[TableGen][NFCI] Speed up generating *GenRegisterInfo.inc files on builds with expensive checks. (llvm#67340)

This is mostly AMDGPU-specific. When the expensive checks are enabled,
generating of AMDGPUGenRegisterInfo.inc currently takes about 20 minutes
on my machine for release+asserts builds, which effectively prevents
such testing from regular use. This patch fixes this by reducing the
time to about 2 minutes.

Generation times for AMDGPUGenRegisterInfo.inc without expensive checks
and other *GenRegisterInfo.inc files with and without the expensive
checks remain approximately the same.

The patch doesn't cause any changes in the contents of the generated
files.

The root cause of the current poor performance is that where glibcxx is
used, enabling the expensive checks defines _GLIBCXX_DEBUG, which
enables various consistency checks in the library. One such check is in
std::binary_search() to make sure the range is ordered. As
CodeGenRegisterClass::contains() relies on std::binary_search() and it
is called very a large number of times from within
CodeGenRegBank::inferMatchingSuperRegClass(), the libcxx checks heavily
affect the runtimes.

Author: kosarev

llvm/utils/TableGen/CodeGenRegisters.cpp

@@ -2297,8 +2297,8 @@ void CodeGenRegBank::inferSubClassWithSubReg(CodeGenRegisterClass *RC) {
 
 void CodeGenRegBank::inferMatchingSuperRegClass(CodeGenRegisterClass *RC,
                                                 std::list<CodeGenRegisterClass>::iterator FirstSubRegRC) {
-  SmallVector<std::pair<const CodeGenRegister*,
-                        const CodeGenRegister*>, 16> SSPairs;
+  DenseMap<const CodeGenRegister *, std::vector<const CodeGenRegister *>>
+      SubToSuperRegs;
   BitVector TopoSigs(getNumTopoSigs());
 
   // Iterate in SubRegIndex numerical order to visit synthetic indices last.
@@ -2310,12 +2310,12 @@ void CodeGenRegBank::inferMatchingSuperRegClass(CodeGenRegisterClass *RC,
       continue;
 
     // Build list of (Super, Sub) pairs for this SubIdx.
-    SSPairs.clear();
+    SubToSuperRegs.clear();
     TopoSigs.reset();
     for (const auto Super : RC->getMembers()) {
       const CodeGenRegister *Sub = Super->getSubRegs().find(&SubIdx)->second;
       assert(Sub && "Missing sub-register");
-      SSPairs.push_back(std::make_pair(Super, Sub));
+      SubToSuperRegs[Sub].push_back(Super);
       TopoSigs.set(Sub->getTopoSig());
     }
 
@@ -2334,16 +2334,20 @@ void CodeGenRegBank::inferMatchingSuperRegClass(CodeGenRegisterClass *RC,
         continue;
       // Compute the subset of RC that maps into SubRC.
       CodeGenRegister::Vec SubSetVec;
-      for (unsigned i = 0, e = SSPairs.size(); i != e; ++i)
-        if (SubRC.contains(SSPairs[i].second))
-          SubSetVec.push_back(SSPairs[i].first);
+      for (const CodeGenRegister *R : SubRC.getMembers()) {
+        auto It = SubToSuperRegs.find(R);
+        if (It != SubToSuperRegs.end()) {
+          const std::vector<const CodeGenRegister *> &SuperRegs = It->second;
+          SubSetVec.insert(SubSetVec.end(), SuperRegs.begin(), SuperRegs.end());
+        }
+      }
 
       if (SubSetVec.empty())
         continue;
 
       // RC injects completely into SubRC.
       sortAndUniqueRegisters(SubSetVec);
-      if (SubSetVec.size() == SSPairs.size()) {
+      if (SubSetVec.size() == RC->getMembers().size()) {
         SubRC.addSuperRegClass(&SubIdx, RC);
         continue;
       }