[llvm] [AArch64][GlobalISel] Lower G_BUILD_VECTOR to G_INSERT_VECTOR_ELT (PR #105686)

[Matt Arsenault via llvm-commits]

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@@ -1054,6 +1054,40 @@ void applyLowerVectorFCMP(MachineInstr &MI, MachineRegisterInfo &MRI,
   MI.eraseFromParent();
 }
 
+// Matches G_BUILD_VECTOR where at least one source operand is not a constant
+bool matchLowerBuildToInsertVecElt(MachineInstr &MI, MachineRegisterInfo &MRI) {
+  auto *GBuildVec = dyn_cast<GBuildVector>(&MI);
+
+  // Check if the values are all constants
+  for (unsigned i = 0; i < GBuildVec->getNumSources(); i++) {
+    auto ConstVal =
+        getAnyConstantVRegValWithLookThrough(GBuildVec->getSourceReg(i), MRI);
+
+    if (!ConstVal.has_value())
+      return true;
+  }
+
+  return false;
+}
+
+void applyLowerBuildToInsertVecElt(MachineInstr &MI, MachineRegisterInfo &MRI,
+                                   MachineIRBuilder &B) {
+  auto *GBuildVec = cast<GBuildVector>(&MI);
+  LLT DstTy = MRI.getType(GBuildVec->getReg(0));
+  Register DstReg = B.buildUndef(DstTy).getReg(0);
+
+  for (unsigned i = 0; i < GBuildVec->getNumSources(); i++) {
+    Register SrcReg = GBuildVec->getSourceReg(i);
+    if (MRI.getVRegDef(SrcReg)->getOpcode() == TargetOpcode::G_IMPLICIT_DEF)
+      continue;
+    Register IdxReg = B.buildConstant(LLT::scalar(64), i).getReg(0);
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arsenm wrote:

auto and remove .getReg(0) 

https://github.com/llvm/llvm-project/pull/105686