[llvm] [AMDGPU][Attributor] Infer `inreg` attribute in `AMDGPUAttributor` (PR #101609)

[Matt Arsenault via llvm-commits]

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@@ -1295,6 +1298,130 @@ struct AAAMDGPUNoAGPR
 
 const char AAAMDGPUNoAGPR::ID = 0;
 
+struct AAAMDGPUUniform : public StateWrapper<BooleanState, AbstractAttribute> {
+  using Base = StateWrapper<BooleanState, AbstractAttribute>;
+  AAAMDGPUUniform(const IRPosition &IRP, Attributor &A) : Base(IRP) {}
+
+  /// Create an abstract attribute view for the position \p IRP.
+  static AAAMDGPUUniform &createForPosition(const IRPosition &IRP,
+                                            Attributor &A);
+
+  /// See AbstractAttribute::getName()
+  StringRef getName() const override { return "AAAMDGPUUniform"; }
+
+  const std::string getAsStr(Attributor *A) const override {
+    return getAssumed() ? "inreg" : "non-inreg";
+  }
+
+  void trackStatistics() const override {}
+
+  /// See AbstractAttribute::getIdAddr()
+  const char *getIdAddr() const override { return &ID; }
+
+  /// This function should return true if the type of the \p AA is
+  /// AAAMDGPUUniform
+  static bool classof(const AbstractAttribute *AA) {
+    return (AA->getIdAddr() == &ID);
+  }
+
+  /// Unique ID (due to the unique address)
+  static const char ID;
+};
+
+const char AAAMDGPUUniform::ID = 0;
+
+struct AAAMDGPUUniformArgument : public AAAMDGPUUniform {
+  AAAMDGPUUniformArgument(const IRPosition &IRP, Attributor &A)
+      : AAAMDGPUUniform(IRP, A) {}
+
+  void initialize(Attributor &A) override {
+    Argument *Arg = getAssociatedArgument();
+    CallingConv::ID CC = Arg->getParent()->getCallingConv();
+    if (Arg->hasAttribute(Attribute::InReg)) {
+      indicateOptimisticFixpoint();
+      return;
+    }
+    if (AMDGPU::isEntryFunctionCC(CC)) {
+      // We only use isArgPassedInSGPR on kernel entry function argument, so the
+      // potential i1 argument change will not affect this.
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arsenm wrote:

Should explain what this means instead of referencing a future change 

https://github.com/llvm/llvm-project/pull/101609