[llvm] [AArch64][GlobalISel] Look through COPY and G_BITCAST while selecting fcvtl2 (fpext) (PR #65463)

[David Green via llvm-commits]

================
@@ -6777,10 +6777,21 @@ AArch64InstructionSelector::selectExtractHigh(MachineOperand &Root) const {
   MachineRegisterInfo &MRI =
       Root.getParent()->getParent()->getParent()->getRegInfo();
 
-  MachineInstr *Extract = getDefIgnoringCopies(Root.getReg(), MRI);
-  if (Extract && Extract->getOpcode() == TargetOpcode::G_UNMERGE_VALUES &&
-      Root.getReg() == Extract->getOperand(1).getReg()) {
-    Register ExtReg = Extract->getOperand(2).getReg();
+  auto Extract = getDefSrcRegIgnoringCopies(Root.getReg(), MRI);
+  while (Extract && Extract->MI->getOpcode() == TargetOpcode::G_BITCAST)
+    Extract =
+        getDefSrcRegIgnoringCopies(Extract->MI->getOperand(1).getReg(), MRI);
+  if (!Extract)
+    return std::nullopt;
+
+  if (Extract->MI->getOpcode() == TargetOpcode::G_UNMERGE_VALUES) {
+    if (Extract->Reg == Extract->MI->getOperand(1).getReg()) {
+      Register ExtReg = Extract->MI->getOperand(2).getReg();
+      return {{[=](MachineInstrBuilder &MIB) { MIB.addUse(ExtReg); }}};
+    }
+  }
+  if (Extract->MI->getOpcode() == TargetOpcode::G_EXTRACT_VECTOR_ELT) {
----------------
davemgreen wrote:

Why is every G_EXTRACT_VECTOR_ELT considered a high extract? Should it be checking that the Extract is a v2s64 and is extracting from index is 1?

https://github.com/llvm/llvm-project/pull/65463