Index: lib/Target/X86/X86ISelDAGToDAG.cpp
===================================================================
--- lib/Target/X86/X86ISelDAGToDAG.cpp
+++ lib/Target/X86/X86ISelDAGToDAG.cpp
@@ -34,6 +34,7 @@
 #include "llvm/Support/raw_ostream.h"
 #include "llvm/Target/TargetMachine.h"
 #include "llvm/Target/TargetOptions.h"
+#include <limits>
 using namespace llvm;
 
 #define DEBUG_TYPE "x86-isel"
@@ -1712,6 +1713,10 @@
     // Quit if not 32-bit imm.
     if ((int32_t)CNVal != CNVal)
       return Val;
+    // Quit if INT32_MIN: it would be negated as it is negative and overflow,
+    // producing an immediate too big to be merged into the sub.
+    if (CNVal == std::numeric_limits<int32_t>::min())
+      return Val;
     // For atomic-load-add, we could do some optimizations.
     if (Op == ADD) {
       // Translate to INC/DEC if ADD by 1 or -1.
@@ -1824,6 +1829,8 @@
           Opc = AtomicOpcTbl[Op][SextConstantI64];
         else if (i64immSExt32(Val.getNode()))
           Opc = AtomicOpcTbl[Op][ConstantI64];
+        else
+          llvm_unreachable("True 64 bits constant in SelectAtomicLoadArith");
       } else
         Opc = AtomicOpcTbl[Op][I64];
       break;
Index: test/CodeGen/X86/pr21099.ll
===================================================================
--- /dev/null
+++ test/CodeGen/X86/pr21099.ll
@@ -0,0 +1,10 @@
+; RUN: llc < %s -O2 -march=x86-64 -verify-machineinstrs | FileCheck %s
+
+define void @pr21099(i64* %p) nounwind uwtable {
+; CHECK-LABEL: pr21099
+; CHECK: movq $-2147483648
+; CHECK-NEXT: lock
+; CHECK-NEXT: addq
+  %1 = atomicrmw add i64* %p, i64 -2147483648 seq_cst
+  ret void
+}