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
@@ -5471,9 +5471,6 @@
         (FMF.noSignedZeros() || CannotBeNegativeZero(Op0, Q.TLI)))
       return Op0;
 
-  if (!isDefaultFPEnvironment(ExBehavior, Rounding))
-    return nullptr;
-
   if (FMF.noNaNs()) {
     // With nnan: X + {+/-}Inf --> {+/-}Inf
     if (match(Op1, m_Inf()))
@@ -5486,22 +5483,27 @@
     // X = -0.0: ( 0.0 - (-0.0)) + (-0.0) == ( 0.0) + (-0.0) == 0.0
     // X =  0.0: (-0.0 - ( 0.0)) + ( 0.0) == (-0.0) + ( 0.0) == 0.0
     // X =  0.0: ( 0.0 - ( 0.0)) + ( 0.0) == ( 0.0) + ( 0.0) == 0.0
-    if (match(Op0, m_FSub(m_AnyZeroFP(), m_Specific(Op1))) ||
-        match(Op1, m_FSub(m_AnyZeroFP(), m_Specific(Op0))))
-      return ConstantFP::getNullValue(Op0->getType());
+    // TODO: Constrained intrinsics can have differing rounding modes.
+    // TODO: This complicates the set of cases.
+    if (isDefaultFPEnvironment(ExBehavior, Rounding))
+      if (match(Op0, m_FSub(m_AnyZeroFP(), m_Specific(Op1))) ||
+          match(Op1, m_FSub(m_AnyZeroFP(), m_Specific(Op0))))
+        return ConstantFP::getNullValue(Op0->getType());
 
-    if (match(Op0, m_FNeg(m_Specific(Op1))) ||
-        match(Op1, m_FNeg(m_Specific(Op0))))
-      return ConstantFP::getNullValue(Op0->getType());
+    if (isDefaultFPEnvironment(ExBehavior, Rounding))
+      if (match(Op0, m_FNeg(m_Specific(Op1))) ||
+          match(Op1, m_FNeg(m_Specific(Op0))))
+        return ConstantFP::getNullValue(Op0->getType());
   }
 
   // (X - Y) + Y --> X
   // Y + (X - Y) --> X
   Value *X;
-  if (FMF.noSignedZeros() && FMF.allowReassoc() &&
-      (match(Op0, m_FSub(m_Value(X), m_Specific(Op1))) ||
-       match(Op1, m_FSub(m_Value(X), m_Specific(Op0)))))
-    return X;
+  if (isDefaultFPEnvironment(ExBehavior, Rounding))
+    if (FMF.noSignedZeros() && FMF.allowReassoc() &&
+        (match(Op0, m_FSub(m_Value(X), m_Specific(Op1))) ||
+         match(Op1, m_FSub(m_Value(X), m_Specific(Op0)))))
+      return X;
 
   return nullptr;
 }
diff --git a/llvm/test/Transforms/InstSimplify/floating-point-arithmetic-strictfp.ll b/llvm/test/Transforms/InstSimplify/floating-point-arithmetic-strictfp.ll
--- a/llvm/test/Transforms/InstSimplify/floating-point-arithmetic-strictfp.ll
+++ b/llvm/test/Transforms/InstSimplify/floating-point-arithmetic-strictfp.ll
@@ -44,6 +44,78 @@
   ret <2 x float> %r
 }
 
+; fadd Inf, X -> Inf
+define double @fadd_nnan_inf_op0_defaultenv(double %x) #0 {
+; CHECK-LABEL: @fadd_nnan_inf_op0_defaultenv(
+; CHECK-NEXT:    ret double 0x7FF0000000000000
+;
+  %r = call nnan double @llvm.experimental.constrained.fadd.f64(double 0x7ff0000000000000, double %x, metadata !"round.tonearest", metadata !"fpexcept.ignore") #0
+  ret double %r
+}
+
+define double @fadd_nnan_inf_op0_dynamic(double %x) #0 {
+; CHECK-LABEL: @fadd_nnan_inf_op0_dynamic(
+; CHECK-NEXT:    [[R:%.*]] = call nnan double @llvm.experimental.constrained.fadd.f64(double 0x7FF0000000000000, double [[X:%.*]], metadata !"round.dynamic", metadata !"fpexcept.ignore") #[[ATTR0]]
+; CHECK-NEXT:    ret double [[R]]
+;
+  %r = call nnan double @llvm.experimental.constrained.fadd.f64(double 0x7ff0000000000000, double %x, metadata !"round.dynamic", metadata !"fpexcept.ignore") #0
+  ret double %r
+}
+
+define double @fadd_nnan_inf_op0_maytrap(double %x) #0 {
+; CHECK-LABEL: @fadd_nnan_inf_op0_maytrap(
+; CHECK-NEXT:    [[R:%.*]] = call nnan double @llvm.experimental.constrained.fadd.f64(double 0x7FF0000000000000, double [[X:%.*]], metadata !"round.tonearest", metadata !"fpexcept.maytrap") #[[ATTR0]]
+; CHECK-NEXT:    ret double [[R]]
+;
+  %r = call nnan double @llvm.experimental.constrained.fadd.f64(double 0x7ff0000000000000, double %x, metadata !"round.tonearest", metadata !"fpexcept.maytrap") #0
+  ret double %r
+}
+
+define double @fadd_nnan_inf_op0_strict(double %x) #0 {
+; CHECK-LABEL: @fadd_nnan_inf_op0_strict(
+; CHECK-NEXT:    [[R:%.*]] = call nnan double @llvm.experimental.constrained.fadd.f64(double 0x7FF0000000000000, double [[X:%.*]], metadata !"round.tonearest", metadata !"fpexcept.strict") #[[ATTR0]]
+; CHECK-NEXT:    ret double [[R]]
+;
+  %r = call nnan double @llvm.experimental.constrained.fadd.f64(double 0x7ff0000000000000, double %x, metadata !"round.tonearest", metadata !"fpexcept.strict") #0
+  ret double %r
+}
+
+; fadd X, Inf -> Inf
+define double @fadd_nnan_inf_op1_defaultenv(double %x) #0 {
+; CHECK-LABEL: @fadd_nnan_inf_op1_defaultenv(
+; CHECK-NEXT:    ret double 0x7FF0000000000000
+;
+  %r = call nnan double @llvm.experimental.constrained.fadd.f64(double %x, double 0x7ff0000000000000, metadata !"round.tonearest", metadata !"fpexcept.ignore") #0
+  ret double %r
+}
+
+define double @fadd_nnan_inf_op1_dynamic(double %x) #0 {
+; CHECK-LABEL: @fadd_nnan_inf_op1_dynamic(
+; CHECK-NEXT:    ret double 0x7FF0000000000000
+;
+  %r = call nnan double @llvm.experimental.constrained.fadd.f64(double %x, double 0x7ff0000000000000, metadata !"round.dynamic", metadata !"fpexcept.ignore") #0
+  ret double %r
+}
+
+define double @fadd_nnan_inf_op1_maytrap(double %x) #0 {
+; CHECK-LABEL: @fadd_nnan_inf_op1_maytrap(
+; CHECK-NEXT:    ret double 0x7FF0000000000000
+;
+  %r = call nnan double @llvm.experimental.constrained.fadd.f64(double %x, double 0x7ff0000000000000, metadata !"round.tonearest", metadata !"fpexcept.maytrap") #0
+  ret double %r
+}
+
+define double @fadd_nnan_inf_op1_strict(double %x) #0 {
+; CHECK-LABEL: @fadd_nnan_inf_op1_strict(
+; CHECK-NEXT:    [[R:%.*]] = call nnan double @llvm.experimental.constrained.fadd.f64(double [[X:%.*]], double 0x7FF0000000000000, metadata !"round.tonearest", metadata !"fpexcept.strict") #[[ATTR0]]
+; CHECK-NEXT:    ret double 0x7FF0000000000000
+;
+  %r = call nnan double @llvm.experimental.constrained.fadd.f64(double %x, double 0x7ff0000000000000, metadata !"round.tonearest", metadata !"fpexcept.strict") #0
+  ret double %r
+}
+
+
+
 ; fsub -0.0, (fsub -0.0, X) ==> X
 define float @fsub_-0_-0_x(float %a) #0 {
 ; CHECK-LABEL: @fsub_-0_-0_x(
@@ -523,6 +595,7 @@
 
 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
+declare double @llvm.experimental.constrained.fadd.f64(double, double, metadata, metadata) #0
 
 declare float @llvm.experimental.constrained.fsub.f32(float, float, metadata, metadata) #0
 declare <2 x float> @llvm.experimental.constrained.fsub.v2f32(<2 x float>, <2 x float>, metadata, metadata) #0