diff --git a/llvm/include/llvm/IR/FPEnv.h b/llvm/include/llvm/IR/FPEnv.h
--- a/llvm/include/llvm/IR/FPEnv.h
+++ b/llvm/include/llvm/IR/FPEnv.h
@@ -58,5 +58,11 @@
 inline bool isDefaultFPEnvironment(fp::ExceptionBehavior EB, RoundingMode RM) {
   return EB == fp::ebIgnore && RM == RoundingMode::NearestTiesToEven;
 }
+
+/// Returns true if the rounding mode RM may be QRM at compile time or
+/// at run time.
+inline bool RoundingModeMayBe(RoundingMode RM, RoundingMode QRM) {
+  return RM == QRM || RM == RoundingMode::Dynamic;
+}
 }
 #endif
diff --git a/llvm/lib/Analysis/InstructionSimplify.cpp b/llvm/lib/Analysis/InstructionSimplify.cpp
--- a/llvm/lib/Analysis/InstructionSimplify.cpp
+++ b/llvm/lib/Analysis/InstructionSimplify.cpp
@@ -4902,17 +4902,22 @@
   if (Constant *C = simplifyFPOp({Op0, Op1}, FMF, Q, ExBehavior, Rounding))
     return C;
 
-  if (!isDefaultFPEnvironment(ExBehavior, Rounding))
-    return nullptr;
-
   // fadd X, -0 ==> X
-  if (match(Op1, m_NegZeroFP()))
-    return Op0;
+  if (isDefaultFPEnvironment(ExBehavior, Rounding) ||
+      (!RoundingModeMayBe(Rounding, RoundingMode::TowardNegative) &&
+       (ExBehavior != fp::ebStrict || FMF.noNaNs())))
+    if (match(Op1, m_NegZeroFP()))
+      return Op0;
 
   // fadd X, 0 ==> X, when we know X is not -0
-  if (match(Op1, m_PosZeroFP()) &&
-      (FMF.noSignedZeros() || CannotBeNegativeZero(Op0, Q.TLI)))
-    return Op0;
+  if (isDefaultFPEnvironment(ExBehavior, Rounding) ||
+      (ExBehavior != fp::ebStrict || FMF.noNaNs()))
+    if (match(Op1, m_PosZeroFP()) &&
+        (FMF.noSignedZeros() || CannotBeNegativeZero(Op0, Q.TLI)))
+      return Op0;
+
+  if (!isDefaultFPEnvironment(ExBehavior, Rounding))
+    return nullptr;
 
   // With nnan: -X + X --> 0.0 (and commuted variant)
   // We don't have to explicitly exclude infinities (ninf): INF + -INF == NaN.
diff --git a/llvm/test/Transforms/InstSimplify/strictfp-fadd.ll b/llvm/test/Transforms/InstSimplify/strictfp-fadd.ll
new file mode 100644
--- /dev/null
+++ b/llvm/test/Transforms/InstSimplify/strictfp-fadd.ll
@@ -0,0 +1,303 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt < %s -instsimplify -S | FileCheck %s
+
+;
+; constrained fadd
+;
+
+;
+; fadd X, -0 ==> X
+;
+
+define float @fadd_x_n0_defaultenv(float %a) #0 {
+; CHECK-LABEL: @fadd_x_n0_defaultenv(
+; CHECK-NEXT:    ret float [[A:%.*]]
+;
+  %ret = call float @llvm.experimental.constrained.fadd.f32(float %a, float -0.0, metadata !"round.tonearest", metadata !"fpexcept.ignore") #0
+  ret float %ret
+}
+
+define <2 x float> @fadd_vec_x_n0_defaultenv(<2 x float> %a) #0 {
+; CHECK-LABEL: @fadd_vec_x_n0_defaultenv(
+; CHECK-NEXT:    ret <2 x float> [[A:%.*]]
+;
+  %ret = call <2 x float> @llvm.experimental.constrained.fadd.v2f32(<2 x float> %a, <2 x float><float -0.0, float -0.0>, metadata !"round.tonearest", metadata !"fpexcept.ignore") #0
+  ret <2 x float> %ret
+}
+
+define float @fadd_x_n0_ebmaytrap(float %a) #0 {
+; CHECK-LABEL: @fadd_x_n0_ebmaytrap(
+; CHECK-NEXT:    ret float [[A:%.*]]
+;
+  %ret = call float @llvm.experimental.constrained.fadd.f32(float %a, float -0.0, metadata !"round.tonearest", metadata !"fpexcept.maytrap") #0
+  ret float %ret
+}
+
+define <2 x float> @fadd_vec_x_n0_ebmaytrap(<2 x float> %a) #0 {
+; CHECK-LABEL: @fadd_vec_x_n0_ebmaytrap(
+; CHECK-NEXT:    ret <2 x float> [[A:%.*]]
+;
+  %ret = call <2 x float> @llvm.experimental.constrained.fadd.v2f32(<2 x float> %a, <2 x float><float -0.0, float -0.0>, metadata !"round.tonearest", metadata !"fpexcept.maytrap") #0
+  ret <2 x float> %ret
+}
+
+define float @fadd_x_n0_ebstrict(float %a) #0 {
+; CHECK-LABEL: @fadd_x_n0_ebstrict(
+; CHECK-NEXT:    [[RET:%.*]] = call float @llvm.experimental.constrained.fadd.f32(float [[A:%.*]], float -0.000000e+00, metadata !"round.tonearest", metadata !"fpexcept.strict") #[[ATTR0:[0-9]+]]
+; CHECK-NEXT:    ret float [[RET]]
+;
+  %ret = call float @llvm.experimental.constrained.fadd.f32(float %a, float -0.0, metadata !"round.tonearest", metadata !"fpexcept.strict") #0
+  ret float %ret
+}
+
+define <2 x float> @fadd_vec_x_n0_ebstrict(<2 x float> %a) #0 {
+; CHECK-LABEL: @fadd_vec_x_n0_ebstrict(
+; CHECK-NEXT:    [[RET:%.*]] = call <2 x float> @llvm.experimental.constrained.fadd.v2f32(<2 x float> [[A:%.*]], <2 x float> <float -0.000000e+00, float -0.000000e+00>, metadata !"round.tonearest", metadata !"fpexcept.strict") #[[ATTR0]]
+; CHECK-NEXT:    ret <2 x float> [[RET]]
+;
+  %ret = call <2 x float> @llvm.experimental.constrained.fadd.v2f32(<2 x float> %a, <2 x float><float -0.0, float -0.0>, metadata !"round.tonearest", metadata !"fpexcept.strict") #0
+  ret <2 x float> %ret
+}
+
+define float @fadd_x_n0_neginf(float %a) #0 {
+; CHECK-LABEL: @fadd_x_n0_neginf(
+; CHECK-NEXT:    [[RET:%.*]] = call float @llvm.experimental.constrained.fadd.f32(float [[A:%.*]], float -0.000000e+00, metadata !"round.downward", metadata !"fpexcept.ignore") #[[ATTR0]]
+; CHECK-NEXT:    ret float [[RET]]
+;
+  %ret = call float @llvm.experimental.constrained.fadd.f32(float %a, float -0.0, metadata !"round.downward", metadata !"fpexcept.ignore") #0
+  ret float %ret
+}
+
+define <2 x float> @fadd_vec_x_n0_neginf(<2 x float> %a) #0 {
+; CHECK-LABEL: @fadd_vec_x_n0_neginf(
+; CHECK-NEXT:    [[RET:%.*]] = call <2 x float> @llvm.experimental.constrained.fadd.v2f32(<2 x float> [[A:%.*]], <2 x float> <float -0.000000e+00, float -0.000000e+00>, metadata !"round.downward", metadata !"fpexcept.ignore") #[[ATTR0]]
+; CHECK-NEXT:    ret <2 x float> [[RET]]
+;
+  %ret = call <2 x float> @llvm.experimental.constrained.fadd.v2f32(<2 x float> %a, <2 x float><float -0.0, float -0.0>, metadata !"round.downward", metadata !"fpexcept.ignore") #0
+  ret <2 x float> %ret
+}
+
+define float @fadd_x_n0_dynamic(float %a) #0 {
+; CHECK-LABEL: @fadd_x_n0_dynamic(
+; CHECK-NEXT:    [[RET:%.*]] = call float @llvm.experimental.constrained.fadd.f32(float [[A:%.*]], float -0.000000e+00, metadata !"round.dynamic", metadata !"fpexcept.ignore") #[[ATTR0]]
+; CHECK-NEXT:    ret float [[RET]]
+;
+  %ret = call float @llvm.experimental.constrained.fadd.f32(float %a, float -0.0, metadata !"round.dynamic", metadata !"fpexcept.ignore") #0
+  ret float %ret
+}
+
+define <2 x float> @fadd_vec_x_n0_dynamic(<2 x float> %a) #0 {
+; CHECK-LABEL: @fadd_vec_x_n0_dynamic(
+; CHECK-NEXT:    [[RET:%.*]] = call <2 x float> @llvm.experimental.constrained.fadd.v2f32(<2 x float> [[A:%.*]], <2 x float> <float -0.000000e+00, float -0.000000e+00>, metadata !"round.dynamic", metadata !"fpexcept.ignore") #[[ATTR0]]
+; CHECK-NEXT:    ret <2 x float> [[RET]]
+;
+  %ret = call <2 x float> @llvm.experimental.constrained.fadd.v2f32(<2 x float> %a, <2 x float><float -0.0, float -0.0>, metadata !"round.dynamic", metadata !"fpexcept.ignore") #0
+  ret <2 x float> %ret
+}
+
+; The rounding mode here needs to not be { tonearest, downward, dynamic }.
+; Test one of the remaining rounding modes and the rest will be fine.
+define float @fadd_x_n0_towardzero(float %a) #0 {
+; CHECK-LABEL: @fadd_x_n0_towardzero(
+; CHECK-NEXT:    ret float [[A:%.*]]
+;
+  %ret = call float @llvm.experimental.constrained.fadd.f32(float %a, float -0.0, metadata !"round.towardzero", metadata !"fpexcept.ignore") #0
+  ret float %ret
+}
+
+; The rounding mode here needs to not be { tonearest, downward, dynamic }.
+; Test one of the remaining rounding modes and the rest will be fine.
+define <2 x float> @fadd_vec_x_n0_towardzero(<2 x float> %a) #0 {
+; CHECK-LABEL: @fadd_vec_x_n0_towardzero(
+; CHECK-NEXT:    ret <2 x float> [[A:%.*]]
+;
+  %ret = call <2 x float> @llvm.experimental.constrained.fadd.v2f32(<2 x float> %a, <2 x float><float -0.0, float -0.0>, metadata !"round.towardzero", metadata !"fpexcept.ignore") #0
+  ret <2 x float> %ret
+}
+
+define float @fadd_nnan_x_n0_ebstrict(float %a) #0 {
+; CHECK-LABEL: @fadd_nnan_x_n0_ebstrict(
+; CHECK-NEXT:    [[RET:%.*]] = call nnan float @llvm.experimental.constrained.fadd.f32(float [[A:%.*]], float -0.000000e+00, metadata !"round.tonearest", metadata !"fpexcept.strict") #[[ATTR0]]
+; CHECK-NEXT:    ret float [[A]]
+;
+  %ret = call nnan float @llvm.experimental.constrained.fadd.f32(float %a, float -0.0, metadata !"round.tonearest", metadata !"fpexcept.strict") #0
+  ret float %ret
+}
+
+define <2 x float> @fadd_vec_nnan_x_n0_ebstrict(<2 x float> %a) #0 {
+; CHECK-LABEL: @fadd_vec_nnan_x_n0_ebstrict(
+; CHECK-NEXT:    [[RET:%.*]] = call nnan <2 x float> @llvm.experimental.constrained.fadd.v2f32(<2 x float> [[A:%.*]], <2 x float> <float -0.000000e+00, float -0.000000e+00>, metadata !"round.tonearest", metadata !"fpexcept.strict") #[[ATTR0]]
+; CHECK-NEXT:    ret <2 x float> [[A]]
+;
+  %ret = call nnan <2 x float> @llvm.experimental.constrained.fadd.v2f32(<2 x float> %a, <2 x float><float -0.0, float -0.0>, metadata !"round.tonearest", metadata !"fpexcept.strict") #0
+  ret <2 x float> %ret
+}
+
+; Test with a fast math flag set but that flag is not "nnan".
+define float @fadd_ninf_x_n0_ebstrict(float %a) #0 {
+; CHECK-LABEL: @fadd_ninf_x_n0_ebstrict(
+; CHECK-NEXT:    [[RET:%.*]] = call ninf float @llvm.experimental.constrained.fadd.f32(float [[A:%.*]], float -0.000000e+00, metadata !"round.tonearest", metadata !"fpexcept.strict") #[[ATTR0]]
+; CHECK-NEXT:    ret float [[RET]]
+;
+  %ret = call ninf float @llvm.experimental.constrained.fadd.f32(float %a, float -0.0, metadata !"round.tonearest", metadata !"fpexcept.strict") #0
+  ret float %ret
+}
+
+; Test with a fast math flag set but that flag is not "nnan".
+define <2 x float> @fadd_vec_ninf_x_n0_ebstrict(<2 x float> %a) #0 {
+; CHECK-LABEL: @fadd_vec_ninf_x_n0_ebstrict(
+; CHECK-NEXT:    [[RET:%.*]] = call ninf <2 x float> @llvm.experimental.constrained.fadd.v2f32(<2 x float> [[A:%.*]], <2 x float> <float -0.000000e+00, float -0.000000e+00>, metadata !"round.tonearest", metadata !"fpexcept.strict") #[[ATTR0]]
+; CHECK-NEXT:    ret <2 x float> [[RET]]
+;
+  %ret = call ninf <2 x float> @llvm.experimental.constrained.fadd.v2f32(<2 x float> %a, <2 x float><float -0.0, float -0.0>, metadata !"round.tonearest", metadata !"fpexcept.strict") #0
+  ret <2 x float> %ret
+}
+
+define float @fadd_n0_x_defaultenv(float %a) #0 {
+; CHECK-LABEL: @fadd_n0_x_defaultenv(
+; CHECK-NEXT:    ret float [[A:%.*]]
+;
+  %ret = call float @llvm.experimental.constrained.fadd.f32(float -0.0, float %a, metadata !"round.tonearest", metadata !"fpexcept.ignore") #0
+  ret float %ret
+}
+
+define <2 x float> @fadd_vec_n0_x_defaultenv(<2 x float> %a) #0 {
+; CHECK-LABEL: @fadd_vec_n0_x_defaultenv(
+; CHECK-NEXT:    ret <2 x float> [[A:%.*]]
+;
+  %ret = call <2 x float> @llvm.experimental.constrained.fadd.v2f32(<2 x float><float -0.0, float -0.0>, <2 x float> %a, metadata !"round.tonearest", metadata !"fpexcept.ignore") #0
+  ret <2 x float> %ret
+}
+
+; TODO: Canonicalize the order of the arguments. Then this will fire.
+define float @fadd_n0_x_ebmaytrap(float %a) #0 {
+; CHECK-LABEL: @fadd_n0_x_ebmaytrap(
+; CHECK-NEXT:    [[RET:%.*]] = call float @llvm.experimental.constrained.fadd.f32(float -0.000000e+00, float [[A:%.*]], metadata !"round.tonearest", metadata !"fpexcept.maytrap") #[[ATTR0]]
+; CHECK-NEXT:    ret float [[RET]]
+;
+  %ret = call float @llvm.experimental.constrained.fadd.f32(float -0.0, float %a, metadata !"round.tonearest", metadata !"fpexcept.maytrap") #0
+  ret float %ret
+}
+
+; TODO: Canonicalize the order of the arguments. Then this will fire.
+define <2 x float> @fadd_vec_n0_x_ebmaytrap(<2 x float> %a) #0 {
+; CHECK-LABEL: @fadd_vec_n0_x_ebmaytrap(
+; CHECK-NEXT:    [[RET:%.*]] = call <2 x float> @llvm.experimental.constrained.fadd.v2f32(<2 x float> <float -0.000000e+00, float -0.000000e+00>, <2 x float> [[A:%.*]], metadata !"round.tonearest", metadata !"fpexcept.maytrap") #[[ATTR0]]
+; CHECK-NEXT:    ret <2 x float> [[RET]]
+;
+  %ret = call <2 x float> @llvm.experimental.constrained.fadd.v2f32(<2 x float><float -0.0, float -0.0>, <2 x float> %a, metadata !"round.tonearest", metadata !"fpexcept.maytrap") #0
+  ret <2 x float> %ret
+}
+
+;
+; fadd X, 0 ==> X, when we know X is not -0
+;
+
+define float @fold_fadd_nsz_x_0_defaultenv(float %a) #0 {
+; CHECK-LABEL: @fold_fadd_nsz_x_0_defaultenv(
+; CHECK-NEXT:    ret float [[A:%.*]]
+;
+  %add = call nsz float @llvm.experimental.constrained.fadd.f32(float %a, float 0.0, metadata !"round.tonearest", metadata !"fpexcept.ignore") #0
+  ret float %add
+}
+
+define <2 x float> @fold_fadd_vec_nsz_x_0_defaultenv(<2 x float> %a) #0 {
+; CHECK-LABEL: @fold_fadd_vec_nsz_x_0_defaultenv(
+; CHECK-NEXT:    ret <2 x float> [[A:%.*]]
+;
+  %add = call nsz <2 x float> @llvm.experimental.constrained.fadd.v2f32(<2 x float> %a, <2 x float> zeroinitializer, metadata !"round.tonearest", metadata !"fpexcept.ignore") #0
+  ret <2 x float> %add
+}
+
+define float @fold_fadd_nsz_x_0_ebmaytrap(float %a) #0 {
+; CHECK-LABEL: @fold_fadd_nsz_x_0_ebmaytrap(
+; CHECK-NEXT:    ret float [[A:%.*]]
+;
+  %add = call nsz float @llvm.experimental.constrained.fadd.f32(float %a, float 0.0, metadata !"round.tonearest", metadata !"fpexcept.maytrap") #0
+  ret float %add
+}
+
+define <2 x float> @fold_fadd_vec_nsz_x_0_ebmaytrap(<2 x float> %a) #0 {
+; CHECK-LABEL: @fold_fadd_vec_nsz_x_0_ebmaytrap(
+; CHECK-NEXT:    ret <2 x float> [[A:%.*]]
+;
+  %add = call nsz <2 x float> @llvm.experimental.constrained.fadd.v2f32(<2 x float> %a, <2 x float> zeroinitializer, metadata !"round.tonearest", metadata !"fpexcept.maytrap") #0
+  ret <2 x float> %add
+}
+
+define float @fold_fadd_nsz_x_0_ebstrict(float %a) #0 {
+; CHECK-LABEL: @fold_fadd_nsz_x_0_ebstrict(
+; CHECK-NEXT:    [[ADD:%.*]] = call nsz float @llvm.experimental.constrained.fadd.f32(float [[A:%.*]], float 0.000000e+00, metadata !"round.tonearest", metadata !"fpexcept.strict") #[[ATTR0]]
+; CHECK-NEXT:    ret float [[ADD]]
+;
+  %add = call nsz float @llvm.experimental.constrained.fadd.f32(float %a, float 0.0, metadata !"round.tonearest", metadata !"fpexcept.strict") #0
+  ret float %add
+}
+
+define <2 x float> @fold_fadd_vec_nsz_x_0_ebstrict(<2 x float> %a) #0 {
+; CHECK-LABEL: @fold_fadd_vec_nsz_x_0_ebstrict(
+; CHECK-NEXT:    [[ADD:%.*]] = call nsz <2 x float> @llvm.experimental.constrained.fadd.v2f32(<2 x float> [[A:%.*]], <2 x float> zeroinitializer, metadata !"round.tonearest", metadata !"fpexcept.strict") #[[ATTR0]]
+; CHECK-NEXT:    ret <2 x float> [[ADD]]
+;
+  %add = call nsz <2 x float> @llvm.experimental.constrained.fadd.v2f32(<2 x float> %a, <2 x float> zeroinitializer, metadata !"round.tonearest", metadata !"fpexcept.strict") #0
+  ret <2 x float> %add
+}
+
+define float @fold_fadd_nsz_nnan_x_0_ebstrict(float %a) #0 {
+; CHECK-LABEL: @fold_fadd_nsz_nnan_x_0_ebstrict(
+; CHECK-NEXT:    [[ADD:%.*]] = call nnan nsz float @llvm.experimental.constrained.fadd.f32(float [[A:%.*]], float 0.000000e+00, metadata !"round.tonearest", metadata !"fpexcept.strict") #[[ATTR0]]
+; CHECK-NEXT:    ret float [[A]]
+;
+  %add = call nsz nnan float @llvm.experimental.constrained.fadd.f32(float %a, float 0.0, metadata !"round.tonearest", metadata !"fpexcept.strict") #0
+  ret float %add
+}
+
+define <2 x float> @fold_fadd_vec_nsz_nnan_x_0_ebstrict(<2 x float> %a) #0 {
+; CHECK-LABEL: @fold_fadd_vec_nsz_nnan_x_0_ebstrict(
+; CHECK-NEXT:    [[ADD:%.*]] = call nnan nsz <2 x float> @llvm.experimental.constrained.fadd.v2f32(<2 x float> [[A:%.*]], <2 x float> zeroinitializer, metadata !"round.tonearest", metadata !"fpexcept.strict") #[[ATTR0]]
+; CHECK-NEXT:    ret <2 x float> [[A]]
+;
+  %add = call nsz nnan <2 x float> @llvm.experimental.constrained.fadd.v2f32(<2 x float> %a, <2 x float> zeroinitializer, metadata !"round.tonearest", metadata !"fpexcept.strict") #0
+  ret <2 x float> %add
+}
+
+define float @fold_fadd_nsz_0_x_defaultenv(float %a) #0 {
+; CHECK-LABEL: @fold_fadd_nsz_0_x_defaultenv(
+; CHECK-NEXT:    ret float [[A:%.*]]
+;
+  %add = call nsz float @llvm.experimental.constrained.fadd.f32(float 0.0, float %a, metadata !"round.tonearest", metadata !"fpexcept.ignore") #0
+  ret float %add
+}
+
+define <2 x float> @fold_fadd_vec_nsz_0_x_defaultenv(<2 x float> %a) #0 {
+; CHECK-LABEL: @fold_fadd_vec_nsz_0_x_defaultenv(
+; CHECK-NEXT:    ret <2 x float> [[A:%.*]]
+;
+  %add = call nsz <2 x float> @llvm.experimental.constrained.fadd.v2f32(<2 x float> zeroinitializer, <2 x float> %a, metadata !"round.tonearest", metadata !"fpexcept.ignore") #0
+  ret <2 x float> %add
+}
+
+; TODO: Canonicalize the order of the arguments. Then this will fire.
+define float @fold_fadd_nsz_0_x_ebmaytrap(float %a) #0 {
+; CHECK-LABEL: @fold_fadd_nsz_0_x_ebmaytrap(
+; CHECK-NEXT:    [[ADD:%.*]] = call nsz float @llvm.experimental.constrained.fadd.f32(float 0.000000e+00, float [[A:%.*]], metadata !"round.tonearest", metadata !"fpexcept.maytrap") #[[ATTR0]]
+; CHECK-NEXT:    ret float [[ADD]]
+;
+  %add = call nsz float @llvm.experimental.constrained.fadd.f32(float 0.0, float %a, metadata !"round.tonearest", metadata !"fpexcept.maytrap") #0
+  ret float %add
+}
+
+; TODO: Canonicalize the order of the arguments. Then this will fire.
+define <2 x float> @fold_fadd_vec_nsz_0_x_ebmaytrap(<2 x float> %a) #0 {
+; CHECK-LABEL: @fold_fadd_vec_nsz_0_x_ebmaytrap(
+; CHECK-NEXT:    [[ADD:%.*]] = call nsz <2 x float> @llvm.experimental.constrained.fadd.v2f32(<2 x float> zeroinitializer, <2 x float> [[A:%.*]], metadata !"round.tonearest", metadata !"fpexcept.maytrap") #[[ATTR0]]
+; CHECK-NEXT:    ret <2 x float> [[ADD]]
+;
+  %add = call nsz <2 x float> @llvm.experimental.constrained.fadd.v2f32(<2 x float> zeroinitializer, <2 x float> %a, metadata !"round.tonearest", metadata !"fpexcept.maytrap") #0
+  ret <2 x float> %add
+}
+
+declare float @llvm.experimental.constrained.fadd.f32(float, float, metadata, metadata) #0
+declare <2 x float> @llvm.experimental.constrained.fadd.v2f32(<2 x float>, <2 x float>, metadata, metadata) #0
+
+attributes #0 = { strictfp }