[llvm-branch-commits] [llvm] RegAlloc: Do not fatal error if there are no registers in the alloc order (PR #119640)

[Matt Arsenault via llvm-branch-commits]

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@@ -192,3 +177,50 @@ void RegAllocBase::enqueue(const LiveInterval *LI) {
                       << " in skipped register class\n");
   }
 }
+
+MCPhysReg RegAllocBase::getErrorAssignment(const TargetRegisterClass &RC,
+                                           const MachineInstr *CtxMI) {
+  MachineFunction &MF = VRM->getMachineFunction();
+
+  // Avoid printing the error for every single instance of the register. It
+  // would be better if this were per register class.
+  bool EmitError = !MF.getProperties().hasProperty(
+      MachineFunctionProperties::Property::FailedRegAlloc);
+  if (EmitError)
+    MF.getProperties().set(MachineFunctionProperties::Property::FailedRegAlloc);
+
+  const Function &Fn = MF.getFunction();
+  LLVMContext &Context = Fn.getContext();
+
+  ArrayRef<MCPhysReg> AllocOrder = RegClassInfo.getOrder(&RC);
+  if (AllocOrder.empty()) {
+    // If the allocation order is empty, it likely means all registers in the
+    // class are reserved. We still to need to pick something, so look at the
+    // underlying class.
+    ArrayRef<MCPhysReg> RawRegs = RC.getRegisters();
+
+    if (EmitError) {
+      DiagnosticInfoRegAllocFailure DI(
+          "no registers from class available to allocate", Fn,
+          CtxMI ? CtxMI->getDebugLoc() : DiagnosticLocation());
+      Context.diagnose(DI);
+    }
+
+    assert(!RawRegs.empty() && "register classes cannot have no registers");
+    return RawRegs.front();
+  }
+
+  if (EmitError) {
----------------
arsenm wrote:

Yes. report_fatal_error is not an acceptable end user error experience. We should produce proper LLVMContext diagnostics for the frontend to intercept, with line locations pointing at the failing instruction. 

AMDGPU has attributes to control the register budget, which can be used to make certain operations uncompilable. It is nicer to have a proper error pointing at the failing instruction 

https://github.com/llvm/llvm-project/pull/119640