Add support for BuiltInFrontFacing

- Remove Primitive::area, it is unused
- Always set front/back masks
- Initialize frontfacing based on front/back mask

Bug: b/118386749
Test: dEQP-VK.glsl.builtin_var.*
Change-Id: I8ccb5a2deb64df2dc4774a3511a5d30c09c30e5f
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/29208
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Tested-by: Chris Forbes <chrisforbes@google.com>
Presubmit-Ready: Chris Forbes <chrisforbes@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>

src/Device/Primitive.hpp

diff --git a/src/Device/Primitive.hpp b/src/Device/Primitive.hpp
index 1e085f7..f72fe32 100644
--- a/src/Device/Primitive.hpp
+++ b/src/Device/Primitive.hpp
@@ -50,8 +50,6 @@
 			PlaneEquation V[MAX_INTERFACE_COMPONENTS];
 		};
 
-		float area;
-
 		// Masks for two-sided stencil
 		int64_t clockwiseMask;
 		int64_t invClockwiseMask;

src/Pipeline/PixelProgram.cpp

diff --git a/src/Pipeline/PixelProgram.cpp b/src/Pipeline/PixelProgram.cpp
index 9eb0ab4..127a148 100644
--- a/src/Pipeline/PixelProgram.cpp
+++ b/src/Pipeline/PixelProgram.cpp
@@ -33,6 +33,14 @@
 		routine.descriptorDynamicOffsets = data + OFFSET(DrawData, descriptorDynamicOffsets);
 		routine.pushConstants = data + OFFSET(DrawData, pushConstants);
 
+		auto it = spirvShader->inputBuiltins.find(spv::BuiltInFrontFacing);
+		if (it != spirvShader->inputBuiltins.end())
+		{
+			ASSERT(it->second.SizeInComponents == 1);
+			auto frontFacing = Int4(*Pointer<Int>(primitive + OFFSET(Primitive, clockwiseMask)));
+			routine.getVariable(it->second.Id)[it->second.FirstComponent] = As<Float4>(frontFacing);
+		}
+
 		auto activeLaneMask = SIMD::Int(0xFFFFFFFF); // TODO: Control this.
 		spirvShader->emit(&routine, activeLaneMask, descriptorSets);
 		spirvShader->emitEpilog(&routine);

src/Pipeline/SetupRoutine.cpp

diff --git a/src/Pipeline/SetupRoutine.cpp b/src/Pipeline/SetupRoutine.cpp
index d090d3d..5e9b714 100644
--- a/src/Pipeline/SetupRoutine.cpp
+++ b/src/Pipeline/SetupRoutine.cpp
@@ -107,32 +107,21 @@
 
 				d = IfThenElse(A > 0.0f, d, Int(0));
 
-				if(state.twoSidedStencil)
-				{
-					If(frontFacing)
-					{
-						*Pointer<Byte8>(primitive + OFFSET(Primitive,clockwiseMask)) = Byte8(0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF);
-						*Pointer<Byte8>(primitive + OFFSET(Primitive,invClockwiseMask)) = Byte8(0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00);
-					}
-					Else
-					{
-						*Pointer<Byte8>(primitive + OFFSET(Primitive,clockwiseMask)) = Byte8(0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00);
-						*Pointer<Byte8>(primitive + OFFSET(Primitive,invClockwiseMask)) = Byte8(0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF);
-					}
-				}
-
-				if(state.vFace)
-				{
-					*Pointer<Float>(primitive + OFFSET(Primitive,area)) = 0.5f * A;
-				}
-			}
-			else
-			{
-				if(state.twoSidedStencil)
+				If(frontFacing)
 				{
 					*Pointer<Byte8>(primitive + OFFSET(Primitive,clockwiseMask)) = Byte8(0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF);
 					*Pointer<Byte8>(primitive + OFFSET(Primitive,invClockwiseMask)) = Byte8(0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00);
 				}
+				Else
+				{
+					*Pointer<Byte8>(primitive + OFFSET(Primitive,clockwiseMask)) = Byte8(0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00);
+					*Pointer<Byte8>(primitive + OFFSET(Primitive,invClockwiseMask)) = Byte8(0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF);
+				}
+			}
+			else
+			{
+				*Pointer<Byte8>(primitive + OFFSET(Primitive,clockwiseMask)) = Byte8(0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF);
+				*Pointer<Byte8>(primitive + OFFSET(Primitive,invClockwiseMask)) = Byte8(0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00);
 			}
 
 			Int n = *Pointer<Int>(polygon + OFFSET(Polygon,n));