diff --git a/docs/LangRef.html b/docs/LangRef.html
index 1392db6..a28d92e 100644
--- a/docs/LangRef.html
+++ b/docs/LangRef.html
@@ -1319,6 +1319,12 @@
       the bits with the least significance have the lowest address
       location.</dd>
 
+  <dt><tt>S<i>size</i></tt></dt>
+  <dd>Specifies the natural alignment of the stack in bits. Alignment promotion
+      of stack variables is limited to the natural stack alignment to avoid
+      dynamic stack realignment. The stack alignment must be a multiple of
+      8-bits, and currently defaults to 128 bits if unspecified.</dd>
+
   <dt><tt>p:<i>size</i>:<i>abi</i>:<i>pref</i></tt></dt>
   <dd>This specifies the <i>size</i> of a pointer and its <i>abi</i> and
       <i>preferred</i> alignments. All sizes are in bits. Specifying
diff --git a/include/llvm/Target/TargetData.h b/include/llvm/Target/TargetData.h
index 315bee9..e34094d 100644
--- a/include/llvm/Target/TargetData.h
+++ b/include/llvm/Target/TargetData.h
@@ -70,6 +70,7 @@
   unsigned      PointerMemSize;        ///< Pointer size in bytes
   unsigned      PointerABIAlign;       ///< Pointer ABI alignment
   unsigned      PointerPrefAlign;      ///< Pointer preferred alignment
+  unsigned      StackNaturalAlign;     ///< Stack natural alignment
 
   SmallVector<unsigned char, 8> LegalIntWidths; ///< Legal Integers.
   
@@ -163,6 +164,11 @@
     return !isLegalInteger(Width);
   }
 
+  /// Returns true if the given alignment exceeds the natural stack alignment.
+  bool exceedsNaturalStackAlignment(unsigned Align) const {
+    return (StackNaturalAlign != 0) && (Align > StackNaturalAlign);
+  }
+
   /// fitsInLegalInteger - This function returns true if the specified type fits
   /// in a native integer type supported by the CPU.  For example, if the CPU
   /// only supports i32 as a native integer type, then i27 fits in a legal
diff --git a/lib/Target/ARM/ARMTargetMachine.cpp b/lib/Target/ARM/ARMTargetMachine.cpp
index f779ede..96b1e89 100644
--- a/lib/Target/ARM/ARMTargetMachine.cpp
+++ b/lib/Target/ARM/ARMTargetMachine.cpp
@@ -53,9 +53,12 @@
   : ARMBaseTargetMachine(T, TT, CPU, FS, RM, CM), InstrInfo(Subtarget),
     DataLayout(Subtarget.isAPCS_ABI() ?
                std::string("e-p:32:32-f64:32:64-i64:32:64-"
-                           "v128:32:128-v64:32:64-n32") :
+                           "v128:32:128-v64:32:64-n32-S32") :
+               Subtarget.isAAPCS_ABI() ?
                std::string("e-p:32:32-f64:64:64-i64:64:64-"
-                           "v128:64:128-v64:64:64-n32")),
+                           "v128:64:128-v64:64:64-n32-S64") :
+               std::string("e-p:32:32-f64:64:64-i64:64:64-"
+                           "v128:64:128-v64:64:64-n32-S32")),
     ELFWriterInfo(*this),
     TLInfo(*this),
     TSInfo(*this),
@@ -75,10 +78,14 @@
     DataLayout(Subtarget.isAPCS_ABI() ?
                std::string("e-p:32:32-f64:32:64-i64:32:64-"
                            "i16:16:32-i8:8:32-i1:8:32-"
-                           "v128:32:128-v64:32:64-a:0:32-n32") :
+                           "v128:32:128-v64:32:64-a:0:32-n32-S32") :
+               Subtarget.isAAPCS_ABI() ?
                std::string("e-p:32:32-f64:64:64-i64:64:64-"
                            "i16:16:32-i8:8:32-i1:8:32-"
-                           "v128:64:128-v64:64:64-a:0:32-n32")),
+                           "v128:64:128-v64:64:64-a:0:32-n32-S64") :
+               std::string("e-p:32:32-f64:64:64-i64:64:64-"
+                           "i16:16:32-i8:8:32-i1:8:32-"
+                           "v128:64:128-v64:64:64-a:0:32-n32-S32")),
     ELFWriterInfo(*this),
     TLInfo(*this),
     TSInfo(*this),
diff --git a/lib/Target/TargetData.cpp b/lib/Target/TargetData.cpp
index 1dfd9a8..bd6a6b6 100644
--- a/lib/Target/TargetData.cpp
+++ b/lib/Target/TargetData.cpp
@@ -139,6 +139,7 @@
   PointerMemSize = 8;
   PointerABIAlign = 8;
   PointerPrefAlign = PointerABIAlign;
+  StackNaturalAlign = 0;
 
   // Default alignments
   setAlignment(INTEGER_ALIGN,   1,  1, 1);   // i1
@@ -218,7 +219,12 @@
         Token = Split.second;
       } while (!Specifier.empty() || !Token.empty());
       break;
-
+    case 'S': // Stack natural alignment.
+      StackNaturalAlign = getInt(Specifier.substr(1));
+      StackNaturalAlign /= 8;
+      // FIXME: Should we really be truncating these alingments and
+      // sizes silently?
+      break;
     default:
       break;
     }
@@ -372,7 +378,9 @@
 
   OS << (LittleEndian ? "e" : "E")
      << "-p:" << PointerMemSize*8 << ':' << PointerABIAlign*8
-     << ':' << PointerPrefAlign*8;
+     << ':' << PointerPrefAlign*8
+     << "-S" << StackNaturalAlign*8;
+
   for (unsigned i = 0, e = Alignments.size(); i != e; ++i) {
     const TargetAlignElem &AI = Alignments[i];
     OS << '-' << (char)AI.AlignType << AI.TypeBitWidth << ':'
diff --git a/lib/Target/X86/README-SSE.txt b/lib/Target/X86/README-SSE.txt
index f16ec02..7d901af 100644
--- a/lib/Target/X86/README-SSE.txt
+++ b/lib/Target/X86/README-SSE.txt
@@ -862,7 +862,7 @@
 
 This IR (from PR6194):
 
-target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64"
+target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64-S128"
 target triple = "x86_64-apple-darwin10.0.0"
 
 %0 = type { double, double }
diff --git a/lib/Target/X86/README.txt b/lib/Target/X86/README.txt
index b98f5fb..b407955 100644
--- a/lib/Target/X86/README.txt
+++ b/lib/Target/X86/README.txt
@@ -1217,7 +1217,7 @@
 
 Take the following:
 
-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"
+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-S128"
 target triple = "i386-apple-darwin8"
 @in_exit.4870.b = internal global i1 false		; <i1*> [#uses=2]
 define fastcc void @abort_gzip() noreturn nounwind  {
diff --git a/lib/Target/X86/X86TargetMachine.cpp b/lib/Target/X86/X86TargetMachine.cpp
index 683d6aa..1dcad35 100644
--- a/lib/Target/X86/X86TargetMachine.cpp
+++ b/lib/Target/X86/X86TargetMachine.cpp
@@ -34,11 +34,11 @@
                                          Reloc::Model RM, CodeModel::Model CM)
   : X86TargetMachine(T, TT, CPU, FS, RM, CM, false),
     DataLayout(getSubtargetImpl()->isTargetDarwin() ?
-               "e-p:32:32-f64:32:64-i64:32:64-f80:128:128-f128:128:128-n8:16:32" :
+               "e-p:32:32-f64:32:64-i64:32:64-f80:128:128-f128:128:128-n8:16:32-S128" :
                (getSubtargetImpl()->isTargetCygMing() ||
                 getSubtargetImpl()->isTargetWindows()) ?
-               "e-p:32:32-f64:64:64-i64:64:64-f80:32:32-f128:128:128-n8:16:32" :
-               "e-p:32:32-f64:32:64-i64:32:64-f80:32:32-f128:128:128-n8:16:32"),
+               "e-p:32:32-f64:64:64-i64:64:64-f80:32:32-f128:128:128-n8:16:32-S32" :
+               "e-p:32:32-f64:32:64-i64:32:64-f80:32:32-f128:128:128-n8:16:32-S32"),
     InstrInfo(*this),
     TSInfo(*this),
     TLInfo(*this),
@@ -50,7 +50,7 @@
                                          StringRef CPU, StringRef FS,
                                          Reloc::Model RM, CodeModel::Model CM)
   : X86TargetMachine(T, TT, CPU, FS, RM, CM, true),
-    DataLayout("e-p:64:64-s:64-f64:64:64-i64:64:64-f80:128:128-f128:128:128-n8:16:32:64"),
+    DataLayout("e-p:64:64-s:64-f64:64:64-i64:64:64-f80:128:128-f128:128:128-n8:16:32:64-S128"),
     InstrInfo(*this),
     TSInfo(*this),
     TLInfo(*this),
diff --git a/lib/Transforms/Utils/Local.cpp b/lib/Transforms/Utils/Local.cpp
index be0aa82..7034feb 100644
--- a/lib/Transforms/Utils/Local.cpp
+++ b/lib/Transforms/Utils/Local.cpp
@@ -721,10 +721,14 @@
 /// their preferred alignment from the beginning.
 ///
 static unsigned enforceKnownAlignment(Value *V, unsigned Align,
-                                      unsigned PrefAlign) {
+                                      unsigned PrefAlign, const TargetData *TD) {
   V = V->stripPointerCasts();
 
   if (AllocaInst *AI = dyn_cast<AllocaInst>(V)) {
+    // If the preferred alignment is greater than the natural stack alignment
+    // then don't round up. This avoids dynamic stack realignment.
+    if (TD && TD->exceedsNaturalStackAlignment(PrefAlign))
+      return Align;
     // If there is a requested alignment and if this is an alloca, round up.
     if (AI->getAlignment() >= PrefAlign)
       return AI->getAlignment();
@@ -775,7 +779,7 @@
   Align = std::min(Align, +Value::MaximumAlignment);
   
   if (PrefAlign > Align)
-    Align = enforceKnownAlignment(V, Align, PrefAlign);
+    Align = enforceKnownAlignment(V, Align, PrefAlign, TD);
     
   // We don't need to make any adjustment.
   return Align;