[llvm] [ARM] CMSE security mitigation on function arguments and returned values (PR #89944)

[Victor Campos via llvm-commits]

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@@ -156,6 +156,17 @@ static const MCPhysReg GPRArgRegs[] = {
   ARM::R0, ARM::R1, ARM::R2, ARM::R3
 };
 
+static SDValue handleCMSEValue(const SDValue &Value, const ISD::InputArg &Arg,
+                               SelectionDAG &DAG, const SDLoc &DL, EVT RegVT) {
+  assert(Arg.ArgVT.isScalarInteger() && RegVT.isScalarInteger());
+  assert(Arg.ArgVT.bitsLT(RegVT));
+  SDValue Trunc = DAG.getNode(ISD::TRUNCATE, DL, Arg.ArgVT, Value);
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vhscampos wrote:

`Value` is of 32-bit type since it originates from a general-purpose register. So in order to clear its higher bits, it needs to be first truncated and then extended back again.

https://github.com/llvm/llvm-project/pull/89944