[CaptureTracking] Allow null to be in either icmp operand

[jdoerfert]

Summary

Before we required the comparison against null to be "canonical", hence
null to be operand #1. This patch allows null to be in either operand,
similar to the handling of loaded globals that follows.

[uenoku]

It looks straight forward. LGTM

llvm/lib/Analysis/CaptureTracking.cpp

while (!Worklist.empty()) {
     case Instruction::GetElementPtr:
     case Instruction::PHI:
     case Instruction::Select:
     case Instruction::AddrSpaceCast:
       // The original value is not captured via this if the new value isn't.
       AddUses(I);
       break;
     case Instruction::ICmp: {
-      if (auto *CPN = dyn_cast<ConstantPointerNull>(I->getOperand(1))) {
+      unsigned Idx = (I->getOperand(0) == V) ? 0 : 1;
+      unsigned OtherIdx = 1 - Idx;
+      if (auto *CPN = dyn_cast<ConstantPointerNull>(I->getOperand(OtherIdx))) {
         // Don't count comparisons of a no-alias return value against null as
         // captures. This allows us to ignore comparisons of malloc results
         // with null, for example.
         if (CPN->getType()->getAddressSpace() == 0)
           if (isNoAliasCall(V->stripPointerCasts()))
             break;
         if (!I->getFunction()->nullPointerIsDefined()) {
-          auto *O = I->getOperand(0)->stripPointerCastsSameRepresentation();
+          auto *O = I->getOperand(Idx)->stripPointerCastsSameRepresentation();
           // An inbounds GEP can either be a valid pointer (pointing into
           // or to the end of an allocation), or be null in the default
           // address space. So for an inbounds GEPs there is no way to let
           // the pointer escape using clever GEP hacking because doing so
           // would make the pointer point outside of the allocated object
           // and thus make the GEP result a poison value.
           if (auto *GEP = dyn_cast<GetElementPtrInst>(O))
             if (GEP->isInBounds())
               break;
           // Comparing a dereferenceable_or_null argument against null
           // cannot lead to pointer escapes, because if it is not null it
           // must be a valid (in-bounds) pointer.
           bool CanBeNull;
-          if (O->getPointerDereferenceableBytes(I->getModule()->getDataLayout(), CanBeNull))
+          if (O->getPointerDereferenceableBytes(I->getModule()->getDataLayout(),
+                                                CanBeNull))
             break;
         }
       }
       // Comparison against value stored in global variable. Given the pointer
       // does not escape, its value cannot be guessed and stored separately in a
       // global variable.
-      unsigned OtherIndex = (I->getOperand(0) == V) ? 1 : 0;
-      auto *LI = dyn_cast<LoadInst>(I->getOperand(OtherIndex));
+      auto *LI = dyn_cast<LoadInst>(I->getOperand(OtherIdx));
       if (LI && isa<GlobalVariable>(LI->getPointerOperand()))
         break;
       // Otherwise, be conservative. There are crazy ways to capture pointers
       // using comparisons.
       if (Tracker->captured(U))
         return;
       break;
     }

llvm/test/Transforms/FunctionAttrs/nocapture.ll

 }
 
 ; CHECK: define i1 @captureICmp(i32* readnone %x)
 define i1 @captureICmp(i32* %x) {
   %1 = icmp eq i32* %x, null
   ret i1 %1
 }
 
+; CHECK: define i1 @captureICmpRev(i32* readnone %x)
+define i1 @captureICmpRev(i32* %x) {
+  %1 = icmp eq i32* null, %x
+  ret i1 %1
+}
+
 ; CHECK: define i1 @nocaptureInboundsGEPICmp(i32* nocapture readnone %x)
 define i1 @nocaptureInboundsGEPICmp(i32* %x) {
   %1 = getelementptr inbounds i32, i32* %x, i32 5
   %2 = bitcast i32* %1 to i8*
   %3 = icmp eq i8* %2, null
   ret i1 %3
 }
 
+; CHECK: define i1 @nocaptureInboundsGEPICmpRev(i32* nocapture readnone %x)
+define i1 @nocaptureInboundsGEPICmpRev(i32* %x) {
+  %1 = getelementptr inbounds i32, i32* %x, i32 5
+  %2 = bitcast i32* %1 to i8*
+  %3 = icmp eq i8* null, %2
+  ret i1 %3
+}
+
 ; CHECK: define i1 @nocaptureDereferenceableOrNullICmp(i32* nocapture readnone dereferenceable_or_null(4) %x)
 define i1 @nocaptureDereferenceableOrNullICmp(i32* dereferenceable_or_null(4) %x) {
   %1 = bitcast i32* %x to i8*
   %2 = icmp eq i8* %1, null
   ret i1 %2
 }
 
 ; CHECK: define i1 @captureDereferenceableOrNullICmp(i32* readnone dereferenceable_or_null(4) %x)
 define i1 @captureDereferenceableOrNullICmp(i32* dereferenceable_or_null(4) %x) "null-pointer-is-valid"="true" {
   %1 = bitcast i32* %x to i8*
   %2 = icmp eq i8* %1, null
   ret i1 %2
 }
 
 declare i8* @llvm.launder.invariant.group.p0i8(i8*)
 declare i8* @llvm.strip.invariant.group.p0i8(i8*)