[llvm] [AMDGPU] Support preloading hidden kernel arguments (PR #98861)

[Matt Arsenault via llvm-commits]

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@@ -1035,6 +1039,14 @@ unsigned GCNSubtarget::getNSAThreshold(const MachineFunction &MF) const {
   return 3;
 }
 
+unsigned AMDGPUSubtarget::getHiddenArgOffset(const Function &F) const {
+  if (!F.hasFnAttribute("amdgpu-hidden-arg-offset"))
+    return F.arg_size();
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arsenm wrote:

Can just use F.getFnAttributeAsParsedInteger, the default value will be used if the attribute doesn't exist 

https://github.com/llvm/llvm-project/pull/98861