[AMDGPU] Simplify getReservedNumSGPRs

[madhur13490]

Summary

This is a followup patch on D103636 where
it seemed checking on amdgpu-calls and
amdgpu-stack-objects is unnecessary. Removing these
checks didn't regress any tests functionally.

[foad]

Adding @Flakebi for awareness.

llvm/lib/Target/AMDGPU/AMDGPUSubtarget.cpp

724–731

This comment is no longer true. SIMachineFunctionInfo.cpp:169 still tests HasCalls and HasStackObjects.

[madhur13490]

address review comments

[madhur13490]

Ping to @arsenm

[sebastian-ne]

llvm/lib/Target/AMDGPU/AMDGPUSubtarget.cpp

732–735

The nested if can be merged into one.

[madhur13490]

optimize conditions

llvm/lib/Target/AMDGPU/AMDGPUSubtarget.cpp

732–735

Actually optimizing it a bit more, checking isAmdHsaOrMesa() is nugatory here. Setting the flag requires enableFlatScratch() to be true always.

llvm/lib/Target/AMDGPU/AMDGPUSubtarget.cpp

 }
 
 unsigned GCNSubtarget::getReservedNumSGPRs(const MachineFunction &MF) const {
   const SIMachineFunctionInfo &MFI = *MF.getInfo<SIMachineFunctionInfo>();
   return getBaseReservedNumSGPRs(MFI.hasFlatScratchInit());
 }
 
 unsigned GCNSubtarget::getReservedNumSGPRs(const Function &F) const {
   // The logic to detect if the function has
-  // flat scratch init is same as how MachineFunctionInfo derives.
+  // flat scratch init is slightly different than how
+  // SIMachineFunctionInfo constructor derives.
+  // We don't use amdgpu-calls, amdgpu-stack-objects
+  // attributes and isAmdHsaOrMesa here as it doesn't really matter.
+  // TODO: Outline this derivation logic and have just
+  // one common function in the backend to avoid duplication.
+  bool isEntry = AMDGPU::isEntryFunctionCC(F.getCallingConv());
   bool FunctionHasFlatScratchInit = false;
-  bool HasCalls = F.hasFnAttribute("amdgpu-calls");
-  bool HasStackObjects = F.hasFnAttribute("amdgpu-stack-objects");
-  if (hasFlatAddressSpace() && AMDGPU::isEntryFunctionCC(F.getCallingConv()) &&
-      (isAmdHsaOrMesa(F) || enableFlatScratch()) &&
-      !flatScratchIsArchitected()) {
-    if (HasCalls || HasStackObjects || enableFlatScratch())
-      FunctionHasFlatScratchInit = true;
+  if (hasFlatAddressSpace() && isEntry && !flatScratchIsArchitected() &&
+      enableFlatScratch()) {
   }
   return getBaseReservedNumSGPRs(FunctionHasFlatScratchInit);
 }
 
 unsigned GCNSubtarget::computeOccupancy(const Function &F, unsigned LDSSize,
                                         unsigned NumSGPRs,
                                         unsigned NumVGPRs) const {
   unsigned Occupancy =