[llvm] Xtensa] Implement windowed call ABI. (PR #130001)

[Matt Arsenault via llvm-commits]

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@@ -179,10 +264,22 @@ void XtensaFrameLowering::emitEpilogue(MachineFunction &MF,
              "Unexpected callee-saved register restore instruction");
 #endif
     }
-
-    BuildMI(MBB, I, DL, TII.get(Xtensa::OR), SP).addReg(FP).addReg(FP);
+    if (STI.isWinABI()) {
+      // In most architectures, we need to explicitly restore the stack pointer
+      // before returning.
+      //
+      // For Xtensa Windowed Register option, it is not needed to explicitly
+      // restore the stack pointer. Reason being is that on function return,
+      // the window of the caller (including the old stack pointer) gets
+      // restored anyways.
+    } else {
+      BuildMI(MBB, I, DL, TII.get(Xtensa::OR), SP).addReg(FP).addReg(FP);
+    }
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arsenm wrote:

Invert condition and avoid comment only block 

https://github.com/llvm/llvm-project/pull/130001