[llvm] [SystemZ] Implement .machine (push|pop) directives (PR #137302)

[Ulrich Weigand via llvm-commits]

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@@ -1382,17 +1395,35 @@ bool SystemZAsmParser::parseDirectiveMachine(SMLoc L) {
       Parser.getTok().isNot(AsmToken::String))
     return TokError("unexpected token in '.machine' directive");
 
-  StringRef CPU = Parser.getTok().getIdentifier();
+  StringRef Id = Parser.getTok().getIdentifier();
+
+  // Parse push and pop directives first
+  if (Id == "push") {
+    // Push the Current FeatureBitSet onto the stack.
+    MachineStack.push_back(CurrentFeatures);
+    getTargetStreamer().emitMachine("push");
+  } else if (Id == "pop") {
+    // If the Stack is not empty, pop the topmost FeatureBitset and use it.
+    if (MachineStack.empty())
+      return TokError("pop without corresponding push in '.machine' directive");
+    CurrentFeatures = MachineStack.back();
+    MachineStack.pop_back();
+    setAvailableFeatures(CurrentFeatures);
+    getTargetStreamer().emitMachine("pop");
+  } else {
+    // Try to interpret the Identifier as a CPU spec and derive the
+    // FeatureBitset from that.
+    MCSubtargetInfo &STI = copySTI();
+    STI.setDefaultFeatures(Id, /*TuneCPU*/ Id, "");
+    CurrentFeatures = STI.getFeatureBits();
+    setAvailableFeatures(ComputeAvailableFeatures(CurrentFeatures));
+    getTargetStreamer().emitMachine(Id);
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uweigand wrote:

Minor nit: looks like you do the same `emitMachine` call in all three branches, so it probably should be moved after the if.

https://github.com/llvm/llvm-project/pull/137302