[AMDGPU] Use more consistemt way to avoid overflow in the scheduler

[rampitec]

Summary

Use more consistent way to avoid overflow when calculating SGPR
and VGPR pressure limits.

[arsenm]

llvm/lib/Target/AMDGPU/GCNSchedStrategy.cpp

93

clamp with min of 0 seems weird. Also could use -=

llvm/lib/Target/AMDGPU/GCNSchedStrategy.cpp

if (!KnownExcessRP) {
     unsigned Addressable = AMDGPU::IsaInfo::getAddressableNumVGPRs(&ST);
     unsigned VGPRBudget = alignDown(Addressable / TargetOccupancy, Granule);
     VGPRBudget = std::max(VGPRBudget, Granule);
     VGPRCriticalLimit = std::min(VGPRBudget, VGPRExcessLimit);
   }
 
   // Subtract error margin and bias from register limits and avoid overflow.
   SGPRCriticalLimit =
-      std::min(SGPRCriticalLimit - SGPRLimitBias, SGPRCriticalLimit);
-  SGPRCriticalLimit =
-      std::min(SGPRCriticalLimit - ErrorMargin, SGPRCriticalLimit);
-  VGPRCriticalLimit =
-      std::min(VGPRCriticalLimit - VGPRLimitBias, VGPRCriticalLimit);
+      SGPRCriticalLimit -
+      std::clamp(SGPRLimitBias + ErrorMargin, 0u, SGPRCriticalLimit);
   VGPRCriticalLimit =
-      std::min(VGPRCriticalLimit - ErrorMargin, VGPRCriticalLimit);
-  SGPRExcessLimit = std::min(SGPRExcessLimit - SGPRLimitBias, SGPRExcessLimit);
-  SGPRExcessLimit = std::min(SGPRExcessLimit - ErrorMargin, SGPRExcessLimit);
-  VGPRExcessLimit = std::min(VGPRExcessLimit - VGPRLimitBias, VGPRExcessLimit);
-  VGPRExcessLimit = std::min(VGPRExcessLimit - ErrorMargin, VGPRExcessLimit);
+      VGPRCriticalLimit -
+      std::clamp(VGPRLimitBias + ErrorMargin, 0u, VGPRCriticalLimit);
+  SGPRExcessLimit =
+      SGPRExcessLimit -
+      std::clamp(SGPRLimitBias + ErrorMargin, 0u, SGPRExcessLimit);
+  VGPRExcessLimit =
+      VGPRExcessLimit -
+      std::clamp(VGPRLimitBias + ErrorMargin, 0u, VGPRExcessLimit);
 
   LLVM_DEBUG(dbgs() << "VGPRCriticalLimit = " << VGPRCriticalLimit
                     << ", VGPRExcessLimit = " << VGPRExcessLimit
                     << ", SGPRCriticalLimit = " << SGPRCriticalLimit
                     << ", SGPRExcessLimit = " << SGPRExcessLimit << "\n\n");
 }
 
 void GCNSchedStrategy::initCandidate(SchedCandidate &Cand, SUnit *SU,