[llvm] [AMDGPU] Skip register uses in AMDGPUResourceUsageAnalysis (PR #133242)

[Matt Arsenault via llvm-commits]

================
@@ -137,274 +137,29 @@ AMDGPUResourceUsageAnalysis::analyzeResourceUsage(
   if (MFI->isStackRealigned())
     Info.PrivateSegmentSize += FrameInfo.getMaxAlign().value();
 
-  Info.UsesVCC =
-      MRI.isPhysRegUsed(AMDGPU::VCC_LO) || MRI.isPhysRegUsed(AMDGPU::VCC_HI);
-
-  // If there are no calls, MachineRegisterInfo can tell us the used register
-  // count easily.
-  // A tail call isn't considered a call for MachineFrameInfo's purposes.
-  if (!FrameInfo.hasCalls() && !FrameInfo.hasTailCall()) {
-    Info.NumVGPR = TRI.getNumUsedPhysRegs(MRI, AMDGPU::VGPR_32RegClass);
-    Info.NumExplicitSGPR = TRI.getNumUsedPhysRegs(MRI, AMDGPU::SGPR_32RegClass);
-    if (ST.hasMAIInsts())
-      Info.NumAGPR = TRI.getNumUsedPhysRegs(MRI, AMDGPU::AGPR_32RegClass);
+  Info.UsesVCC = MRI.isPhysRegUsed(AMDGPU::VCC);
+
+  Info.NumVGPR = TRI.getNumDefinedPhysRegs(MRI, AMDGPU::VGPR_32RegClass);
+  Info.NumExplicitSGPR =
+      TRI.getNumDefinedPhysRegs(MRI, AMDGPU::SGPR_32RegClass);
+  if (ST.hasMAIInsts())
+    Info.NumAGPR = TRI.getNumDefinedPhysRegs(MRI, AMDGPU::AGPR_32RegClass);
+
+  // Count any user or system SGPRs that are actually used.
+  for (int I = MFI->getNumPreloadedSGPRs() - 1; I >= 0; I--)
----------------
arsenm wrote:

Braces, but also should just take the raw number of preloaded SGPRs to start with. What happens if you don't allocate preloaded registers that were requested? If we wanted to trim out unused preloaded registers, it should have happened earlier (or, we wouldn't have preloaded in the first place). This could also break the debugger use case if it expects to find something there 

https://github.com/llvm/llvm-project/pull/133242