Add a feature flag for slow 32-byte unaligned memory accesses [x86].

This patch adds a feature flag to avoid unaligned 32-byte load/store AVX codegen
for Sandy Bridge and Ivy Bridge. There is no functionality change intended for 
those chips. Previously, the absence of AVX2 was being used as a proxy to detect
this feature. But that hindered codegen for AVX-enabled AMD chips such as btver2
that do not have the 32-byte unaligned access slowdown.

Performance measurements are included in PR21541 ( http://llvm.org/bugs/show_bug.cgi?id=21541 ).

Differential Revision: http://reviews.llvm.org/D6355



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222544 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Target/X86/X86.td

@@ -82,6 +82,9 @@ def FeatureSlowSHLD : SubtargetFeature<"slow-shld", "IsSHLDSlow", "true",
 def FeatureFastUAMem : SubtargetFeature<"fast-unaligned-mem",
                                         "IsUAMemFast", "true",
                                         "Fast unaligned memory access">;
+def FeatureSlowUAMem32 : SubtargetFeature<"slow-unaligned-mem-32",
+                            "IsUAMem32Slow", "true",
+                            "Slow unaligned 32-byte memory access">;
 def FeatureSSE4A   : SubtargetFeature<"sse4a", "HasSSE4A", "true",
                                       "Support SSE 4a instructions",
                                       [FeatureSSE3]>;
@@ -271,12 +274,14 @@ def : ProcessorModel<"westmere", SandyBridgeModel,
 // rather than a superset.
 def : ProcessorModel<"corei7-avx", SandyBridgeModel,
                      [FeatureAVX, FeatureCMPXCHG16B, FeatureFastUAMem,
-                      FeaturePOPCNT, FeatureAES, FeaturePCLMUL]>;
+                      FeatureSlowUAMem32, FeaturePOPCNT, FeatureAES,
+                      FeaturePCLMUL]>;
 // Ivy Bridge
 def : ProcessorModel<"core-avx-i", SandyBridgeModel,
                      [FeatureAVX, FeatureCMPXCHG16B, FeatureFastUAMem,
-                      FeaturePOPCNT, FeatureAES, FeaturePCLMUL, FeatureRDRAND,
+                      FeatureSlowUAMem32, FeaturePOPCNT, FeatureAES,
-                      FeatureF16C, FeatureFSGSBase]>;
+                      FeaturePCLMUL, FeatureRDRAND, FeatureF16C,
+                      FeatureFSGSBase]>;
 
 // Haswell
 def : ProcessorModel<"core-avx2", HaswellModel,

lib/Target/X86/X86ISelLowering.cpp

@@ -24376,11 +24376,12 @@ static SDValue PerformLOADCombine(SDNode *N, SelectionDAG &DAG,
   SDLoc dl(Ld);
   const TargetLowering &TLI = DAG.getTargetLoweringInfo();
 
-  // On Sandybridge unaligned 256bit loads are inefficient.
+  // For chips with slow 32-byte unaligned loads, break the 32-byte operation
+  // into two 16-byte operations.
   ISD::LoadExtType Ext = Ld->getExtensionType();
   unsigned Alignment = Ld->getAlignment();
   bool IsAligned = Alignment == 0 || Alignment >= MemVT.getSizeInBits()/8;
-  if (RegVT.is256BitVector() && !Subtarget->hasInt256() &&
+  if (RegVT.is256BitVector() && Subtarget->isUnalignedMem32Slow() &&
       !DCI.isBeforeLegalizeOps() && !IsAligned && Ext == ISD::NON_EXTLOAD) {
     unsigned NumElems = RegVT.getVectorNumElements();
     if (NumElems < 2)
@@ -24423,13 +24424,11 @@ static SDValue PerformSTORECombine(SDNode *N, SelectionDAG &DAG,
   SDValue StoredVal = St->getOperand(1);
   const TargetLowering &TLI = DAG.getTargetLoweringInfo();
 
-  // If we are saving a concatenation of two XMM registers, perform two stores.
+  // If we are saving a concatenation of two XMM registers and 32-byte stores
-  // On Sandy Bridge, 256-bit memory operations are executed by two
+  // are slow, such as on Sandy Bridge, perform two 16-byte stores.
-  // 128-bit ports. However, on Haswell it is better to issue a single 256-bit
-  // memory  operation.
   unsigned Alignment = St->getAlignment();
   bool IsAligned = Alignment == 0 || Alignment >= VT.getSizeInBits()/8;
-  if (VT.is256BitVector() && !Subtarget->hasInt256() &&
+  if (VT.is256BitVector() && Subtarget->isUnalignedMem32Slow() &&
       StVT == VT && !IsAligned) {
     unsigned NumElems = VT.getVectorNumElements();
     if (NumElems < 2)

lib/Target/X86/X86Subtarget.cpp

@@ -264,6 +264,7 @@ void X86Subtarget::initializeEnvironment() {
   IsBTMemSlow = false;
   IsSHLDSlow = false;
   IsUAMemFast = false;
+  IsUAMem32Slow = false;
   HasVectorUAMem = false;
   HasCmpxchg16b = false;
   UseLeaForSP = false;

lib/Target/X86/X86Subtarget.h

@@ -159,6 +159,9 @@ protected:
   /// IsUAMemFast - True if unaligned memory access is fast.
   bool IsUAMemFast;
 
+  /// True if unaligned 32-byte memory accesses are slow.
+  bool IsUAMem32Slow;
+  
   /// HasVectorUAMem - True if SIMD operations can have unaligned memory
   /// operands. This may require setting a feature bit in the processor.
   bool HasVectorUAMem;
@@ -374,6 +377,7 @@ public:
   bool isBTMemSlow() const { return IsBTMemSlow; }
   bool isSHLDSlow() const { return IsSHLDSlow; }
   bool isUnalignedMemAccessFast() const { return IsUAMemFast; }
+  bool isUnalignedMem32Slow() const { return IsUAMem32Slow; }
   bool hasVectorUAMem() const { return HasVectorUAMem; }
   bool hasCmpxchg16b() const { return HasCmpxchg16b; }
   bool useLeaForSP() const { return UseLeaForSP; }

test/CodeGen/X86/2012-05-19-avx2-store.ll

@@ -1,13 +0,0 @@
-; RUN: llc < %s -mtriple=x86_64-apple-darwin -mcpu=corei7-avx -mattr=+avx2 | FileCheck %s
-
-define void @double_save(<4 x i32>* %Ap, <4 x i32>* %Bp, <8 x i32>* %P) nounwind ssp {
-entry:
-  ; CHECK: vmovaps
-  ; CHECK: vinsertf128 $1, ([[A0:%rdi|%rsi]]),
-  ; CHECK: vmovups
-  %A = load <4 x i32>* %Ap
-  %B = load <4 x i32>* %Bp
-  %Z = shufflevector <4 x i32>%A, <4 x i32>%B, <8 x i32> <i32 0, i32 1, i32 2, i32 3, i32 4, i32 5, i32 6, i32 7>
-  store <8 x i32> %Z, <8 x i32>* %P, align 16
-  ret void
-}

test/CodeGen/X86/unaligned-32-byte-memops.ll

@@ -0,0 +1,46 @@
+; RUN: llc < %s -mtriple=x86_64-apple-darwin -mcpu=corei7-avx | FileCheck %s --check-prefix=SANDYB
+; RUN: llc < %s -mtriple=x86_64-apple-darwin -mcpu=core-avx-i | FileCheck %s --check-prefix=SANDYB
+; RUN: llc < %s -mtriple=x86_64-apple-darwin -mcpu=btver2 | FileCheck %s --check-prefix=BTVER2
+; RUN: llc < %s -mtriple=x86_64-apple-darwin -mcpu=core-avx2 | FileCheck %s --check-prefix=HASWELL
+
+; On Sandy Bridge or Ivy Bridge, we should not generate an unaligned 32-byte load
+; because that is slower than two 16-byte loads. 
+; Other AVX-capable chips don't have that problem.
+
+define <8 x float> @load32bytes(<8 x float>* %Ap) {
+  ; CHECK-LABEL: load32bytes
+
+  ; SANDYB: vmovaps
+  ; SANDYB: vinsertf128
+  ; SANDYB: retq
+
+  ; BTVER2: vmovups
+  ; BTVER2: retq
+
+  ; HASWELL: vmovups
+  ; HASWELL: retq
+
+  %A = load <8 x float>* %Ap, align 16
+  ret <8 x float> %A
+}
+
+; On Sandy Bridge or Ivy Bridge, we should not generate an unaligned 32-byte store
+; because that is slowerthan two 16-byte stores. 
+; Other AVX-capable chips don't have that problem.
+
+define void @store32bytes(<8 x float> %A, <8 x float>* %P) {
+  ; CHECK-LABEL: store32bytes
+
+  ; SANDYB: vextractf128
+  ; SANDYB: vmovaps
+  ; SANDYB: retq
+
+  ; BTVER2: vmovups
+  ; BTVER2: retq
+
+  ; HASWELL: vmovups
+  ; HASWELL: retq
+
+  store <8 x float> %A, <8 x float>* %P, align 16
+  ret void
+}