Add constant evaluation for fmod to CodeGen

tools/clang/lib/CodeGen/CGHLSLMSFinishCodeGen.cpp

@@ -2223,6 +2223,15 @@ Value *TryEvalIntrinsic(CallInst *CI, IntrinsicOp intriOp,
     };
     return EvalTernaryIntrinsic(CI, clampF, clampD, clampI);
   } break;
+  case IntrinsicOp::IOP_fmod: {
+    auto fmodF = [](float a, float b) -> float {
+      return a - b * std::truncf(a / b);
+    };
+    auto fmodD = [](double a, double b) -> double {
+      return a - b * std::trunc(a / b);
+    };
+    return EvalBinaryIntrinsic(CI, fmodF, fmodD);
+  } break;
   default:
     return nullptr;
   }

tools/clang/test/HLSLFileCheck/hlsl/intrinsics/basic/FMod_const_prop.hlsl

@@ -0,0 +1,20 @@
+// RUN: %dxc -T lib_6_x -fcgl %s -E main | %FileCheck %s
+
+// Ensure fmod is constant propagated during codegen
+// CHECK: call void {{.*}}results{{.*}}(float 2.500000e+00, float -2.500000e+00, float 2.500000e+00, float -2.500000e+00)
+// CHECK: call void {{.*}}results{{.*}}(float 2.500000e+00, float -2.500000e+00, float 2.500000e+00, float -2.500000e+00)
+
+void results(float a, float b, float c, float d);
+
+void main() {
+    results(
+        fmod(5.5, 3.0),
+        fmod(-5.5, 3.0),
+        fmod(5.5, -3.0),
+        fmod(-5.5, -3.0));
+    results(
+        fmod(5.5f, 3.0f),
+        fmod(-5.5f, 3.0f),
+        fmod(5.5f, -3.0f),
+        fmod(-5.5f, -3.0f));
+}