glsl: Add ldexp_to_arith lowering pass.

Reviewed-by: Paul Berry <[email protected]>

1 files changed, 128 insertions, 0 deletions

diff --git a/src/glsl/lower_instructions.cpp b/src/glsl/lower_instructions.cpp
index d32ec80d6bb..cb530489593 100644
--- a/src/glsl/lower_instructions.cpp
+++ b/src/glsl/lower_instructions.cpp
@@ -37,6 +37,7 @@
  * - POW_TO_EXP2
  * - LOG_TO_LOG2
  * - MOD_TO_FRACT
+ * - LDEXP_TO_ARITH
  * - LRP_TO_ARITH
  * - BITFIELD_INSERT_TO_BFM_BFI
  *
@@ -82,6 +83,10 @@
  * if we have to break it down like this anyway, it gives an
  * opportunity to do things like constant fold the (1.0 / op1) easily.
  *
+ * LDEXP_TO_ARITH:
+ * -------------
+ * Converts ir_binop_ldexp to arithmetic and bit operations.
+ *
  * LRP_TO_ARITH:
  * -------------
  * Converts ir_triop_lrp to (op0 * (1.0f - op2)) + (op1 * op2).
@@ -125,6 +130,7 @@ private:
    void log_to_log2(ir_expression *);
    void lrp_to_arith(ir_expression *);
    void bitfield_insert_to_bfm_bfi(ir_expression *);
+   void ldexp_to_arith(ir_expression *);
 };
 
 /**
@@ -332,6 +338,123 @@ lower_instructions_visitor::bitfield_insert_to_bfm_bfi(ir_expression *ir)
    this->progress = true;
 }
 
+void
+lower_instructions_visitor::ldexp_to_arith(ir_expression *ir)
+{
+   /* Translates
+    *    ir_binop_ldexp x exp
+    * into
+    *
+    *    extracted_biased_exp = rshift(bitcast_f2i(abs(x)), exp_shift);
+    *    resulting_biased_exp = extracted_biased_exp + exp;
+    *
+    *    if (resulting_biased_exp < 1) {
+    *       return copysign(0.0, x);
+    *    }
+    *
+    *    return bitcast_u2f((bitcast_f2u(x) & sign_mantissa_mask) |
+    *                       lshift(i2u(resulting_biased_exp), exp_shift));
+    *
+    * which we can't actually implement as such, since the GLSL IR doesn't
+    * have vectorized if-statements. We actually implement it without branches
+    * using conditional-select:
+    *
+    *    extracted_biased_exp = rshift(bitcast_f2i(abs(x)), exp_shift);
+    *    resulting_biased_exp = extracted_biased_exp + exp;
+    *
+    *    is_not_zero_or_underflow = gequal(resulting_biased_exp, 1);
+    *    x = csel(is_not_zero_or_underflow, x, copysign(0.0f, x));
+    *    resulting_biased_exp = csel(is_not_zero_or_underflow,
+    *                                resulting_biased_exp, 0);
+    *
+    *    return bitcast_u2f((bitcast_f2u(x) & sign_mantissa_mask) |
+    *                       lshift(i2u(resulting_biased_exp), exp_shift));
+    */
+
+   const unsigned vec_elem = ir->type->vector_elements;
+
+   /* Types */
+   const glsl_type *ivec = glsl_type::get_instance(GLSL_TYPE_INT, vec_elem, 1);
+   const glsl_type *bvec = glsl_type::get_instance(GLSL_TYPE_BOOL, vec_elem, 1);
+
+   /* Constants */
+   ir_constant *zeroi = ir_constant::zero(ir, ivec);
+   ir_constant *zerof = ir_constant::zero(ir, ir->type);
+
+   ir_constant *sign_mantissa_mask = new(ir) ir_constant(0x807fffffu, vec_elem);
+   ir_constant *sign_mask = new(ir) ir_constant(0x80000000u, vec_elem);
+
+   ir_constant *exp_shift = new(ir) ir_constant(23u, vec_elem);
+
+   /* Temporary variables */
+   ir_variable *x = new(ir) ir_variable(ir->type, "x", ir_var_temporary);
+   ir_variable *exp = new(ir) ir_variable(ivec, "exp", ir_var_temporary);
+
+   ir_variable *zero_sign_x = new(ir) ir_variable(ir->type, "zero_sign_x",
+                                                  ir_var_temporary);
+
+   ir_variable *extracted_biased_exp =
+      new(ir) ir_variable(ivec, "extracted_biased_exp", ir_var_temporary);
+   ir_variable *resulting_biased_exp =
+      new(ir) ir_variable(ivec, "resulting_biased_exp", ir_var_temporary);
+
+   ir_variable *is_not_zero_or_underflow =
+      new(ir) ir_variable(bvec, "is_not_zero_or_underflow", ir_var_temporary);
+
+   ir_instruction &i = *base_ir;
+
+   /* Copy <x> and <exp> arguments. */
+   i.insert_before(x);
+   i.insert_before(assign(x, ir->operands[0]));
+   i.insert_before(exp);
+   i.insert_before(assign(exp, ir->operands[1]));
+
+   /* Extract the biased exponent from <x>. */
+   i.insert_before(extracted_biased_exp);
+   i.insert_before(assign(extracted_biased_exp,
+                          rshift(bitcast_f2i(abs(x)), exp_shift)));
+
+   i.insert_before(resulting_biased_exp);
+   i.insert_before(assign(resulting_biased_exp,
+                          add(extracted_biased_exp, exp)));
+
+   /* Test if result is ±0.0, subnormal, or underflow by checking if the
+    * resulting biased exponent would be less than 0x1. If so, the result is
+    * 0.0 with the sign of x. (Actually, invert the conditions so that
+    * immediate values are the second arguments, which is better for i965)
+    */
+   i.insert_before(zero_sign_x);
+   i.insert_before(assign(zero_sign_x,
+                          bitcast_u2f(bit_or(bit_and(bitcast_f2u(x), sign_mask),
+                                             bitcast_f2u(zerof)))));
+
+   i.insert_before(is_not_zero_or_underflow);
+   i.insert_before(assign(is_not_zero_or_underflow,
+                          gequal(resulting_biased_exp,
+                                  new(ir) ir_constant(0x1, vec_elem))));
+   i.insert_before(assign(x, csel(is_not_zero_or_underflow,
+                                  x, zero_sign_x)));
+   i.insert_before(assign(resulting_biased_exp,
+                          csel(is_not_zero_or_underflow,
+                               resulting_biased_exp, zeroi)));
+
+   /* We could test for overflows by checking if the resulting biased exponent
+    * would be greater than 0xFE. Turns out we don't need to because the GLSL
+    * spec says:
+    *
+    *    "If this product is too large to be represented in the
+    *     floating-point type, the result is undefined."
+    */
+
+   ir_constant *exp_shift_clone = exp_shift->clone(ir, NULL);
+   ir->operation = ir_unop_bitcast_u2f;
+   ir->operands[0] = bit_or(bit_and(bitcast_f2u(x), sign_mantissa_mask),
+                            lshift(i2u(resulting_biased_exp), exp_shift_clone));
+   ir->operands[1] = NULL;
+
+   this->progress = true;
+}
+
 ir_visitor_status
 lower_instructions_visitor::visit_leave(ir_expression *ir)
 {
@@ -378,6 +501,11 @@ lower_instructions_visitor::visit_leave(ir_expression *ir)
          bitfield_insert_to_bfm_bfi(ir);
       break;
 
+   case ir_binop_ldexp:
+      if (lowering(LDEXP_TO_ARITH))
+         ldexp_to_arith(ir);
+      break;
+
    default:
       return visit_continue;
    }