Reverse subregister saved loops in register usage info collector.

[airlied]

Summary

On AMDGPU we have 70 register classes, so iterating over all 70
each time and exiting is costly on the CPU, this flips the loop
around so that it loops over the 70 register classes first,
and exits without doing the inner loop if needed.

On my test just starting radv this takes
RegUsageInfoCollector::runOnMachineFunction
from 6.0% of total time to 2.7% of total time,
and reduces the startup from 2.24s to 2.19s

[arsenm]

ping

[MatzeB]

Adding more people; still reading up on why the covered logic was necessary in the first place...

[MatzeB]

I fear the covered check will hit us perf wise either way (+CC some people from our GPU team) and that we need to find a different solution long term (like the Targets announcing the saved registers themselfes instead of the generic code piecing together the information...).
Anyway if this works for you I'd be fine with this patch as a stopgap.

[airlied]

Rebased onto latest master. I require someone else to push this as I don't have commit rights.

[xbolva00]

Please upload with context

llvm/trunk/lib/CodeGen/RegUsageInfoCollector.cpp

computeCalleeSavedRegs(BitVector &SavedRegs, MachineFunction &MF) {
   for (unsigned i = 0; CSRegs[i]; ++i) {
     unsigned Reg = CSRegs[i];
     if (SavedRegs.test(Reg))
       for (MCSubRegIterator SR(Reg, &TRI, false); SR.isValid(); ++SR)
         SavedRegs.set(*SR);
   }
 
   // Insert any register fully saved via subregisters.
+  for (const TargetRegisterClass *RC : TRI.regclasses()) {
+    if (!RC->CoveredBySubRegs)
+       continue;
+
     for (unsigned PReg = 1, PRegE = TRI.getNumRegs(); PReg < PRegE; ++PReg) {
       if (SavedRegs.test(PReg))
         continue;
 
       // Check if PReg is fully covered by its subregs.
-    bool CoveredBySubRegs = false;
-    for (const TargetRegisterClass *RC : TRI.regclasses())
-      if (RC->CoveredBySubRegs && RC->contains(PReg)) {
-        CoveredBySubRegs = true;
-        break;
-      }
-    if (!CoveredBySubRegs)
+      if (!RC->contains(PReg))
         continue;
 
       // Add PReg to SavedRegs if all subregs are saved.
       bool AllSubRegsSaved = true;
       for (MCSubRegIterator SR(PReg, &TRI, false); SR.isValid(); ++SR)
         if (!SavedRegs.test(*SR)) {
           AllSubRegsSaved = false;
           break;
         }
       if (AllSubRegsSaved)
         SavedRegs.set(PReg);
     }
   }
+}