i965: fix textureGrad for cubemaps

Fixes bugs exposed by commit
2b1cdb0eddb73f62e4848d4b64840067f1f70865 in:
   ES3-CTS.gtf.GL3Tests.shadow.shadow_execution_frag

No regressions observed in deqp, CTS or Piglit.

v2: address review feedback from Iago Toral:
   - move rho calculation to else branch
   - optimize dx and dy calculation
   - fix documentation inconsistensies

Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
Signed-off-by: Kevin Rogovin <kevin.rogovin@intel.com>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=91114
Cc: "10.6 11.0" <mesa-stable@lists.freedesktop.org>
(cherry picked from commit 7f8815bcb9af9b4b374ad7bd6e7cfa7529a6c980)

1 files changed, 182 insertions, 19 deletions

diff --git a/src/mesa/drivers/dri/i965/brw_lower_texture_gradients.cpp b/src/mesa/drivers/dri/i965/brw_lower_texture_gradients.cpp
index 7a5f9834423..d571ecd4394 100644
--- a/src/mesa/drivers/dri/i965/brw_lower_texture_gradients.cpp
+++ b/src/mesa/drivers/dri/i965/brw_lower_texture_gradients.cpp
@@ -48,6 +48,7 @@ public:
 
 private:
    void emit(ir_variable *, ir_rvalue *);
+   ir_variable *temp(void *ctx, const glsl_type *type, const char *name);
 };
 
 /**
@@ -60,6 +61,17 @@ lower_texture_grad_visitor::emit(ir_variable *var, ir_rvalue *value)
    base_ir->insert_before(assign(var, value));
 }
 
+/**
+ * Emit a temporary variable declaration
+ */
+ir_variable *
+lower_texture_grad_visitor::temp(void *ctx, const glsl_type *type, const char *name)
+{
+   ir_variable *var = new(ctx) ir_variable(type, name, ir_var_temporary);
+   base_ir->insert_before(var);
+   return var;
+}
+
 static const glsl_type *
 txs_type(const glsl_type *type)
 {
@@ -144,28 +156,179 @@ lower_texture_grad_visitor::visit_leave(ir_texture *ir)
       new(mem_ctx) ir_variable(grad_type, "dPdy", ir_var_temporary);
    emit(dPdy, mul(size, ir->lod_info.grad.dPdy));
 
-   /* Calculate rho from equation 3.20 of the GL 3.0 specification. */
-   ir_rvalue *rho;
-   if (dPdx->type->is_scalar()) {
-      rho = expr(ir_binop_max, expr(ir_unop_abs, dPdx),
-			       expr(ir_unop_abs, dPdy));
-   } else {
-      rho = expr(ir_binop_max, expr(ir_unop_sqrt, dot(dPdx, dPdx)),
-			       expr(ir_unop_sqrt, dot(dPdy, dPdy)));
-   }
-
-   /* lambda_base = log2(rho).  We're ignoring GL state biases for now.
-    *
-    * For cube maps the result of these formulas is giving us a value of rho
-    * that is twice the value we should use, so divide it by 2 or,
-    * alternatively, remove one unit from the result of the log2 computation.
-    */
    ir->op = ir_txl;
    if (ir->sampler->type->sampler_dimensionality == GLSL_SAMPLER_DIM_CUBE) {
-      ir->lod_info.lod = expr(ir_binop_add,
-                              expr(ir_unop_log2, rho),
-                              new(mem_ctx) ir_constant(-1.0f));
+      /* Cubemap texture lookups first generate a texture coordinate normalized
+       * to [-1, 1] on the appropiate face. The appropiate face is determined
+       * by which component has largest magnitude and its sign. The texture
+       * coordinate is the quotient of the remaining texture coordinates against
+       * that absolute value of the component of largest magnitude. This
+       * division requires that the computing of the derivative of the texel
+       * coordinate must use the quotient rule. The high level GLSL code is as
+       * follows:
+       *
+       * Step 1: selection
+       *
+       * vec3 abs_p, Q, dQdx, dQdy;
+       * abs_p = abs(ir->coordinate);
+       * if (abs_p.x >= max(abs_p.y, abs_p.z)) {
+       *    Q = ir->coordinate.yzx;
+       *    dQdx = ir->lod_info.grad.dPdx.yzx;
+       *    dQdy = ir->lod_info.grad.dPdy.yzx;
+       * }
+       * if (abs_p.y >= max(abs_p.x, abs_p.z)) {
+       *    Q = ir->coordinate.xzy;
+       *    dQdx = ir->lod_info.grad.dPdx.xzy;
+       *    dQdy = ir->lod_info.grad.dPdy.xzy;
+       * }
+       * if (abs_p.z >= max(abs_p.x, abs_p.y)) {
+       *    Q = ir->coordinate;
+       *    dQdx = ir->lod_info.grad.dPdx;
+       *    dQdy = ir->lod_info.grad.dPdy;
+       * }
+       *
+       * Step 2: use quotient rule to compute derivative. The normalized to
+       * [-1, 1] texel coordinate is given by Q.xy / (sign(Q.z) * Q.z). We are
+       * only concerned with the magnitudes of the derivatives whose values are
+       * not affected by the sign. We drop the sign from the computation.
+       *
+       * vec2 dx, dy;
+       * float recip;
+       *
+       * recip = 1.0 / Q.z;
+       * dx = recip * ( dQdx.xy - Q.xy * (dQdx.z * recip) );
+       * dy = recip * ( dQdy.xy - Q.xy * (dQdy.z * recip) );
+       *
+       * Step 3: compute LOD. At this point we have the derivatives of the
+       * texture coordinates normalized to [-1,1]. We take the LOD to be
+       *  result = log2(max(sqrt(dot(dx, dx)), sqrt(dy, dy)) * 0.5 * L)
+       *         = -1.0 + log2(max(sqrt(dot(dx, dx)), sqrt(dy, dy)) * L)
+       *         = -1.0 + log2(sqrt(max(dot(dx, dx), dot(dy,dy))) * L)
+       *         = -1.0 + log2(sqrt(L * L * max(dot(dx, dx), dot(dy,dy))))
+       *         = -1.0 + 0.5 * log2(L * L * max(dot(dx, dx), dot(dy,dy)))
+       * where L is the dimension of the cubemap. The code is:
+       *
+       * float M, result;
+       * M = max(dot(dx, dx), dot(dy, dy));
+       * L = textureSize(sampler, 0).x;
+       * result = -1.0 + 0.5 * log2(L * L * M);
+       */
+
+/* Helpers to make code more human readable. */
+#define EMIT(instr) base_ir->insert_before(instr)
+#define THEN(irif, instr) irif->then_instructions.push_tail(instr)
+#define CLONE(x) x->clone(mem_ctx, NULL)
+
+      ir_variable *abs_p = temp(mem_ctx, glsl_type::vec3_type, "abs_p");
+
+      EMIT(assign(abs_p, swizzle_for_size(abs(CLONE(ir->coordinate)), 3)));
+
+      ir_variable *Q = temp(mem_ctx, glsl_type::vec3_type, "Q");
+      ir_variable *dQdx = temp(mem_ctx, glsl_type::vec3_type, "dQdx");
+      ir_variable *dQdy = temp(mem_ctx, glsl_type::vec3_type, "dQdy");
+
+      /* unmodified dPdx, dPdy values */
+      ir_rvalue *dPdx = ir->lod_info.grad.dPdx;
+      ir_rvalue *dPdy = ir->lod_info.grad.dPdy;
+
+      /* 1. compute selector */
+
+      /* if (abs_p.x >= max(abs_p.y, abs_p.z))  ... */
+      ir_if *branch_x =
+         new(mem_ctx) ir_if(gequal(swizzle_x(abs_p),
+                                   max2(swizzle_y(abs_p), swizzle_z(abs_p))));
+
+      /* Q = p.yzx;
+       * dQdx = dPdx.yzx;
+       * dQdy = dPdy.yzx;
+       */
+      int yzx = MAKE_SWIZZLE4(SWIZZLE_Y, SWIZZLE_Z, SWIZZLE_X, 0);
+      THEN(branch_x, assign(Q, swizzle(CLONE(ir->coordinate), yzx, 3)));
+      THEN(branch_x, assign(dQdx, swizzle(CLONE(dPdx), yzx, 3)));
+      THEN(branch_x, assign(dQdy, swizzle(CLONE(dPdy), yzx, 3)));
+      EMIT(branch_x);
+
+      /* if (abs_p.y >= max(abs_p.x, abs_p.z)) */
+      ir_if *branch_y =
+         new(mem_ctx) ir_if(gequal(swizzle_y(abs_p),
+                                   max2(swizzle_x(abs_p), swizzle_z(abs_p))));
+
+      /* Q = p.xzy;
+       * dQdx = dPdx.xzy;
+       * dQdy = dPdy.xzy;
+       */
+      int xzy = MAKE_SWIZZLE4(SWIZZLE_X, SWIZZLE_Z, SWIZZLE_Y, 0);
+      THEN(branch_y, assign(Q, swizzle(CLONE(ir->coordinate), xzy, 3)));
+      THEN(branch_y, assign(dQdx, swizzle(CLONE(dPdx), xzy, 3)));
+      THEN(branch_y, assign(dQdy, swizzle(CLONE(dPdy), xzy, 3)));
+      EMIT(branch_y);
+
+      /* if (abs_p.z >= max(abs_p.x, abs_p.y)) */
+      ir_if *branch_z =
+         new(mem_ctx) ir_if(gequal(swizzle_z(abs_p),
+                            max2(swizzle_x(abs_p), swizzle_y(abs_p))));
+
+      /* Q = p;
+       * dQdx = dPdx;
+       * dQdy = dPdy;
+       */
+      THEN(branch_z, assign(Q, swizzle_for_size(CLONE(ir->coordinate), 3)));
+      THEN(branch_z, assign(dQdx, CLONE(dPdx)));
+      THEN(branch_z, assign(dQdy, CLONE(dPdy)));
+      EMIT(branch_z);
+
+      /* 2. quotient rule */
+      ir_variable *recip = temp(mem_ctx, glsl_type::float_type, "recip");
+      EMIT(assign(recip, div(new(mem_ctx) ir_constant(1.0f), swizzle_z(Q))));
+
+      ir_variable *dx = temp(mem_ctx, glsl_type::vec2_type, "dx");
+      ir_variable *dy = temp(mem_ctx, glsl_type::vec2_type, "dy");
+
+      /* tmp = Q.xy * recip;
+       * dx = recip * ( dQdx.xy - (tmp * dQdx.z) );
+       * dy = recip * ( dQdy.xy - (tmp * dQdy.z) );
+       */
+      ir_variable *tmp = temp(mem_ctx, glsl_type::vec2_type, "tmp");
+      EMIT(assign(tmp, mul(swizzle_xy(Q), recip)));
+      EMIT(assign(dx, mul(recip, sub(swizzle_xy(dQdx),
+                                     mul(tmp, swizzle_z(dQdx))))));
+      EMIT(assign(dy, mul(recip, sub(swizzle_xy(dQdy),
+                                     mul(tmp, swizzle_z(dQdy))))));
+
+      /* M = max(dot(dx, dx), dot(dy, dy)); */
+      ir_variable *M = temp(mem_ctx, glsl_type::float_type, "M");
+      EMIT(assign(M, max2(dot(dx, dx), dot(dy, dy))));
+
+      /* size has textureSize() of LOD 0 */
+      ir_variable *L = temp(mem_ctx, glsl_type::float_type, "L");
+      EMIT(assign(L, swizzle_x(size)));
+
+      ir_variable *result = temp(mem_ctx, glsl_type::float_type, "result");
+
+      /* result = -1.0 + 0.5 * log2(L * L * M); */
+      EMIT(assign(result,
+                  add(new(mem_ctx)ir_constant(-1.0f),
+                      mul(new(mem_ctx)ir_constant(0.5f),
+                          expr(ir_unop_log2, mul(mul(L, L), M))))));
+
+      /* 3. final assignment of parameters to textureLod call */
+      ir->lod_info.lod = new (mem_ctx) ir_dereference_variable(result);
+
+#undef THEN
+#undef EMIT
+
    } else {
+      /* Calculate rho from equation 3.20 of the GL 3.0 specification. */
+      ir_rvalue *rho;
+      if (dPdx->type->is_scalar()) {
+         rho = expr(ir_binop_max, expr(ir_unop_abs, dPdx),
+                    expr(ir_unop_abs, dPdy));
+      } else {
+         rho = expr(ir_binop_max, expr(ir_unop_sqrt, dot(dPdx, dPdx)),
+                    expr(ir_unop_sqrt, dot(dPdy, dPdy)));
+      }
+
+      /* lambda_base = log2(rho).  We're ignoring GL state biases for now. */
       ir->lod_info.lod = expr(ir_unop_log2, rho);
    }