Index: llvm/lib/Analysis/ConstantFolding.cpp
===================================================================
--- llvm/lib/Analysis/ConstantFolding.cpp
+++ llvm/lib/Analysis/ConstantFolding.cpp
@@ -1000,6 +1000,45 @@
   return C;
 }
 
+// Check whether a constant is a floating point denormal that should be flushed
+// to zero according to the denormal handling mode set in the function
+// attributes. If so, return a zero with the correct sign, otherwise return the
+// original constant. Inputs and outputs to floating point instructions can have
+// their mode set seperately, so this information is also needed.
+Constant *FlushFPConstant(Constant *Operand, const llvm::Function *F,
+                          bool IsOutput) {
+  if (F == nullptr)
+    return Operand;
+  if (auto *CFP = dyn_cast<ConstantFP>(Operand)) {
+    const APFloat &APF = CFP->getValueAPF();
+    Type *Ty = CFP->getType();
+    DenormalMode DenormMode = F->getDenormalMode(Ty->getFltSemantics());
+    llvm::DenormalMode::DenormalModeKind Mode =
+        IsOutput ? DenormMode.Output : DenormMode.Input;
+    switch (Mode) {
+    default:
+      llvm_unreachable("unknown denormal mode");
+      return Operand;
+    case llvm::DenormalMode::IEEE:
+      return Operand;
+    case llvm::DenormalMode::PreserveSign:
+      if (APF.isDenormal()) {
+        return ConstantFP::get(
+            Ty->getContext(),
+            APFloat::getZero(Ty->getFltSemantics(), APF.isNegative()));
+      }
+      return Operand;
+    case llvm::DenormalMode::PositiveZero:
+      if (APF.isDenormal()) {
+        return ConstantFP::get(Ty->getContext(),
+                               APFloat::getZero(Ty->getFltSemantics(), false));
+      }
+      return Operand;
+    }
+  }
+  return Operand;
+}
+
 /// Attempt to constant fold an instruction with the
 /// specified opcode and operands.  If successful, the constant result is
 /// returned, if not, null is returned.  Note that this function can fail when
@@ -1011,6 +1050,39 @@
                                        const TargetLibraryInfo *TLI) {
   Type *DestTy = InstOrCE->getType();
 
+  switch (Opcode) {
+  default:
+    break;
+  case Instruction::FNeg:
+  case Instruction::FAdd:
+  case Instruction::FSub:
+  case Instruction::FMul:
+  case Instruction::FDiv:
+  case Instruction::FRem:
+    // For floating point instructions, use the denormal handling mode set by
+    // function attributes to check whether the inputs to constant folding
+    // should be treated as zero, and/or whether the outputs should be flushed
+    // to zero.
+    if (auto *I = dyn_cast<Instruction>(InstOrCE)) {
+      if (auto *BB = I->getParent()) {
+        if (auto *F = BB->getParent()) {
+          if (Instruction::isUnaryOp(Opcode)) {
+            Constant *Op0 = FlushFPConstant(Ops[0], F, false);
+            Constant *C = ConstantFoldUnaryOpOperand(Opcode, Op0, DL);
+            return FlushFPConstant(C, F, true);
+          }
+          if (Instruction::isBinaryOp(Opcode)) {
+            Constant *Op0 = FlushFPConstant(Ops[0], F, false);
+            Constant *Op1 = FlushFPConstant(Ops[1], F, false);
+            Constant *C = ConstantFoldBinaryOpOperands(Opcode, Op0, Op1, DL);
+            return FlushFPConstant(C, F, true);
+          }
+        }
+      }
+    }
+    break;
+  }
+
   if (Instruction::isUnaryOp(Opcode))
     return ConstantFoldUnaryOpOperand(Opcode, Ops[0], DL);
 
Index: llvm/test/Transforms/InstCombine/AArch64/constant-fold-fp-denormal.ll
===================================================================
--- /dev/null
+++ llvm/test/Transforms/InstCombine/AArch64/constant-fold-fp-denormal.ll
@@ -0,0 +1,72 @@
+; RUN: opt -S -instcombine < %s | FileCheck %s
+
+target triple = "aarch64--linux-gnu"
+
+define float @test_float() #0 {
+; CHECK-LABEL: @test_float(
+; CHECK-NEXT:    ret float 0x3800000000000000
+  %mul = fmul float 0x3810000000000000, 5.000000e-01
+  ret float %mul
+}
+
+define double @test_double() #0 {
+; CHECK-LABEL: @test_double(
+; CHECK-NEXT:    ret double 0x8000000000000
+  %mul = fmul double 0x10000000000000, 5.000000e-01
+  ret double %mul
+}
+
+define float @test_float_positive() #1 {
+; CHECK-LABEL: @test_float_positive(
+; CHECK-NEXT:    ret float 0.000000e+00
+  %mul = fmul float 0x3810000000000000, 5.000000e-01
+  ret float %mul
+}
+
+define double @test_double_positive() #1 {
+; CHECK-LABEL: @test_double_positive(
+; CHECK-NEXT:    ret double 0.000000e+00
+  %mul = fmul double 0x10000000000000, 5.000000e-01
+  ret double %mul
+}
+
+define float @test_float_preserve() #2 {
+; CHECK-LABEL: @test_float_preserve(
+; CHECK-NEXT:    ret float -0.000000e+00
+  %mul = fmul float 0x3810000000000000, -5.000000e-01
+  ret float %mul
+}
+
+define double @test_double_preserve() #2 {
+; CHECK-LABEL: @test_double_preserve(
+; CHECK-NEXT:    ret double -0.000000e+00
+  %mul = fmul double 0x10000000000000, -5.000000e-01
+  ret double %mul
+}
+
+define float @test_float_positive_f32() #3 {
+; CHECK-LABEL: @test_float_positive_f32(
+; CHECK-NEXT:    ret float 0.000000e+00
+  %mul = fmul float 0x3810000000000000, 5.000000e-01
+  ret float %mul
+}
+
+define double @test_double_positive_f32() #3 {
+; CHECK-LABEL: @test_double_positive_f32(
+; CHECK-NEXT:    ret double 0x8000000000000
+  %mul = fmul double 0x10000000000000, 5.000000e-01
+  ret double %mul
+}
+
+define double @test_double_input() #4 {
+; CHECK-LABEL: @test_double_input(
+; CHECK-NEXT:    ret double 0.000000e+00
+  %mul = fmul double 0x8000000000000, 1.000000e+10
+  ret double %mul
+}
+
+attributes #0 = { nounwind "denormal-fp-math"="ieee,ieee" }
+attributes #1 = { nounwind "denormal-fp-math"="positive-zero,ieee" }
+attributes #2 = { nounwind "denormal-fp-math"="preserve-sign,ieee" }
+attributes #3 = { nounwind "denormal-fp-math"="ieee,ieee" "denormal-fp-math-f32"="positive-zero,ieee" }
+attributes #4 = { nounwind "denormal-fp-math"="ieee,positive-zero" }
Index: llvm/test/Transforms/InstCombine/ARM/constant-fold-fp-denormal.ll
===================================================================
--- /dev/null
+++ llvm/test/Transforms/InstCombine/ARM/constant-fold-fp-denormal.ll
@@ -0,0 +1,72 @@
+; RUN: opt -S -instcombine < %s | FileCheck %s
+
+target triple = "armv8-arm-none-eabi"
+
+define float @test_float() #0 {
+; CHECK-LABEL: @test_float(
+; CHECK-NEXT:    ret float 0x3800000000000000
+  %mul = fmul float 0x3810000000000000, 5.000000e-01
+  ret float %mul
+}
+
+define double @test_double() #0 {
+; CHECK-LABEL: @test_double(
+; CHECK-NEXT:    ret double 0x8000000000000
+  %mul = fmul double 0x10000000000000, 5.000000e-01
+  ret double %mul
+}
+
+define float @test_float_positive() #1 {
+; CHECK-LABEL: @test_float_positive(
+; CHECK-NEXT:    ret float 0.000000e+00
+  %mul = fmul float 0x3810000000000000, 5.000000e-01
+  ret float %mul
+}
+
+define double @test_double_positive() #1 {
+; CHECK-LABEL: @test_double_positive(
+; CHECK-NEXT:    ret double 0.000000e+00
+  %mul = fmul double 0x10000000000000, 5.000000e-01
+  ret double %mul
+}
+
+define float @test_float_preserve() #2 {
+; CHECK-LABEL: @test_float_preserve(
+; CHECK-NEXT:    ret float -0.000000e+00
+  %mul = fmul float 0x3810000000000000, -5.000000e-01
+  ret float %mul
+}
+
+define double @test_double_preserve() #2 {
+; CHECK-LABEL: @test_double_preserve(
+; CHECK-NEXT:    ret double -0.000000e+00
+  %mul = fmul double 0x10000000000000, -5.000000e-01
+  ret double %mul
+}
+
+define float @test_float_positive_f32() #3 {
+; CHECK-LABEL: @test_float_positive_f32(
+; CHECK-NEXT:    ret float 0.000000e+00
+  %mul = fmul float 0x3810000000000000, 5.000000e-01
+  ret float %mul
+}
+
+define double @test_double_positive_f32() #3 {
+; CHECK-LABEL: @test_double_positive_f32(
+; CHECK-NEXT:    ret double 0x8000000000000
+  %mul = fmul double 0x10000000000000, 5.000000e-01
+  ret double %mul
+}
+
+define double @test_double_input() #4 {
+; CHECK-LABEL: @test_double_input(
+; CHECK-NEXT:    ret double 0.000000e+00
+  %mul = fmul double 0x8000000000000, 1.000000e+10
+  ret double %mul
+}
+
+attributes #0 = { nounwind "denormal-fp-math"="ieee,ieee" }
+attributes #1 = { nounwind "denormal-fp-math"="positive-zero,ieee" }
+attributes #2 = { nounwind "denormal-fp-math"="preserve-sign,ieee" }
+attributes #3 = { nounwind "denormal-fp-math"="ieee,ieee" "denormal-fp-math-f32"="positive-zero,ieee" }
+attributes #4 = { nounwind "denormal-fp-math"="ieee,positive-zero" }