[SPIR-V] Implement Shader Model 6.8 Extended Command Information (microsoft#6843)

Compile `SV_StartVertexLocation` as `BaseVertex` and
`SV_StartInstanceLocation` as `BaseInstance`.

Author: cassiebeckley

docs/SPIR-V.rst

@@ -1519,6 +1519,10 @@ some system-value (SV) semantic strings will be translated into SPIR-V
 |                           |             | with                                   |                       |                             |
 |                           |             | ``-fvk-support-nonzero-base-instance`` |                       |                             |
 +---------------------------+-------------+----------------------------------------+-----------------------+-----------------------------+
+| SV_StartVertexLocation    | VSIn        | ``BaseVertex``                         | N/A                   | ``Shader``                  |
++---------------------------+-------------+----------------------------------------+-----------------------+-----------------------------+
+| SV_StartInstanceLocation  | VSIn        | ``BaseInstance``                       | N/A                   | ``Shader``                  |
++---------------------------+-------------+----------------------------------------+-----------------------+-----------------------------+
 | SV_Depth                  | PSOut       | ``FragDepth``                          | N/A                   | ``Shader``                  |
 +---------------------------+-------------+----------------------------------------+-----------------------+-----------------------------+
 | SV_DepthGreaterEqual      | PSOut       | ``FragDepth``                          | ``DepthGreater``      | ``Shader``                  |

tools/clang/lib/SPIRV/DeclResultIdMapper.cpp

@@ -4218,6 +4218,22 @@ SpirvVariable *DeclResultIdMapper::createSpirvStageVar(
       llvm_unreachable("invalid usage of SV_InstanceID sneaked in");
     }
   }
+  // According to DXIL spec, the StartVertexLocation SV can only be used by
+  // VSIn. According to Vulkan spec, the BaseVertex BuiltIn can only be used by
+  // VSIn.
+  case hlsl::Semantic::Kind::StartVertexLocation: {
+    stageVar->setIsSpirvBuiltin();
+    return spvBuilder.addStageBuiltinVar(type, sc, BuiltIn::BaseVertex,
+                                         isPrecise, srcLoc);
+  }
+  // According to DXIL spec, the StartInstanceLocation SV can only be used by
+  // VSIn. According to Vulkan spec, the BaseInstance BuiltIn can only be used
+  // by VSIn.
+  case hlsl::Semantic::Kind::StartInstanceLocation: {
+    stageVar->setIsSpirvBuiltin();
+    return spvBuilder.addStageBuiltinVar(type, sc, BuiltIn::BaseInstance,
+                                         isPrecise, srcLoc);
+  }
   // According to DXIL spec, the Depth{|GreaterEqual|LessEqual} SV can only be
   // used by PSOut.
   // According to Vulkan spec, the FragDepth BuiltIn can only be used by PSOut.

tools/clang/test/CodeGenSPIRV/semantic.start-instance-location.hlsl

@@ -0,0 +1,12 @@
+// RUN: %dxc -T vs_6_8 -E main -fcgl  %s -spirv | FileCheck %s
+
+// CHECK:                   OpEntryPoint Vertex %main "main"
+// CHECK-SAME:              %gl_BaseInstance
+
+// CHECK:                   OpDecorate %gl_BaseInstance BuiltIn BaseInstance
+
+// CHECK: %gl_BaseInstance = OpVariable %_ptr_Input_int Input
+
+int main(int input: SV_StartInstanceLocation) : A {
+    return input;
+}

tools/clang/test/CodeGenSPIRV/semantic.start-vertex-location.hlsl

@@ -0,0 +1,11 @@
+// RUN: %dxc -T vs_6_8 -E main -fcgl  %s -spirv | FileCheck %s
+
+// CHECK:                   OpEntryPoint Vertex %main "main" [[baseVertex:%[0-9]+]]
+
+// CHECK:                   OpDecorate [[baseVertex]] BuiltIn BaseVertex
+
+// CHECK: [[baseVertex]] = OpVariable %_ptr_Input_int Input
+
+int main(int input: SV_StartVertexLocation) : A {
+    return input;
+}