[llvm] [AMDGPU] Add support for preloading implicit kernel arguments (PR #83817)

[Matt Arsenault via llvm-commits]

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@@ -5495,6 +5495,34 @@ bool AMDGPULegalizerInfo::legalizeLDSKernelId(MachineInstr &MI,
   return true;
 }
 
+bool AMDGPULegalizerInfo::legalizePreloadImplicitarg(
+    MachineInstr &MI, MachineRegisterInfo &MRI, MachineIRBuilder &B) const {
+  assert(ST.hasKernargPreload());
+  MachineFunction &MF = B.getMF();
+  Register OrigReg = MI.getOperand(0).getReg();
+  const SIMachineFunctionInfo *MFI = MF.getInfo<SIMachineFunctionInfo>();
+  // A unique identifier defined as the offset from start of implicit args added
+  // to the number of formal args.
+  unsigned ImplictArgIdx =
+      MI.getOperand(2).getImm() + MF.getFunction().arg_size();
+  auto &ArgDesc =
+      MFI->getArgInfo().PreloadKernArgs.find(ImplictArgIdx)->getSecond();
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arsenm wrote:

I think representing this in terms of indexes of implicit arguments will be confusing. The arguments do not have uniform size. Can this be expressed as a raw byte offset instead? 

https://github.com/llvm/llvm-project/pull/83817