Add a natural stack alignment field to TargetData, and prevent InstCombine from

promoting allocas to preferred alignments that exceed the natural
alignment. This avoids some potentially expensive dynamic stack realignments.

The natural stack alignment is set in target data strings via the "S<size>"
option. Size is in bits and must be a multiple of 8. The natural stack alignment
defaults to "unspecified" (represented by a zero value), and the "unspecified"
value does not prevent any alignment promotions. Target maintainers that care
about avoiding promotions should explicitly add the "S<size>" option to their
target data strings.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@141599 91177308-0d34-0410-b5e6-96231b3b80d8

8 files changed, 45 insertions, 14 deletions

diff --git a/docs/LangRef.html b/docs/LangRef.html
index 1392db6e60..a28d92efa1 100644
--- a/docs/LangRef.html
+++ b/docs/LangRef.html
@@ -1319,6 +1319,12 @@ target datalayout = "<i>layout specification</i>"
       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 315bee96ba..e34094ddb7 100644
--- a/include/llvm/Target/TargetData.h
+++ b/include/llvm/Target/TargetData.h
@@ -70,6 +70,7 @@ private:
   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 @@ public:
     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 f779ede3e3..96b1e89b0d 100644
--- a/lib/Target/ARM/ARMTargetMachine.cpp
+++ b/lib/Target/ARM/ARMTargetMachine.cpp
@@ -53,9 +53,12 @@ ARMTargetMachine::ARMTargetMachine(const Target &T, StringRef TT,
   : 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 @@ ThumbTargetMachine::ThumbTargetMachine(const Target &T, StringRef TT,
     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-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")),
+                           "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 1dfd9a83da..bd6a6b67be 100644
--- a/lib/Target/TargetData.cpp
+++ b/lib/Target/TargetData.cpp
@@ -139,6 +139,7 @@ void TargetData::init(StringRef Desc) {
   PointerMemSize = 8;
   PointerABIAlign = 8;
   PointerPrefAlign = PointerABIAlign;
+  StackNaturalAlign = 0;
 
   // Default alignments
   setAlignment(INTEGER_ALIGN,   1,  1, 1);   // i1
@@ -218,7 +219,12 @@ void TargetData::init(StringRef Desc) {
         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 @@ std::string TargetData::getStringRepresentation() const {
 
   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 f16ec029e9..7d901afae4 100644
--- a/lib/Target/X86/README-SSE.txt
+++ b/lib/Target/X86/README-SSE.txt
@@ -862,7 +862,7 @@ define float @bar(float %x) nounwind {
 
 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 b98f5fbb99..b407955060 100644
--- a/lib/Target/X86/README.txt
+++ b/lib/Target/X86/README.txt
@@ -1217,7 +1217,7 @@ Also check why xmm7 is not used at all in the function.
 
 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 683d6aa0dd..1dcad35c2e 100644
--- a/lib/Target/X86/X86TargetMachine.cpp
+++ b/lib/Target/X86/X86TargetMachine.cpp
@@ -34,11 +34,11 @@ X86_32TargetMachine::X86_32TargetMachine(const Target &T, StringRef TT,
                                          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 @@ X86_64TargetMachine::X86_64TargetMachine(const Target &T, StringRef TT,
                                          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 be0aa82500..7034feb227 100644
--- a/lib/Transforms/Utils/Local.cpp
+++ b/lib/Transforms/Utils/Local.cpp
@@ -721,10 +721,14 @@ bool llvm::EliminateDuplicatePHINodes(BasicBlock *BB) {
 /// 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 @@ unsigned llvm::getOrEnforceKnownAlignment(Value *V, unsigned PrefAlign,
   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;