diff --git a/clang/lib/Headers/hlsl/hlsl_intrinsics.h b/clang/lib/Headers/hlsl/hlsl_intrinsics.h
--- a/clang/lib/Headers/hlsl/hlsl_intrinsics.h
+++ b/clang/lib/Headers/hlsl/hlsl_intrinsics.h
@@ -12,4 +12,12 @@
 __attribute__((clang_builtin_alias(__builtin_hlsl_wave_active_count_bits))) uint
 WaveActiveCountBits(bool bBit);
 
+
+__attribute__((clang_builtin_alias(__builtin_acos))) double acos(double In);
+__attribute__((clang_builtin_alias(__builtin_acosf))) float acos(float In);
+__attribute__((clang_builtin_alias(__builtin_acosl))) long double acos(long double In);
+//__attribute__((clang_builtin_alias(__builtin_acosf128))) __float128 acos(__float128 In);
+
+
+
 #endif //_HLSL_HLSL_INTRINSICS_H_
diff --git a/clang/test/CodeGenHLSL/builtins/acos.hlsl b/clang/test/CodeGenHLSL/builtins/acos.hlsl
new file mode 100644
--- /dev/null
+++ b/clang/test/CodeGenHLSL/builtins/acos.hlsl
@@ -0,0 +1,34 @@
+// RUN: %clang_cc1 -std=hlsl2021 -finclude-default-header -x hlsl -triple \
+// RUN:   dxil-pc-shadermodel6.3-library %s -emit-llvm -disable-llvm-passes \
+// RUN:   -o - | FileCheck %s
+
+double acos_d(double x)
+{
+  return acos(x);
+}
+
+// CHECK: define noundef double @"?acos_d@@YANN@Z"(
+// CHECK: call noundef double @acos(double noundef %0) #2
+
+float acos_f(float x)
+{
+  return acos(x);
+}
+
+// CHECK: define noundef float @"?acos_f@@YAMM@Z"(
+// CHECK: call noundef float @acosf(float noundef %0)
+
+long double acos_ld(long double x)
+{
+  return acos(x);
+}
+
+// CHECK: define noundef double @"?acos_ld@@YAOO@Z"(
+// CHECK: call noundef double @acosl(double noundef %0)
+
+/*
+long long double acos_lld(long long double x)
+{
+  return acos(x);
+}
+*/
\ No newline at end of file