[SelectionDAG] Use unaligned store to move AVX registers onto stack for extractelement

Prior to this patch, SelectionDAG generated aligned move onto stacks for AVX registers
when the function was marked as a no-realign-stack function. This lead to misalignment
between the stack and the instruction generated. This patch fixes the issue.

Fixes llvm#77730

Author: Nirhar

llvm/lib/CodeGen/SelectionDAG/LegalizeDAG.cpp

@@ -21,6 +21,7 @@
 #include "llvm/Analysis/ConstantFolding.h"
 #include "llvm/Analysis/TargetLibraryInfo.h"
 #include "llvm/CodeGen/ISDOpcodes.h"
+#include "llvm/CodeGen/MachineFrameInfo.h"
 #include "llvm/CodeGen/MachineFunction.h"
 #include "llvm/CodeGen/MachineJumpTableInfo.h"
 #include "llvm/CodeGen/MachineMemOperand.h"
@@ -1425,9 +1426,16 @@ SDValue SelectionDAGLegalize::ExpandExtractFromVectorThroughStack(SDValue Op) {
 
   if (!Ch.getNode()) {
     // Store the value to a temporary stack slot, then LOAD the returned part.
+    auto &MF = DAG.getMachineFunction();
+    auto &MFI = MF.getFrameInfo();
     StackPtr = DAG.CreateStackTemporary(VecVT);
-    Ch = DAG.getStore(DAG.getEntryNode(), dl, Vec, StackPtr,
-                      MachinePointerInfo());
+    int FI = cast<FrameIndexSDNode>(StackPtr.getNode())->getIndex();
+    MachinePointerInfo PtrInfo = MachinePointerInfo::getFixedStack(MF, FI);
+    MachineMemOperand *StoreMMO =
+        MF.getMachineMemOperand(PtrInfo, MachineMemOperand::MOStore,
+                                MFI.getObjectSize(FI), MFI.getObjectAlign(FI));
+
+    Ch = DAG.getStore(DAG.getEntryNode(), dl, Vec, StackPtr, StoreMMO);
   }
 
   SDValue NewLoad;

llvm/test/CodeGen/X86/unaligned_extract_from_vector_through_stack.ll

@@ -0,0 +1,20 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
+; RUN: llc < %s -mtriple=x86_64-unknown-linux-gnu | FileCheck %s
+
+define i32 @foo(i32 %arg1) #0 {
+; CHECK-LABEL: foo:
+; CHECK:       # %bb.0: # %entry
+; CHECK-NEXT:    # kill: def $edi killed $edi def $rdi
+; CHECK-NEXT:    vxorps %xmm0, %xmm0, %xmm0
+; CHECK-NEXT:    vmovups %ymm0, -{{[0-9]+}}(%rsp)
+; CHECK-NEXT:    andl $31, %edi
+; CHECK-NEXT:    movzbl -40(%rsp,%rdi), %eax
+; CHECK-NEXT:    vzeroupper
+; CHECK-NEXT:    retq
+entry:
+  %a = extractelement <32 x i8> zeroinitializer, i32 %arg1
+  %b = zext i8 %a to i32
+  ret i32 %b
+}
+
+attributes #0 = { "no-realign-stack" "target-cpu"="skylake-avx512" }