[PATCH] [x86] Fold extract_vector_elt of a load into the load's address computation.

[Michael Spencer]

Hi grosbach,

An address only use of an extract element of a load can be simplified to a
load. Without this the result of the extract element is spilled to the
stack so that an address is available.

I actually think this is more general, but I'm not sure where it would go, or if the load is valid for all targets.

http://reviews.llvm.org/D3375

Files:
  lib/Target/X86/X86ISelLowering.cpp
  test/CodeGen/X86/vec_splat.ll

Index: lib/Target/X86/X86ISelLowering.cpp
===================================================================
--- lib/Target/X86/X86ISelLowering.cpp
+++ lib/Target/X86/X86ISelLowering.cpp
@@ -5677,6 +5677,25 @@
     }
   }
 
+  // Try to fold extract_vector_elt of a load into the load's address
+  // computation.
+  if (Ld.getOpcode() == ISD::EXTRACT_VECTOR_ELT &&
+      ISD::isNormalLoad(Ld.getOperand(0).getNode())) {
+    auto Size = Ld.getSimpleValueType().getStoreSize();
+    SDValue RealLoad = Ld.getOperand(0);
+    SDValue Index = Ld.getOperand(1);
+    SDValue Addr = RealLoad.getOperand(1);
+    SDValue Offset =
+        DAG.getNode(ISD::MUL, dl, Index.getSimpleValueType(), Index,
+                    DAG.getConstant(Size, Index.getSimpleValueType()));
+    SDValue NewAddr =
+        DAG.getNode(ISD::ADD, dl, Addr.getSimpleValueType(), Addr, Offset);
+    LoadSDNode *Load = cast<LoadSDNode>(RealLoad);
+    Ld = DAG.getLoad(Ld.getSimpleValueType(), dl, Load->getChain(), NewAddr,
+                     DAG.getMachineFunction().getMachineMemOperand(
+                         Load->getMemOperand(), 0, Size));
+  }
+
   bool IsLoad = ISD::isNormalLoad(Ld.getNode());
   unsigned ScalarSize = Ld.getValueType().getSizeInBits();
 
Index: test/CodeGen/X86/vec_splat.ll
===================================================================
--- test/CodeGen/X86/vec_splat.ll
+++ test/CodeGen/X86/vec_splat.ll
@@ -1,5 +1,6 @@
 ; RUN: llc < %s -march=x86 -mcpu=pentium4 -mattr=+sse2 | FileCheck %s -check-prefix=SSE2
 ; RUN: llc < %s -march=x86 -mcpu=pentium4 -mattr=+sse3 | FileCheck %s -check-prefix=SSE3
+; RUN: llc < %s -march=x86-64 -mattr=+avx | FileCheck %s -check-prefix=AVX
 
 define void @test_v4sf(<4 x float>* %P, <4 x float>* %Q, float %X) nounwind {
 	%tmp = insertelement <4 x float> zeroinitializer, float %X, i32 0		; <<4 x float>> [#uses=1]
@@ -32,3 +33,20 @@
 ; SSE3-LABEL: test_v2sd:
 ; SSE3: movddup
 }
+
+; Fold extract of a load into the load's address computation. This avoids spilling to the stack.
+define <4 x float> @load_extract_splat(<4 x float>* nocapture readonly %ptr, i64 %i, i64 %j) nounwind {
+  %1 = getelementptr inbounds <4 x float>* %ptr, i64 %i
+  %2 = load <4 x float>* %1, align 16
+  %3 = trunc i64 %j to i32
+  %4 = extractelement <4 x float> %2, i32 %3
+  %5 = insertelement <4 x float> undef, float %4, i32 0
+  %6 = insertelement <4 x float> %5, float %4, i32 1
+  %7 = insertelement <4 x float> %6, float %4, i32 2
+  %8 = insertelement <4 x float> %7, float %4, i32 3
+  ret <4 x float> %8
+  
+; AVX-LABEL: load_extract_splat
+; AVX-NOT: rsp
+; AVX: vbroadcastss
+}
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