Fix RegMask calculation for alias registers

[vivekvpandya]

Summary

This patch fixes a very subtle bug in regmask calculation. Thanks to zan jyu Wong <[e-mail]> for bringing this to notice.
For example if CL is only clobbered than CH should not be marked clobbered but CX, RCX and ECX should be mark clobbered. Previously for each modified register all of its aliases are marked clobbered by markRegClobbred() in RegUsageInfoCollector.cpp but that is wrong because when CL is clobbered then MRI::isPhysRegModified() will return true for CL, CX, ECX, RCX which is correct behavior but then for CX, EXC, RCX we mark CH also clobbered as CH is aliased to CX,ECX,RCX so markRegClobbred() is not required because isPhysRegModified already take cares of proper aliasing register. A very simple test case has been added to verify this change.
Please find relevant bug report here : https://llvm.org/bugs/show_bug.cgi?id=28567

[vivekvpandya]

ping !

[MatzeB]

LGTM

[MatzeB]

Should I commit this for you Vivek?

[vivekvpandya]

Sorry @MatzeB I don't have commit access. Usually @mehdi_amini commits for me. If you are not able to find time I guess @mehdi_amini will do before this week ends.

[MatzeB]

no problem, I'll commit it.

lib/CodeGen/RegUsageInfoCollector.cpp

const char *getPassName() const override {
     return "Register Usage Information Collector Pass";
   }
 
   void getAnalysisUsage(AnalysisUsage &AU) const override;
 
   bool runOnMachineFunction(MachineFunction &MF) override;
 
   static char ID;
-
-private:
-  void markRegClobbered(const TargetRegisterInfo *TRI, uint32_t *RegMask,
-                        unsigned PReg);
 };
 } // end of anonymous namespace
 
 char RegUsageInfoCollector::ID = 0;
 
 INITIALIZE_PASS_BEGIN(RegUsageInfoCollector, "RegUsageInfoCollector",
                       "Register Usage Information Collector", false, false)
 INITIALIZE_PASS_DEPENDENCY(PhysicalRegisterUsageInfo)
 INITIALIZE_PASS_END(RegUsageInfoCollector, "RegUsageInfoCollector",
                     "Register Usage Information Collector", false, false)
 
 FunctionPass *llvm::createRegUsageInfoCollector() {
   return new RegUsageInfoCollector();
 }
 
-void RegUsageInfoCollector::markRegClobbered(const TargetRegisterInfo *TRI,
-                                             uint32_t *RegMask, unsigned PReg) {
-  // If PReg is clobbered then all of its alias are also clobbered.
-  for (MCRegAliasIterator AI(PReg, TRI, true); AI.isValid(); ++AI)
-    RegMask[*AI / 32] &= ~(1u << *AI % 32);
-}
-
 void RegUsageInfoCollector::getAnalysisUsage(AnalysisUsage &AU) const {
   AU.addRequired<PhysicalRegisterUsageInfo>();
   AU.setPreservesAll();
   MachineFunctionPass::getAnalysisUsage(AU);
 }
 
 bool RegUsageInfoCollector::runOnMachineFunction(MachineFunction &MF) {
   MachineRegisterInfo *MRI = &MF.getRegInfo();
bool RegUsageInfoCollector::runOnMachineFunction(MachineFunction &MF) {
   PhysicalRegisterUsageInfo *PRUI = &getAnalysis<PhysicalRegisterUsageInfo>();
 
   PRUI->setTargetMachine(&TM);
 
   DEBUG(dbgs() << "Clobbered Registers: ");
 
   for (unsigned PReg = 1, PRegE = TRI->getNumRegs(); PReg < PRegE; ++PReg)
     if (MRI->isPhysRegModified(PReg, true))
-      markRegClobbered(TRI, &RegMask[0], PReg);
+      RegMask[PReg / 32] &= ~(1u << PReg % 32);
 
   if (!TargetFrameLowering::isSafeForNoCSROpt(F)) {
     const uint32_t *CallPreservedMask =
         TRI->getCallPreservedMask(MF, F->getCallingConv());
     // Set callee saved register as preserved.
     for (unsigned i = 0; i < RegMaskSize; ++i)
       RegMask[i] = RegMask[i] | CallPreservedMask[i];
   } else {

test/CodeGen/X86/ipra-reg-alias.ll

+; RUN: llc -enable-ipra -print-regusage -o /dev/null 2>&1 < %s | FileCheck %s
+target triple = "x86_64--"
+
+define i8 @main(i8 %X) {
+  %inc = add i8 %X, 1
+  %inc2 = mul i8 %inc, 5
+; Here only CL is clobbred so CH should not be clobbred, but CX, ECX and RCX
+; should be clobbered.
+; CHECK: main Clobbered Registers: AH AL AX CL CX EAX ECX EFLAGS RAX RCX
+  ret i8 %inc2
+}
+