
+      factory.emit(assign(u2, bit_and(u, constant(0xffffu)), WRITEMASK_X));
+
+      /* u2.y = u >> 16u; */
+      factory.emit(assign(u2, rshift(u, constant(16u)), WRITEMASK_Y));
+
+      return deref(u2).val;
+   }
+
+   /**
+    * \brief Lower a packSnorm2x16 expression.
+    *
+    * \param vec2_rval is packSnorm2x16's input
+    * \return packSnorm2x16's output as a uint rvalue
+    */
+   ir_rvalue*
+   lower_pack_snorm_2x16(ir_rvalue *vec2_rval)
+   {
+      /* From page 88 (94 of pdf) of the GLSL ES 3.00 spec:
+       *
+       *    highp uint packSnorm2x16(vec2 v)
+       *    --------------------------------
+       *    First, converts each component of the normalized floating-point value
+       *    v into 16-bit integer values. Then, the results are packed into the
+       *    returned 32-bit unsigned integer.
+       *
+       *    The conversion for component c of v to fixed point is done as
+       *    follows:
+       *
+       *       packSnorm2x16: round(clamp(c, -1, +1) * 32767.0)
+       *
+       *    The first component of the vector will be written to the least
+       *    significant bits of the output; the last component will be written to
+       *    the most significant bits.
+       *
+       * This function generates IR that approximates the following pseudo-GLSL:
+       *
+       *     return pack_uvec2_to_uint(
+       *         uvec2(ivec2(
+       *           round(clamp(VEC2_RVALUE, -1.0f, 1.0f) * 32767.0f))));
+       *
+       * It is necessary to first convert the vec2 to ivec2 rather than directly
+       * converting vec2 to uvec2 because the latter conversion is undefined.
+       * From page 56 (62 of pdf) of the GLSL ES 3.00 spec: "It is undefined to
+       * convert a negative floating point value to an uint".
+       */
+      assert(vec2_rval->type == glsl_type::vec2_type);
+
+      ir_rvalue *result = pack_uvec2_to_uint(
+            i2u(f2i(round_even(mul(clamp(vec2_rval,
+                                         constant(-1.0f),
+                                         constant(1.0f)),
+                                   constant(32767.0f))))));
+
+      assert(result->type == glsl_type::uint_type);
+      return result;
+   }
+
+   /**
+    * \brief Lower an unpackSnorm2x16 expression.
+    *
+    * \param uint_rval is unpackSnorm2x16's input
+    * \return unpackSnorm2x16's output as a vec2 rvalue
+    */
+   ir_rvalue*
+   lower_unpack_snorm_2x16(ir_rvalue *uint_rval)
+   {
+      /* From page 88 (94 of pdf) of the GLSL ES 3.00 spec:
+       *
+       *    highp vec2 unpackSnorm2x16 (highp uint p)
+       *    -----------------------------------------
+       *    First, unpacks a single 32-bit unsigned integer p into a pair of
+       *    16-bit unsigned integers. Then, each component is converted to
+       *    a normalized floating-point value to generate the returned
+       *    two-component vector.
+       *
+       *    The conversion for unpacked fixed-point value f to floating point is
+       *    done as follows:
+       *
+       *       unpackSnorm2x16: clamp(f / 32767.0, -1,+1)
+       *
+       *    The first component of the returned vector will be extracted from the
+       *    least significant bits of the input; the last component will be
+       *    extracted from the most significant bits.
+       *
+       * This function generates IR that approximates the following pseudo-GLSL:
+       *
+       *    return clamp(
+       *       ((ivec2(unpack_uint_to_uvec2(UINT_RVALUE)) << 16) >> 16) / 32767.0f,
+       *       -1.0f, 1.0f);
+       *
+       * The above IR may appear unnecessarily complex, but the intermediate
+       * conversion to ivec2 and the bit shifts are necessary to correctly unpack
+       * negative floats.
+       *
+       * To see why, consider packing and then unpacking vec2(-1.0, 0.0).
+       * packSnorm2x16 encodes -1.0 as the int16 0xffff. During unpacking, we
+       * place that int16 into an int32, which results in the *positive* integer
+       * 0x0000ffff.  The int16's sign bit becomes, in the int32, the rather
+       * unimportant bit 16. We must now extend the int16's sign bit into bits
+       * 17-32, which is accomplished by left-shifting then right-shifting.
+       */
+
+      assert(uint_rval->type == glsl_type::uint_type);
+
+      ir_rvalue *result =
+        clamp(div(i2f(rshift(lshift(u2i(unpack_uint_to_uvec2(uint_rval)),
+                                    constant(16)),
+                             constant(16u))),
+                  constant(32767.0f)),
+              constant(-1.0f),
+              constant(1.0f));
+
+      assert(result->type == glsl_type::vec2_type);
+      return result;
+   }
+
+   /**
+    * \brief Lower a packUnorm2x16 expression.
+    *
+    * \param vec2_rval is packUnorm2x16's input
+    * \return packUnorm2x16's output as a uint rvalue
+    */
+   ir_rvalue*
+   lower_pack_unorm_2x16(ir_rvalue *vec2_rval)
+   {
+      /* From page 88 (94 of pdf) of the GLSL ES 3.00 spec:
+       *
+       *    highp uint packUnorm2x16 (vec2 v)
+       *    ---------------------------------
+       *    First, converts each component of the normalized floating-point value
+       *    v into 16-bit integer values. Then, the results are packed into the
+       *    returned 32-bit unsigned integer.
+       *
+       *    The conversion for component c of v to fixed point is done as
+       *    follows:
+       *
+       *       packUnorm2x16: round(clamp(c, 0, +1) * 65535.0)
+       *
+       *    The first component of the vector will be written to the least
+       *    significant bits of the output; the last component will be written to
+       *    the most significant bits.
+       *
+       * This function generates IR that approximates the following pseudo-GLSL:
+       *
+       *     return pack_uvec2_to_uint(uvec2(
+       *                round(clamp(VEC2_RVALUE, 0.0f, 1.0f) * 65535.0f)));
+       *
+       * Here it is safe to directly convert the vec2 to uvec2 because the the
+       * vec2 has been clamped to a non-negative range.
+       */
+
+      assert(vec2_rval->type == glsl_type::vec2_type);
+
+      ir_rvalue *result = pack_uvec2_to_uint(
+         f2u(round_even(mul(saturate(vec2_rval), constant(65535.0f)))));
+
+      assert(result->type == glsl_type::uint_type);
+      return result;
+   }
+
+   /**
+    * \brief Lower an unpackUnorm2x16 expression.
+    *
+    * \param uint_rval is unpackUnorm2x16's input
+    * \return unpackUnorm2x16's output as a vec2 rvalue
+    */
+   ir_rvalue*
+   lower_unpack_unorm_2x16(ir_rvalue *uint_rval)
+   {
+      /* From page 89 (95 of pdf) of the GLSL ES 3.00 spec:
+       *
+       *    highp vec2 unpackUnorm2x16 (highp uint p)
+       *    -----------------------------------------
+       *    First, unpacks a single 32-bit unsigned integer p into a pair of
+       *    16-bit unsigned integers. Then, each component is converted to
+       *    a normalized floating-point value to generate the returned
+       *    two-component vector.
+       *
+       *    The conversion for unpacked fixed-point value f to floating point is
+       *    done as follows:
+       *
+       *       unpackUnorm2x16: f / 65535.0
+       *
+       *    The first component of the returned vector will be extracted from the
+       *    least significant bits of the input; the last component will be
+       *    extracted from the most significant bits.
+       *
+       * This function generates IR that approximates the following pseudo-GLSL:
+       *
+       *     return vec2(unpack_uint_to_uvec2(UINT_RVALUE)) / 65535.0;
+       */
+
+      assert(uint_rval->type == glsl_type::uint_type);
+
+      ir_rvalue *result = div(u2f(unpack_uint_to_uvec2(uint_rval)),
+                              constant(65535.0f));
+
+      assert(result->type == glsl_type::vec2_type);
+      return result;
+   }
+
+   /**
+    * \brief Lower the component-wise calculation of packHalf2x16.
+    *
+    * \param f_rval is one component of packHafl2x16's input
+    * \param e_rval is the unshifted exponent bits of f_rval
+    * \param m_rval is the unshifted mantissa bits of f_rval
+    *
+    * \return a uint rvalue that encodes a float16 in its lower 16 bits
+    */
+   ir_rvalue*
+   pack_half_1x16_nosign(ir_rvalue *f_rval,
+                         ir_rvalue *e_rval,
+                         ir_rvalue *m_rval)
+   {
+      assert(e_rval->type == glsl_type::uint_type);
+      assert(m_rval->type == glsl_type::uint_type);
+
+      /* uint u16; */
+      ir_variable *u16 = factory.make_temp(glsl_type::uint_type,
+                                           "tmp_pack_half_1x16_u16");
+
+      /* float f = FLOAT_RVAL; */
+      ir_variable *f = factory.make_temp(glsl_type::float_type,
+                                          "tmp_pack_half_1x16_f");
+      factory.emit(assign(f, f_rval));
+
+      /* uint e = E_RVAL; */
+      ir_variable *e = factory.make_temp(glsl_type::uint_type,
+                                          "tmp_pack_half_1x16_e");
+      factory.emit(assign(e, e_rval));
+
+      /* uint m = M_RVAL; */
+      ir_variable *m = factory.make_temp(glsl_type::uint_type,
+                                          "tmp_pack_half_1x16_m");
+      factory.emit(assign(m, m_rval));
+
+      /* Preliminaries
+       * -------------
+       *
+       * For a float16, the bit layout is:
+       *
+       *   sign:     15
+       *   exponent: 10:14
+       *   mantissa: 0:9
+       *
+       * Let f16 be a float16 value. The sign, exponent, and mantissa
+       * determine its value thus:
+       *
+       *   if e16 = 0 and m16 = 0, then zero:       (-1)^s16 * 0                               (1)
+       *   if e16 = 0 and m16!= 0, then subnormal:  (-1)^s16 * 2^(e16 - 14) * (m16 / 2^10)     (2)
+       *   if 0 < e16 < 31, then normal:            (-1)^s16 * 2^(e16 - 15) * (1 + m16 / 2^10) (3)
+       *   if e16 = 31 and m16 = 0, then infinite:  (-1)^s16 * inf                             (4)
+       *   if e16 = 31 and m16 != 0, then           NaN                                        (5)
+       *
+       * where 0 <= m16 < 2^10.
+       *
+       * For a float32, the bit layout is:
+       *
+       *   sign:     31
+       *   exponent: 23:30
+       *   mantissa: 0:22
+       *
+       * Let f32 be a float32 value. The sign, exponent, and mantissa
+       * determine its value thus:
+       *
+       *   if e32 = 0 and m32 = 0, then zero:        (-1)^s * 0                                (10)
+       *   if e32 = 0 and m32 != 0, then subnormal:  (-1)^s * 2^(e32 - 126) * (m32 / 2^23)     (11)
+       *   if 0 < e32 < 255, then normal:            (-1)^s * 2^(e32 - 127) * (1 + m32 / 2^23) (12)
+       *   if e32 = 255 and m32 = 0, then infinite:  (-1)^s * inf                              (13)
+       *   if e32 = 255 and m32 != 0, then           NaN                                       (14)
+       *
+       * where 0 <= m32 < 2^23.
+       *
+       * The minimum and maximum normal float16 values are
+       *
+       *   min_norm16 = 2^(1 - 15) * (1 + 0 / 2^10) = 2^(-14)   (20)
+       *   max_norm16 = 2^(30 - 15) * (1 + 1023 / 2^10)         (21)
+       *
+       * The step at max_norm16 is
+       *
+       *   max_step16 = 2^5                                     (22)
+       *
+       * Observe that the float16 boundary values in equations 20-21 lie in the
+       * range of normal float32 values.
+       *
+       *
+       * Rounding Behavior
+       * -----------------
+       * Not all float32 values can be exactly represented as a float16. We
+       * round all such intermediate float32 values to the nearest float16; if
+       * the float32 is exactly between to float16 values, we round to the one
+       * with an even mantissa. This rounding behavior has several benefits:
+       *
+       *   - It has no sign bias.
+       *
+       *   - It reproduces the behavior of real hardware: opcode F32TO16 in Intel's
+       *     GPU ISA.
+       *
+       *   - By reproducing the behavior of the GPU (at least on Intel hardware),
+       *     compile-time evaluation of constant packHalf2x16 GLSL expressions will
+       *     result in the same value as if the expression were executed on the
+       *     GPU.
+       *
+       * Calculation
+       * -----------
+       * Our task is to compute s16, e16, m16 given f32.  Since this function
+       * ignores the sign bit, assume that s32 = s16 = 0.  There are several
+       * cases consider.
+       */
+
+      factory.emit(
+
+         /* Case 1) f32 is NaN
+          *
+          *   The resultant f16 will also be NaN.
+          */
+
+         /* if (e32 == 255 && m32 != 0) { */
+         if_tree(logic_and(equal(e, constant(0xffu << 23u)),
+                           logic_not(equal(m, constant(0u)))),
+
+            assign(u16, constant(0x7fffu)),
+
+         /* Case 2) f32 lies in the range [0, min_norm16).
+          *
+          *   The resultant float16 will be either zero, subnormal, or normal.
+          *
+          *   Solving
+          *
+          *     f32 = min_norm16       (30)
+          *
+          *   gives
+          *
+          *     e32 = 113 and m32 = 0  (31)
+          *
+          *   Therefore this case occurs if and only if
+          *
+          *     e32 < 113              (32)
+          */
+
+         /* } else if (e32 < 113) { */
+         if_tree(less(e, constant(113u << 23u)),
+
+            /* u16 = uint(round_to_even(abs(f32) * float(1u << 24u))); */
+            assign(u16, f2u(round_even(mul(expr(ir_unop_abs, f),
+                                           constant((float) (1 << 24)))))),
+
+         /* Case 3) f32 lies in the range
+          *         [min_norm16, max_norm16 + max_step16).
+          *
+          *   The resultant float16 will be either normal or infinite.
+          *
+          *   Solving
+          *
+          *     f32 = max_norm16 + max_step16           (40)
+          *         = 2^15 * (1 + 1023 / 2^10) + 2^5    (41)
+          *         = 2^16                              (42)
+          *   gives
+          *
+          *     e32 = 143 and m32 = 0                   (43)
+          *
+          *   We already solved the boundary condition f32 = min_norm16 above
+          *   in equation 31. Therefore this case occurs if and only if
+          *
+          *     113 <= e32 and e32 < 143
+          */
+
+         /* } else if (e32 < 143) { */
+         if_tree(less(e, constant(143u << 23u)),
+
+            /* The addition below handles the case where the mantissa rounds
+             * up to 1024 and bumps the exponent.
+             *
+             * u16 = ((e - (112u << 23u)) >> 13u)
+             *     + round_to_even((float(m) / (1u << 13u));
+             */
+            assign(u16, add(rshift(sub(e, constant(112u << 23u)),
+                                   constant(13u)),
+                            f2u(round_even(
+                                  div(u2f(m), constant((float) (1 << 13))))))),
+
+         /* Case 4) f32 lies in the range [max_norm16 + max_step16, inf].
+          *
+          *   The resultant float16 will be infinite.
+          *
+          *   The cases above caught all float32 values in the range
+          *   [0, max_norm16 + max_step16), so this is the fall-through case.
+          */
+
+         /* } else { */
+
+            assign(u16, constant(31u << 10u))))));
+
+         /* } */
+
+       return deref(u16).val;
+   }
+
+   /**
+    * \brief Lower a packHalf2x16 expression.
+    *
+    * \param vec2_rval is packHalf2x16's input
+    * \return packHalf2x16's output as a uint rvalue
+    */
+   ir_rvalue*
+   lower_pack_half_2x16(ir_rvalue *vec2_rval)
+   {
+      /* From page 89 (95 of pdf) of the GLSL ES 3.00 spec:
+       *
+       *    highp uint packHalf2x16 (mediump vec2 v)
+       *    ----------------------------------------
+       *    Returns an unsigned integer obtained by converting the components of
+       *    a two-component floating-point vector to the 16-bit floating-point
+       *    representation found in the OpenGL ES Specification, and then packing
+       *    these two 16-bit integers into a 32-bit unsigned integer.
+       *
+       *    The first vector component specifies the 16 least- significant bits
+       *    of the result; the second component specifies the 16 most-significant
+       *    bits.
+       */
+
+      assert(vec2_rval->type == glsl_type::vec2_type);
+
+      /* vec2 f = VEC2_RVAL; */
+      ir_variable *f = factory.make_temp(glsl_type::vec2_type,
+                                         "tmp_pack_half_2x16_f");
+      factory.emit(assign(f, vec2_rval));
+
+      /* uvec2 f32 = bitcast_f2u(f); */
+      ir_variable *f32 = factory.make_temp(glsl_type::uvec2_type,
+                                            "tmp_pack_half_2x16_f32");
+      factory.emit(assign(f32, expr(ir_unop_bitcast_f2u, f)));
+
+      /* uvec2 f16; */
+      ir_variable *f16 = factory.make_temp(glsl_type::uvec2_type,
+                                        "tmp_pack_half_2x16_f16");
+
+      /* Get f32's unshifted exponent bits.
+       *
+       *   uvec2 e = f32 & 0x7f800000u;
+       */
+      ir_variable *e = factory.make_temp(glsl_type::uvec2_type,
+                                          "tmp_pack_half_2x16_e");
+      factory.emit(assign(e, bit_and(f32, constant(0x7f800000u))));
+
+      /* Get f32's unshifted mantissa bits.
+       *
+       *   uvec2 m = f32 & 0x007fffffu;
+       */
+      ir_variable *m = factory.make_temp(glsl_type::uvec2_type,
+                                          "tmp_pack_half_2x16_m");
+      factory.emit(assign(m, bit_and(f32, constant(0x007fffffu))));
+
+      /* Set f16's exponent and mantissa bits.
+       *
+       *   f16.x = pack_half_1x16_nosign(e.x, m.x);
+       *   f16.y = pack_half_1y16_nosign(e.y, m.y);
+       */
+      factory.emit(assign(f16, pack_half_1x16_nosign(swizzle_x(f),
+                                                     swizzle_x(e),
+                                                     swizzle_x(m)),
+                           WRITEMASK_X));
+      factory.emit(assign(f16, pack_half_1x16_nosign(swizzle_y(f),
+                                                     swizzle_y(e),
+                                                     swizzle_y(m)),
+                           WRITEMASK_Y));
+
+      /* Set f16's sign bits.
+       *
+       *   f16 |= (f32 & (1u << 31u) >> 16u;
+       */
+      factory.emit(
+         assign(f16, bit_or(f16,
+                            rshift(bit_and(f32, constant(1u << 31u)),
+                                   constant(16u)))));
+
+
+      /* return (f16.y << 16u) | f16.x; */
+      ir_rvalue *result = bit_or(lshift(swizzle_y(f16),
+                                        constant(16u)),
+                                 swizzle_x(f16));
+
+      assert(result->type == glsl_type::uint_type);
+      return result;
+   }
+
+   /**
+    * \brief Split packHalf2x16's vec2 operand into two floats.
+    *
+    * \param vec2_rval is packHalf2x16's input
+    * \return a uint rvalue
+    *
+    * Some code generators, such as the i965 fragment shader, require that all
+    * vector expressions be lowered to a sequence of scalar expressions.
+    * However, packHalf2x16 cannot be scalarized by the same mechanism as
+    * a true vector operation because its input and output have a differing
+    * number of vector components.
+    *
+    * This method scalarizes packHalf2x16 by transforming it from an unary
+    * operation having vector input to a binary operation having scalar input.
+    * That is, it transforms
+    *
+    *    packHalf2x16(VEC2_RVAL);
+    *
+    * into
+    *
+    *    vec2 v = VEC2_RVAL;
+    *    return packHalf2x16_split(v.x, v.y);
+    */
+   ir_rvalue*
+   split_pack_half_2x16(ir_rvalue *vec2_rval)
+   {
+      assert(vec2_rval->type == glsl_type::vec2_type);
+
+      ir_variable *v = factory.make_temp(glsl_type::vec2_type,
+                                         "tmp_split_pack_half_2x16_v");
+      factory.emit(assign(v, vec2_rval));
+
+      return expr(ir_binop_pack_half_2x16_split, swizzle_x(v), swizzle_y(v));
+   }
+
+   /**
+    * \brief Lower the component-wise calculation of unpackHalf2x16.
+    *
+    * Given a uint that encodes a float16 in its lower 16 bits, this function
+    * returns a uint that encodes a float32 with the same value. The sign bit
+    * of the float16 is ignored.
+    *
+    * \param e_rval is the unshifted exponent bits of a float16
+    * \param m_rval is the unshifted mantissa bits of a float16
+    * \param a uint rvalue that encodes a float32
+    */
+   ir_rvalue*
+   unpack_half_1x16_nosign(ir_rvalue *e_rval, ir_rvalue *m_rval)
+   {
+      assert(e_rval->type == glsl_type::uint_type);
+      assert(m_rval->type == glsl_type::uint_type);
+
+      /* uint u32; */
+      ir_variable *u32 = factory.make_temp(glsl_type::uint_type,
+                                           "tmp_unpack_half_1x16_u32");
+
+      /* uint e = E_RVAL; */
+      ir_variable *e = factory.make_temp(glsl_type::uint_type,
+                                          "tmp_unpack_half_1x16_e");
+      factory.emit(assign(e, e_rval));
+
+      /* uint m = M_RVAL; */
+      ir_variable *m = factory.make_temp(glsl_type::uint_type,
+                                          "tmp_unpack_half_1x16_m");
+      factory.emit(assign(m, m_rval));
+
+      /* Preliminaries
+       * -------------
+       *
+       * For a float16, the bit layout is:
+       *
+       *   sign:     15
+       *   exponent: 10:14
+       *   mantissa: 0:9
+       *
+       * Let f16 be a float16 value. The sign, exponent, and mantissa
+       * determine its value thus:
+       *
+       *   if e16 = 0 and m16 = 0, then zero:       (-1)^s16 * 0                               (1)
+       *   if e16 = 0 and m16!= 0, then subnormal:  (-1)^s16 * 2^(e16 - 14) * (m16 / 2^10)     (2)
+       *   if 0 < e16 < 31, then normal:            (-1)^s16 * 2^(e16 - 15) * (1 + m16 / 2^10) (3)
+       *   if e16 = 31 and m16 = 0, then infinite:  (-1)^s16 * inf                             (4)
+       *   if e16 = 31 and m16 != 0, then           NaN                                        (5)
+       *
+       * where 0 <= m16 < 2^10.
+       *
+       * For a float32, the bit layout is:
+       *
+       *   sign: 31
+       *   exponent: 23:30
+       *   mantissa: 0:22
+       *
+       * Let f32 be a float32 value. The sign, exponent, and mantissa
+       * determine its value thus:
+       *
+       *   if e32 = 0 and m32 = 0, then zero:        (-1)^s * 0                                (10)
+       *   if e32 = 0 and m32 != 0, then subnormal:  (-1)^s * 2^(e32 - 126) * (m32 / 2^23)     (11)
+       *   if 0 < e32 < 255, then normal:            (-1)^s * 2^(e32 - 127) * (1 + m32 / 2^23) (12)
+       *   if e32 = 255 and m32 = 0, then infinite:  (-1)^s * inf                              (13)
+       *   if e32 = 255 and m32 != 0, then           NaN                                       (14)
+       *
+       * where 0 <= m32 < 2^23.
+       *
+       * Calculation
+       * -----------
+       * Our task is to compute s32, e32, m32 given f16.  Since this function
+       * ignores the sign bit, assume that s32 = s16 = 0.  There are several
+       * cases consider.
+       */
+
+      factory.emit(
+
+         /* Case 1) f16 is zero or subnormal.
+          *
+          *   The simplest method of calcuating f32 in this case is
+          *
+          *     f32 = f16                       (20)
+          *         = 2^(-14) * (m16 / 2^10)    (21)
+          *         = m16 / 2^(-24)             (22)
+          */
+
+         /* if (e16 == 0) { */
+         if_tree(equal(e, constant(0u)),
+
+            /* u32 = bitcast_f2u(float(m) / float(1 << 24)); */
+            assign(u32, expr(ir_unop_bitcast_f2u,
+                                div(u2f(m), constant((float)(1 << 24))))),
+
+         /* Case 2) f16 is normal.
+          *
+          *   The equation
+          *
+          *     f32 = f16                              (30)
+          *     2^(e32 - 127) * (1 + m32 / 2^23) =     (31)
+          *       2^(e16 - 15) * (1 + m16 / 2^10)
+          *
+          *   can be decomposed into two
+          *
+          *     2^(e32 - 127) = 2^(e16 - 15)           (32)
+          *     1 + m32 / 2^23 = 1 + m16 / 2^10        (33)
+          *
+          *   which solve to
+          *
+          *     e32 = e16 + 112                        (34)
+          *     m32 = m16 * 2^13                       (35)
+          */
+
+         /* } else if (e16 < 31)) { */
+         if_tree(less(e, constant(31u << 10u)),
+
+              /* u32 = ((e + (112 << 10)) | m) << 13;
+               */
+              assign(u32, lshift(bit_or(add(e, constant(112u << 10u)), m),
+                                 constant(13u))),
+
+
+         /* Case 3) f16 is infinite. */
+         if_tree(equal(m, constant(0u)),
+
+                 assign(u32, constant(255u << 23u)),
+
+         /* Case 4) f16 is NaN. */
+         /* } else { */
+
+            assign(u32, constant(0x7fffffffu))))));
+
+         /* } */
+
+      return deref(u32).val;
+   }
+
+   /**
+    * \brief Lower an unpackHalf2x16 expression.
+    *
+    * \param uint_rval is unpackHalf2x16's input
+    * \return unpackHalf2x16's output as a vec2 rvalue
+    */
+   ir_rvalue*
+   lower_unpack_half_2x16(ir_rvalue *uint_rval)
+   {
+      /* From page 89 (95 of pdf) of the GLSL ES 3.00 spec:
+       *
+       *    mediump vec2 unpackHalf2x16 (highp uint v)
+       *    ------------------------------------------
+       *    Returns a two-component floating-point vector with components
+       *    obtained by unpacking a 32-bit unsigned integer into a pair of 16-bit
+       *    values, interpreting those values as 16-bit floating-point numbers
+       *    according to the OpenGL ES Specification, and converting them to
+       *    32-bit floating-point values.
+       *
+       *    The first component of the vector is obtained from the
+       *    16 least-significant bits of v; the second component is obtained
+       *    from the 16 most-significant bits of v.
+       */
+      assert(uint_rval->type == glsl_type::uint_type);
+
+      /* uint u = RVALUE;
+       * uvec2 f16 = uvec2(u.x & 0xffff, u.y >> 16);
+       */
+      ir_variable *f16 = factory.make_temp(glsl_type::uvec2_type,
+                                            "tmp_unpack_half_2x16_f16");
+      factory.emit(assign(f16, unpack_uint_to_uvec2(uint_rval)));
+
+      /* uvec2 f32; */
+      ir_variable *f32 = factory.make_temp(glsl_type::uvec2_type,
+                                            "tmp_unpack_half_2x16_f32");
+
+      /* Get f16's unshifted exponent bits.
+       *
+       *    uvec2 e = f16 & 0x7c00u;
+       */
+      ir_variable *e = factory.make_temp(glsl_type::uvec2_type,
+                                          "tmp_unpack_half_2x16_e");
+      factory.emit(assign(e, bit_and(f16, constant(0x7c00u))));
+
+      /* Get f16's unshifted mantissa bits.
+       *
+       *    uvec2 m = f16 & 0x03ffu;
+       */
+      ir_variable *m = factory.make_temp(glsl_type::uvec2_type,
+                                          "tmp_unpack_half_2x16_m");
+      factory.emit(assign(m, bit_and(f16, constant(0x03ffu))));
+
+      /* Set f32's exponent and mantissa bits.
+       *
+       *   f32.x = unpack_half_1x16_nosign(e.x, m.x);
+       *   f32.y = unpack_half_1x16_nosign(e.y, m.y);
+       */
+      factory.emit(assign(f32, unpack_half_1x16_nosign(swizzle_x(e),
+                                                       swizzle_x(m)),
+                           WRITEMASK_X));
+      factory.emit(assign(f32, unpack_half_1x16_nosign(swizzle_y(e),
+                                                       swizzle_y(m)),
+                           WRITEMASK_Y));
+
+      /* Set f32's sign bit.
+       *
+       *    f32 |= (f16 & 0x8000u) << 16u;
+       */
+      factory.emit(assign(f32, bit_or(f32,
+                                       lshift(bit_and(f16,
+                                                      constant(0x8000u)),
+                                              constant(16u)))));
+
+      /* return bitcast_u2f(f32); */
+      ir_rvalue *result = expr(ir_unop_bitcast_u2f, f32);
+      assert(result->type == glsl_type::vec2_type);
+      return result;
+   }
+
+   /**
+    * \brief Split unpackHalf2x16 into two operations.
+    *
+    * \param uint_rval is unpackHalf2x16's input
+    * \return a vec2 rvalue
+    *
+    * Some code generators, such as the i965 fragment shader, require that all
+    * vector expressions be lowered to a sequence of scalar expressions.
+    * However, unpackHalf2x16 cannot be scalarized by the same method as
+    * a true vector operation because the number of components of its input
+    * and output differ.
+    *
+    * This method scalarizes unpackHalf2x16 by transforming it from a single
+    * operation having vec2 output to a pair of operations each having float
+    * output. That is, it transforms
+    *
+    *   unpackHalf2x16(UINT_RVAL)
+    *
+    * into
+    *
+    *   uint u = UINT_RVAL;
+    *   vec2 v;
+    *
+    *   v.x = unpackHalf2x16_split_x(u);
+    *   v.y = unpackHalf2x16_split_y(u);
+    *
+    *   return v;
+    */
+   ir_rvalue*
+   split_unpack_half_2x16(ir_rvalue *uint_rval)
+   {
+      assert(uint_rval->type == glsl_type::uint_type);
+
+      /* uint u = uint_rval; */
+      ir_variable *u = factory.make_temp(glsl_type::uint_type,
+                                          "tmp_split_unpack_half_2x16_u");
+      factory.emit(assign(u, uint_rval));
+
+      /* vec2 v; */
+      ir_variable *v = factory.make_temp(glsl_type::vec2_type,
+                                          "tmp_split_unpack_half_2x16_v");
+
+      /* v.x = unpack_half_2x16_split_x(u); */
+      factory.emit(assign(v, expr(ir_unop_unpack_half_2x16_split_x, u),
+                           WRITEMASK_X));
+
+      /* v.y = unpack_half_2x16_split_y(u); */
+      factory.emit(assign(v, expr(ir_unop_unpack_half_2x16_split_y, u),
+                           WRITEMASK_Y));
+
+      return deref(v).val;
+   }
+};
+
+} // namespace anonymous
+
+/**
+ * \brief Lower the builtin packing functions.
+ *
+ * \param op_mask is a bitmask of `enum lower_packing_builtins_op`.
+ */
+bool
+lower_packing_builtins(exec_list *instructions, int op_mask)
+{
+   lower_packing_builtins_visitor v(op_mask);
+   visit_list_elements(&v, instructions, true);
+   return v.get_progress();
+}