diff --git a/lib/Target/X86/X86FloatingPoint.cpp b/lib/Target/X86/X86FloatingPoint.cpp
index 6eed6abd43e..6f619a9e567 100644
--- a/lib/Target/X86/X86FloatingPoint.cpp
+++ b/lib/Target/X86/X86FloatingPoint.cpp
@@ -406,6 +406,10 @@ bool FPS::processBasicBlock(MachineFunction &MF, MachineBasicBlock &BB) {
     if (MI->isCopy() && isFPCopy(MI))
       FPInstClass = X86II::SpecialFP;
 
+    if (MI->isImplicitDef() &&
+        X86::RFP80RegClass.contains(MI->getOperand(0).getReg()))
+      FPInstClass = X86II::SpecialFP;
+
     if (FPInstClass == X86II::NotFP)
       continue;  // Efficiently ignore non-fp insts!
 
@@ -1369,6 +1373,15 @@ void FPS::handleSpecialFP(MachineBasicBlock::iterator &I) {
     break;
   }
 
+  case TargetOpcode::IMPLICIT_DEF: {
+    // All FP registers must be explicitly defined, so load a 0 instead.
+    unsigned Reg = MI->getOperand(0).getReg() - X86::FP0;
+    DEBUG(dbgs() << "Emitting LD_F0 for implicit FP" << Reg << '\n');
+    BuildMI(*MBB, I, MI->getDebugLoc(), TII->get(X86::LD_F0));
+    pushReg(Reg);
+    break;
+  }
+
   case X86::FpPOP_RETVAL: {
     // The FpPOP_RETVAL instruction is used after calls that return a value on
     // the floating point stack. We cannot model this with ST defs since CALL
diff --git a/test/CodeGen/X86/fp-stack-O0-crash.ll b/test/CodeGen/X86/fp-stack-O0-crash.ll
index 9b629c08652..ae83a02c674 100644
--- a/test/CodeGen/X86/fp-stack-O0-crash.ll
+++ b/test/CodeGen/X86/fp-stack-O0-crash.ll
@@ -1,4 +1,4 @@
-; RUN: llc %s -O0 -fast-isel -regalloc=fast -o -
+; RUN: llc %s -O0 -fast-isel -regalloc=fast -mcpu=i386 -o -
 ; PR4767
 
 target datalayout = "e-p:32:32:32-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:32:64-f32:32:32-f64:32:64-v64:64:64-v128:128:128-a0:0:64-f80:128:128"
@@ -28,3 +28,22 @@ if.then:                                          ; preds = %cond.false, %cond.t
 if.end:                                           ; preds = %if.then, %cond.false, %cond.true
   ret void
 }
+
+; PR10575
+; This produces a FP0 = IMPLICIT_DEF instruction.
+define void @__m_rankmerge_MOD_dindexmerge_() nounwind {
+entry:
+  br label %"20"
+
+"20":                                             ; preds = %"23", %entry
+    %0 = phi double [ undef, %entry ], [ %0, %"23" ]
+    %1 = phi double [ 0.000000e+00, %entry ], [ %2, %"23" ]
+    br i1 undef, label %"21", label %"23"
+
+"21":                                             ; preds = %"20"
+    ret void
+
+"23":                                             ; preds = %"20"
+    %2 = select i1 undef, double %0, double %1
+    br label %"20"
+}