Index: clang/lib/Basic/Targets/AMDGPU.cpp
===================================================================
--- clang/lib/Basic/Targets/AMDGPU.cpp
+++ clang/lib/Basic/Targets/AMDGPU.cpp
@@ -407,6 +407,9 @@
     }
   }
 
+  if (AllowAMDGPUUnsafeFPAtomics)
+    Builder.defineMacro("__AMDGCN_UNSAFE_FP_ATOMICS__");
+
   // TODO: __HAS_FMAF__, __HAS_LDEXPF__, __HAS_FP64__ are deprecated and will be
   // removed in the near future.
   if (hasFMAF())
Index: clang/test/Driver/amdgpu-macros.cl
===================================================================
--- clang/test/Driver/amdgpu-macros.cl
+++ clang/test/Driver/amdgpu-macros.cl
@@ -104,6 +104,7 @@
 // RUN: %clang -E -dM -target amdgcn -mcpu=gfx904 %s 2>&1 | FileCheck --check-prefixes=ARCH-GCN,FAST_FMAF %s -DWAVEFRONT_SIZE=64 -DCPU=gfx904
 // RUN: %clang -E -dM -target amdgcn -mcpu=gfx906 %s 2>&1 | FileCheck --check-prefixes=ARCH-GCN,FAST_FMAF %s -DWAVEFRONT_SIZE=64 -DCPU=gfx906
 // RUN: %clang -E -dM -target amdgcn -mcpu=gfx908 %s 2>&1 | FileCheck --check-prefixes=ARCH-GCN,FAST_FMAF %s -DWAVEFRONT_SIZE=64 -DCPU=gfx908
+// RUN: %clang -E -dM -target amdgcn -mcpu=gfx908 -munsafe-fp-atomics %s 2>&1 | FileCheck --check-prefixes=ARCH-GCN,UNSAFEFPATOMIC %s -DWAVEFRONT_SIZE=64 -DCPU=gfx908
 // RUN: %clang -E -dM -target amdgcn -mcpu=gfx909 %s 2>&1 | FileCheck --check-prefixes=ARCH-GCN,FAST_FMAF %s -DWAVEFRONT_SIZE=64 -DCPU=gfx909
 // RUN: %clang -E -dM -target amdgcn -mcpu=gfx90a %s 2>&1 | FileCheck --check-prefixes=ARCH-GCN,FAST_FMAF %s -DWAVEFRONT_SIZE=64 -DCPU=gfx90a
 // RUN: %clang -E -dM -target amdgcn -mcpu=gfx90c %s 2>&1 | FileCheck --check-prefixes=ARCH-GCN,FAST_FMAF %s -DWAVEFRONT_SIZE=64 -DCPU=gfx90c
@@ -134,6 +135,7 @@
 // ARCH-GCN-DAG: #define __[[CPU]]__ 1
 // ARCH-GCN-DAG: #define __amdgcn_processor__ "[[CPU]]"
 // ARCH-GCN-DAG: #define __AMDGCN_WAVEFRONT_SIZE [[WAVEFRONT_SIZE]]
+// UNSAFEFPATOMIC-DAG: #define __AMDGCN_UNSAFE_FP_ATOMICS__ 1
 
 // RUN: %clang -E -dM -target amdgcn -mcpu=gfx906 -mwavefrontsize64 \
 // RUN:   %s 2>&1 | FileCheck --check-prefix=WAVE64 %s