diff --git a/clang/include/clang/Basic/DiagnosticFrontendKinds.td b/clang/include/clang/Basic/DiagnosticFrontendKinds.td
--- a/clang/include/clang/Basic/DiagnosticFrontendKinds.td
+++ b/clang/include/clang/Basic/DiagnosticFrontendKinds.td
@@ -48,6 +48,10 @@
     "overriding currently unsupported use of floating point exceptions "
     "on this target">, InGroup<UnsupportedFPOpt>;
 
+def err_incompatible_fp_eval_method_options : Error<
+    "option 'ffp-eval-method' cannot be used with option "
+    "%select{'fapprox-func'|'mreassociate'|'freciprocal'}0">;
+
 def remark_fe_backend_optimization_remark : Remark<"%0">, BackendInfo,
     InGroup<BackendOptimizationRemark>;
 def remark_fe_backend_optimization_remark_missed : Remark<"%0">, BackendInfo,
diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -6491,6 +6491,10 @@
   "comparing floating point with == or != is unsafe">,
   InGroup<DiagGroup<"float-equal">>, DefaultIgnore;
 
+def err_setting_eval_method_used_in_unsafe_context : Error <
+  "%select{'#pragma clang fp eval_method'|option 'ffp-eval-method'}0 cannot be used with "
+  "%select{option 'fapprox-func'|option 'mreassociate'|option 'freciprocal'|option 'ffp-eval-method'|'#pragma clang fp reassociate'}1">;
+
 def warn_remainder_division_by_zero : Warning<
   "%select{remainder|division}0 by zero is undefined">,
   InGroup<DivZero>;
diff --git a/clang/lib/Frontend/CompilerInvocation.cpp b/clang/lib/Frontend/CompilerInvocation.cpp
--- a/clang/lib/Frontend/CompilerInvocation.cpp
+++ b/clang/lib/Frontend/CompilerInvocation.cpp
@@ -503,6 +503,22 @@
     Diags.Report(diag::warn_ignored_hip_only_option)
         << Args.getLastArg(OPT_gpu_max_threads_per_block_EQ)->getAsString(Args);
 
+  // When these options are used, the compiler is allowed to apply
+  // optimizations that may affect the final result. For example
+  // (x+y)+z is transformed to x+(y+z) but may not give the same
+  // final result; it's not value safe.
+  // Another example can be to simplify x/x to 1.0 but x could be 0.0, INF
+  // or NaN. Final result may then differ. An error is issued when the eval
+  // method is set with one of these options.
+  if (Args.hasArg(OPT_ffp_eval_method_EQ)) {
+    if (LangOpts.ApproxFunc)
+      Diags.Report(diag::err_incompatible_fp_eval_method_options) << 0;
+    if (LangOpts.AllowFPReassoc)
+      Diags.Report(diag::err_incompatible_fp_eval_method_options) << 1;
+    if (LangOpts.AllowRecip)
+      Diags.Report(diag::err_incompatible_fp_eval_method_options) << 2;
+  }
+
   // -cl-strict-aliasing needs to emit diagnostic in the case where CL > 1.0.
   // This option should be deprecated for CL > 1.0 because
   // this option was added for compatibility with OpenCL 1.0.
diff --git a/clang/lib/Sema/SemaAttr.cpp b/clang/lib/Sema/SemaAttr.cpp
--- a/clang/lib/Sema/SemaAttr.cpp
+++ b/clang/lib/Sema/SemaAttr.cpp
@@ -486,6 +486,12 @@
     NewFPFeatures.setFPEvalMethodOverride(LangOptions::FEM_Extended);
     break;
   }
+  if (getLangOpts().ApproxFunc)
+    Diag(Loc, diag::err_setting_eval_method_used_in_unsafe_context) << 0 << 0;
+  if (getLangOpts().AllowFPReassoc)
+    Diag(Loc, diag::err_setting_eval_method_used_in_unsafe_context) << 0 << 1;
+  if (getLangOpts().AllowRecip)
+    Diag(Loc, diag::err_setting_eval_method_used_in_unsafe_context) << 0 << 2;
   FpPragmaStack.Act(Loc, PSK_Set, StringRef(), NewFPFeatures);
   CurFPFeatures = NewFPFeatures.applyOverrides(getLangOpts());
   PP.setCurrentFPEvalMethod(Loc, Value);
@@ -1153,6 +1159,23 @@
 }
 
 void Sema::ActOnPragmaFPReassociate(SourceLocation Loc, bool IsEnabled) {
+  if (IsEnabled) {
+    // For value unsafe context, combining this pragma with eval method
+    // setting is not recommended. See comment in function FixupInvocation#506.
+    int Reason = -1;
+    if (getLangOpts().getFPEvalMethod() != LangOptions::FEM_UnsetOnCommandLine)
+      // Eval method set using the option 'ffp-eval-method'.
+      Reason = 1;
+    if (PP.getLastFPEvalPragmaLocation().isValid())
+      // Eval method set using the '#pragma clang fp eval_method'.
+      // We could have both an option and a pragma used to the set the eval
+      // method. The pragma overrides the option in the command line. The Reason
+      // of the diagnostic is overriden too.
+      Reason = 0;
+    if (Reason != -1)
+      Diag(Loc, diag::err_setting_eval_method_used_in_unsafe_context)
+          << Reason << 4;
+  }
   FPOptionsOverride NewFPFeatures = CurFPFeatureOverrides();
   NewFPFeatures.setAllowFPReassociateOverride(IsEnabled);
   FpPragmaStack.Act(Loc, PSK_Set, StringRef(), NewFPFeatures);
diff --git a/clang/test/CodeGen/X86/32bit-behavior.c b/clang/test/CodeGen/X86/32bit-behavior.c
--- a/clang/test/CodeGen/X86/32bit-behavior.c
+++ b/clang/test/CodeGen/X86/32bit-behavior.c
@@ -14,22 +14,6 @@
 // RUN: -emit-llvm -o - %s -ffp-eval-method=extended \
 // RUN: | FileCheck -check-prefix=CHECK-DBL %s
 
-// SSE Fast Math
-// RUN: %clang_cc1 -fexperimental-strict-floating-point \
-// RUN: -triple i386-pc-windows -target-cpu pentium4 -target-feature +sse \
-// RUN: -emit-llvm -o - %s -ffp-eval-method=source \
-// RUN: -ffast-math | FileCheck -check-prefix=CHECK-FM-SRC %s
-
-// RUN: %clang_cc1 -fexperimental-strict-floating-point \
-// RUN: -triple i386-pc-windows -target-cpu pentium4 -target-feature +sse \
-// RUN: -emit-llvm -o - %s -ffp-eval-method=double \
-// RUN: -ffast-math | FileCheck -check-prefix=CHECK-FM %s
-
-// RUN: %clang_cc1 -fexperimental-strict-floating-point \
-// RUN: -triple i386-pc-windows -target-cpu pentium4 -target-feature +sse \
-// RUN: -emit-llvm -o - %s -ffp-eval-method=extended \
-// RUN: -ffast-math | FileCheck -check-prefix=CHECK-FM %s
-
 // NO SSE
 // RUN: %clang_cc1 -fexperimental-strict-floating-point \
 // RUN: -triple i386-pc-windows -target-cpu pentium4 -target-feature -sse \
@@ -46,22 +30,6 @@
 // RUN: -emit-llvm -o - %s -ffp-eval-method=extended \
 // RUN: | FileCheck -check-prefix=CHECK-DBL %s
 
-// NO SSE Fast Math
-// RUN: %clang_cc1 -fexperimental-strict-floating-point \
-// RUN: -triple i386-pc-windows -target-cpu pentium4 -target-feature -sse \
-// RUN: -emit-llvm -o - %s -ffp-eval-method=source \
-// RUN: -ffast-math | FileCheck -check-prefix=CHECK %s
-
-// RUN: %clang_cc1 -fexperimental-strict-floating-point \
-// RUN: -triple i386-pc-windows -target-cpu pentium4 -target-feature -sse \
-// RUN: -emit-llvm -o - %s -ffp-eval-method=double \
-// RUN: -ffast-math | FileCheck -check-prefix=CHECK-DBL-FM %s
-
-// RUN: %clang_cc1 -fexperimental-strict-floating-point \
-// RUN: -triple i386-pc-windows -target-cpu pentium4 -target-feature -sse \
-// RUN: -emit-llvm -o - %s -ffp-eval-method=extended \
-// RUN: -ffast-math | FileCheck -check-prefix=CHECK-DBL-FM %s
-
 float addit(float a, float b, float c) {
   // CHECK-SRC: load float, float*
   // CHECK-SRC: load float, float*
@@ -69,21 +37,6 @@
   // CHECK-SRC: load float, float*
   // CHECK-SRC: fadd float
 
-  // CHECK-FM-SRC: load float, float*
-  // CHECK-FM-SRC: load float, float*
-  // CHECK-FM-SRC: fadd reassoc nnan ninf nsz arcp afn float
-  // CHECK-FM-SRC: load float, float*
-  // CHECK-FM-SRC: fadd reassoc nnan ninf nsz arcp afn float
-
-  // CHECK-FM: load float, float*
-  // CHECK-FM: fpext float {{.*}} to double
-  // CHECK-FM: load float, float*
-  // CHECK-FM: fpext float {{.*}} to double
-  // CHECK-FM: fadd reassoc nnan ninf nsz arcp afn double
-  // CHECK-FM: load float, float*
-  // CHECK-FM: fadd reassoc nnan ninf nsz arcp afn double
-  // CHECK-FM: fptrunc double {{.*}} to float
-
   // CHECK-DBL: load float, float*
   // CHECK-DBL: fpext float {{.*}} to double
   // CHECK-DBL: load float, float*
@@ -94,16 +47,6 @@
   // CHECK-DBL: fadd double
   // CHECK-DBL: fptrunc double {{.*}} to float
 
-  // CHECK-DBL-FM: load float, float*
-  // CHECK-DBL-FM: fpext float {{.*}} to double
-  // CHECK-DBL-FM: load float, float*
-  // CHECK-DBL-FM: fpext float {{.*}} to double
-  // CHECK-DBL-FM: fadd reassoc nnan ninf nsz arcp afn double
-  // CHECK-DBL-FM: load float, float*
-  // CHECK-DBL-FM: fpext float {{.*}} to double
-  // CHECK-DBL-FM: fadd reassoc nnan ninf nsz arcp afn double
-  // CHECK-DBL-FM: fptrunc double {{.*}} to float
-
   // CHECK: ret float
   return a + b + c;
 }
diff --git a/clang/test/Driver/eval-method-with-unsafe-math.c b/clang/test/Driver/eval-method-with-unsafe-math.c
new file mode 100644
--- /dev/null
+++ b/clang/test/Driver/eval-method-with-unsafe-math.c
@@ -0,0 +1,29 @@
+// RUN: not %clang -Xclang -fexperimental-strict-floating-point \
+// RUN: -Xclang -triple -Xclang x86_64-linux-gnu -fapprox-func \
+// RUN: -Xclang -verify -ffp-eval-method=source %s 2>&1  \
+// RUN: | FileCheck %s --check-prefixes=CHECK-FUNC
+
+// RUN: not %clang -Xclang -fexperimental-strict-floating-point \
+// RUN: -Xclang -triple -Xclang x86_64-linux-gnu -Xclang -mreassociate \
+// RUN: -ffp-eval-method=source -Xclang -verify %s 2>&1 \
+// RUN: | FileCheck %s --check-prefixes=CHECK-ASSOC
+
+// RUN: not %clang -Xclang -fexperimental-strict-floating-point \
+// RUN: -Xclang -triple -Xclang x86_64-linux-gnu -Xclang -freciprocal-math \
+// RUN: -ffp-eval-method=source -Xclang -verify %s 2>&1 \
+// RUN: | FileCheck %s --check-prefixes=CHECK-RECPR
+
+// RUN: not %clang -Xclang -fexperimental-strict-floating-point \
+// RUN: -Xclang -triple -Xclang x86_64-linux-gnu -Xclang -freciprocal-math \
+// RUN: -Xclang -mreassociate -ffp-eval-method=source -Xclang -verify %s 2>&1 \
+// RUN: | FileCheck %s --check-prefixes=CHECK-ASSOC,CHECK-RECPR
+
+// RUN: not %clang -Xclang -fexperimental-strict-floating-point \
+// RUN: -Xclang -triple -Xclang x86_64-linux-gnu -Xclang -freciprocal-math \
+// RUN: -Xclang -mreassociate -fapprox-func -ffp-eval-method=source \
+// RUN: -Xclang -verify %s 2>&1 \
+// RUN: | FileCheck %s --check-prefixes=CHECK-ASSOC,CHECK-RECPR,CHECK-FUNC
+
+// CHECK-FUNC: (frontend): option 'ffp-eval-method' cannot be used with option 'fapprox-func'
+// CHECK-ASSOC: (frontend): option 'ffp-eval-method' cannot be used with option 'mreassociate'
+// CHECK-RECPR: (frontend): option 'ffp-eval-method' cannot be used with option 'freciprocal'
diff --git a/clang/test/Sema/eval-method-with-unsafe-math.c b/clang/test/Sema/eval-method-with-unsafe-math.c
new file mode 100644
--- /dev/null
+++ b/clang/test/Sema/eval-method-with-unsafe-math.c
@@ -0,0 +1,56 @@
+// RUN: not %clang_cc1 -fexperimental-strict-floating-point \
+// RUN: -triple x86_64-linux-gnu  \
+// RUN: -verify %s 2>&1 | FileCheck %s --check-prefixes=CHECK-PRGM
+
+// RUN: not %clang_cc1 -fexperimental-strict-floating-point \
+// RUN: -triple x86_64-linux-gnu  -freciprocal-math \
+// RUN: -verify %s 2>&1 | FileCheck %s --check-prefixes=CHECK-RECPR,CHECK-PRGM
+
+// RUN: not %clang_cc1 -fexperimental-strict-floating-point \
+// RUN: -triple x86_64-linux-gnu  -mreassociate \
+// RUN: -verify %s 2>&1 | FileCheck %s --check-prefixes=CHECK-ASSOC,CHECK-PRGM
+
+// RUN: not %clang_cc1 -fexperimental-strict-floating-point \
+// RUN: -triple x86_64-linux-gnu  -fapprox-func \
+// RUN: -verify %s 2>&1 | FileCheck %s --check-prefixes=CHECK-FUNC,CHECK-PRGM
+
+// RUN: not %clang_cc1 -fexperimental-strict-floating-point \
+// RUN: -triple x86_64-linux-gnu -freciprocal-math -mreassociate -verify \
+// RUN: %s 2>&1 | FileCheck %s --check-prefixes=CHECK-ASSOC,CHECK-RECPR,CHECK-PRGM
+
+// RUN: not %clang_cc1 -fexperimental-strict-floating-point \
+// RUN: -triple x86_64-linux-gnu -freciprocal-math -mreassociate -fapprox-func \
+// RUN: -verify %s 2>&1 \
+// RUN: | FileCheck %s --check-prefixes=CHECK-FUNC,CHECK-ASSOC,CHECK-RECPR,CHECK-PRGM
+
+// RUN: not %clang_cc1 -fexperimental-strict-floating-point \
+// RUN: -triple x86_64-linux-gnu -ffp-eval-method=source \
+// RUN: -verify %s 2>&1 | FileCheck %s --check-prefixes=CHECK-FFP-OPT,CHECK-PRGM
+
+// expected-no-diagnostics
+
+float f1(float a, float b, float c) {
+  a = b + c;
+  return a * b + c;
+}
+
+float f2(float a, float b, float c) {
+  // CHECK-FFP-OPT: option 'ffp-eval-method' cannot be used with '#pragma clang fp reassociate'
+#pragma clang fp reassociate(on)
+  return (a + b) + c;
+}
+
+float f3(float a, float b, float c) {
+#pragma clang fp reassociate(off)
+  return (a - b) - c;
+}
+
+float f4(float a, float b, float c) {
+#pragma clang fp eval_method(double)
+  // CHECK-FUNC: '#pragma clang fp eval_method' cannot be used with option 'fapprox-func'
+  // CHECK-ASSOC: '#pragma clang fp eval_method' cannot be used with option 'mreassociate'
+  // CHECK-RECPR: '#pragma clang fp eval_method' cannot be used with option 'freciprocal'
+  // CHECK-PRGM: '#pragma clang fp eval_method' cannot be used with '#pragma clang fp reassociate'
+#pragma clang fp reassociate(on)
+  return (a * c) - (b * c);
+}