IPRA: Fix RegMask calculation for alias registers

This patch fixes a very subtle bug in regmask calculation. Thanks to zan
jyu Wong <zyfwong@gmail.com> 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 :
http://llvm.org/PR28567

Patch by Vivek Pandya <vivekvpandya@gmail.com>

Differential Revision: https://reviews.llvm.org/D22400

llvm-svn: 276235

llvm/lib/CodeGen/RegUsageInfoCollector.cpp

@@ -57,10 +57,6 @@ class RegUsageInfoCollector : public MachineFunctionPass {
   bool runOnMachineFunction(MachineFunction &MF) override;
 
   static char ID;
-
-private:
-  void markRegClobbered(const TargetRegisterInfo *TRI, uint32_t *RegMask,
-                        unsigned PReg);
 };
 } // end of anonymous namespace
 
@@ -76,13 +72,6 @@ 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();
@@ -116,7 +105,7 @@ bool RegUsageInfoCollector::runOnMachineFunction(MachineFunction &MF) {
 
   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 =

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

@@ -0,0 +1,12 @@
+; 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
+}
+