writer/spirv: Implement interpolate attributes

Add the SampleRateShading capability if the sampling type is `sample`.

Bug: tint:746
Change-Id: I20fb25913f5c0919b6d16a9d0e9fc8b1551ff7ea
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/56247
Kokoro: Kokoro <noreply+kokoro@google.com>
Auto-Submit: James Price <jrprice@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>

src/writer/spirv/builder.cc

@@ -842,6 +842,8 @@ bool Builder::GenerateGlobalVariable(ast::Variable* var) {
       push_annot(spv::Op::OpDecorate,
                  {Operand::Int(var_id), Operand::Int(SpvDecorationLocation),
                   Operand::Int(location->value())});
+    } else if (auto* interpolate = deco->As<ast::InterpolateDecoration>()) {
+      AddInterpolationDecorations(var_id, interpolate);
     } else if (auto* binding = deco->As<ast::BindingDecoration>()) {
       push_annot(spv::Op::OpDecorate,
                  {Operand::Int(var_id), Operand::Int(SpvDecorationBinding),
@@ -3986,6 +3988,37 @@ SpvBuiltIn Builder::ConvertBuiltin(ast::Builtin builtin,
   return SpvBuiltInMax;
 }
 
+void Builder::AddInterpolationDecorations(
+    uint32_t id,
+    ast::InterpolateDecoration* interpolate) {
+  switch (interpolate->type()) {
+    case ast::InterpolationType::kLinear:
+      push_annot(spv::Op::OpDecorate,
+                 {Operand::Int(id), Operand::Int(SpvDecorationNoPerspective)});
+      break;
+    case ast::InterpolationType::kFlat:
+      push_annot(spv::Op::OpDecorate,
+                 {Operand::Int(id), Operand::Int(SpvDecorationFlat)});
+      break;
+    case ast::InterpolationType::kPerspective:
+      break;
+  }
+  switch (interpolate->sampling()) {
+    case ast::InterpolationSampling::kCentroid:
+      push_annot(spv::Op::OpDecorate,
+                 {Operand::Int(id), Operand::Int(SpvDecorationCentroid)});
+      break;
+    case ast::InterpolationSampling::kSample:
+      push_capability(SpvCapabilitySampleRateShading);
+      push_annot(spv::Op::OpDecorate,
+                 {Operand::Int(id), Operand::Int(SpvDecorationSample)});
+      break;
+    case ast::InterpolationSampling::kCenter:
+    case ast::InterpolationSampling::kNone:
+      break;
+  }
+}
+
 SpvImageFormat Builder::convert_image_format_to_spv(
     const ast::ImageFormat format) {
   switch (format) {

src/writer/spirv/builder.h

@@ -27,6 +27,7 @@
 #include "src/ast/continue_statement.h"
 #include "src/ast/discard_statement.h"
 #include "src/ast/if_statement.h"
+#include "src/ast/interpolate_decoration.h"
 #include "src/ast/loop_statement.h"
 #include "src/ast/return_statement.h"
 #include "src/ast/switch_statement.h"
@@ -208,6 +209,13 @@ class Builder {
   /// @returns the SPIR-V builtin or SpvBuiltInMax on error.
   SpvBuiltIn ConvertBuiltin(ast::Builtin builtin, ast::StorageClass storage);
 
+  /// Converts an interpolate attribute to SPIR-V decorations and pushes a
+  /// capability if needed.
+  /// @param id the id to decorate
+  /// @param interpolate the interpolation attribute to convert
+  void AddInterpolationDecorations(uint32_t id,
+                                   ast::InterpolateDecoration* interpolate);
+
   /// Generates a label for the given id. Emits an error and returns false if
   /// we're currently outside a function.
   /// @param id the id to use for the label