[HLSL] Implement the lit intrinsic (#134171)

Closes #99135.

Tasks completed:
- Wrote implementation in `hlsl_intrinsics.h`/`hlsl_intrinsic_helpers.h`
- Added codegen tests to `clang/test/CodeGenHLSL/builtins/lit.hlsl`

Author: kmpeng

clang/lib/Headers/hlsl/hlsl_intrinsic_helpers.h

@@ -114,6 +114,18 @@ constexpr vector<T, N> smoothstep_vec_impl(vector<T, N> Min, vector<T, N> Max,
 #endif
 }
 
+template <typename T> constexpr vector<T, 4> lit_impl(T NDotL, T NDotH, T M) {
+  bool DiffuseCond = NDotL < 0;
+  T Diffuse = select<T>(DiffuseCond, 0, NDotL);
+  vector<T, 4> Result = {1, Diffuse, 0, 1};
+  // clang-format off
+  bool SpecularCond = or(DiffuseCond, (NDotH < 0));
+  // clang-format on
+  T SpecularExp = exp(log(NDotH) * M);
+  Result[2] = select<T>(SpecularCond, 0, SpecularExp);
+  return Result;
+}
+
 } // namespace __detail
 } // namespace hlsl
 

clang/lib/Headers/hlsl/hlsl_intrinsics.h

@@ -292,6 +292,30 @@ const inline float length(__detail::HLSL_FIXED_VECTOR<float, N> X) {
   return __detail::length_vec_impl(X);
 }
 
+//===----------------------------------------------------------------------===//
+// lit builtins
+//===----------------------------------------------------------------------===//
+
+/// \fn vector<T, 4> lit(T NDotL, T NDotH, T M)
+/// \brief Returns a lighting coefficient vector.
+/// \param NDotL The dot product of the normalized surface normal and the
+/// light vector.
+/// \param NDotH The dot product of the half-angle vector and the surface
+/// normal.
+/// \param M A specular exponent.
+///
+/// This function returns a lighting coefficient vector (ambient, diffuse,
+/// specular, 1).
+
+_HLSL_16BIT_AVAILABILITY(shadermodel, 6.2)
+const inline half4 lit(half NDotL, half NDotH, half M) {
+  return __detail::lit_impl(NDotL, NDotH, M);
+}
+
+const inline float4 lit(float NDotL, float NDotH, float M) {
+  return __detail::lit_impl(NDotL, NDotH, M);
+}
+
 //===----------------------------------------------------------------------===//
 // D3DCOLORtoUBYTE4 builtin
 //===----------------------------------------------------------------------===//

clang/test/CodeGenHLSL/builtins/lit.hlsl

@@ -0,0 +1,31 @@
+// RUN: %clang_cc1 -finclude-default-header -x hlsl -triple dxil-pc-shadermodel6.3-library %s -fnative-half-type -emit-llvm -o - | FileCheck %s
+
+// CHECK-LABEL: test_lit_half
+// CHECK: %cmp.i = fcmp reassoc nnan ninf nsz arcp afn olt half %{{.*}}, 0xH0000
+// CHECK: %hlsl.select.i = select reassoc nnan ninf nsz arcp afn i1 %{{.*}}, half 0xH0000, half %{{.*}}
+// CHECK: %vecinit.i = insertelement <4 x half> <half 0xH3C00, half poison, half poison, half poison>, half %{{.*}}, i32 1
+// CHECK: %vecinit2.i = insertelement <4 x half> %{{.*}}, half 0xH3C00, i32 3
+// CHECK: %cmp4.i = fcmp reassoc nnan ninf nsz arcp afn olt half %{{.*}}, 0xH0000
+// CHECK: %hlsl.or.i = or i1 %{{.*}}, %cmp4.i
+// CHECK: %elt.log.i = call reassoc nnan ninf nsz arcp afn half @llvm.log.f16(half %{{.*}})
+// CHECK: %mul.i = fmul reassoc nnan ninf nsz arcp afn half %elt.log.i, %{{.*}}
+// CHECK: %elt.exp.i = call reassoc nnan ninf nsz arcp afn half @llvm.exp.f16(half %mul.i)
+// CHECK: %hlsl.select7.i = select reassoc nnan ninf nsz arcp afn i1 %{{.*}}, half 0xH0000, half %{{.*}}
+// CHECK: %vecins.i = insertelement <4 x half> %{{.*}}, half %hlsl.select7.i, i32 2
+// CHECK: ret <4 x half> %{{.*}}
+half4 test_lit_half(half NDotL, half NDotH, half M) { return lit(NDotL, NDotH, M); }
+
+// CHECK-LABEL: test_lit_float
+// CHECK: %cmp.i = fcmp reassoc nnan ninf nsz arcp afn olt float %{{.*}}, 0.000000e+00
+// CHECK: %hlsl.select.i = select reassoc nnan ninf nsz arcp afn i1 %{{.*}}, float 0.000000e+00, float %{{.*}}
+// CHECK: %vecinit.i = insertelement <4 x float> <float 1.000000e+00, float poison, float poison, float poison>, float %{{.*}}, i32 1
+// CHECK: %vecinit2.i = insertelement <4 x float> %{{.*}}, float 1.000000e+00, i32 3
+// CHECK: %cmp4.i = fcmp reassoc nnan ninf nsz arcp afn olt float %{{.*}}, 0.000000e+00
+// CHECK: %hlsl.or.i = or i1 %{{.*}}, %cmp4.i
+// CHECK: %elt.log.i = call reassoc nnan ninf nsz arcp afn float @llvm.log.f32(float %{{.*}})
+// CHECK: %mul.i = fmul reassoc nnan ninf nsz arcp afn float %elt.log.i, %{{.*}}
+// CHECK: %elt.exp.i = call reassoc nnan ninf nsz arcp afn float @llvm.exp.f32(float %mul.i)
+// CHECK: %hlsl.select7.i = select reassoc nnan ninf nsz arcp afn i1 %{{.*}}, float 0.000000e+00, float %{{.*}}
+// CHECK: %vecins.i = insertelement <4 x float> %{{.*}}, float %hlsl.select7.i, i32 2
+// CHECK: ret <4 x float> %{{.*}}
+float4 test_lit_float(float NDotL, float NDotH, float M) { return lit(NDotL, NDotH, M); }