isKnownNonNull: recognize GEP inbounds as non-null

[nlopes]

Summary

A GEP inbounds cannot return a null pointer in address space 0 because:

1. the input is non-zero
2. arithmetic cannot overflow

This patch makes isKnownNonNull() recognize this fact.

[nlopes]

Just realized there's a complicated isGEPKnownNonNull() in ValueTracking.cpp, which is use solely by isKnownNonZero().
I don't believe such a thing is necessary, since the input pointer to a GEP inbounds has to be non-null in address space 0. If others agree I'll also remove that function in a subsequent patch.

[efriedma]

See https://reviews.llvm.org/D28026

[nlopes]

In D35887#823029, @efriedma wrote:

See https://reviews.llvm.org/D28026

Ah, thanks for the pointer.
Then I'll abandon this patch. I'm not sure why I understand why we have isKnownNotNull and isKnownNotZero. The difference seems to be that the latter is recursive and the former is not. I dunno if that was intended or not.
The current situation is that not all tricks are duplicated in the two functions, so the clients miss out if they use isKnownNotNull().
Any opinions or pointers to historical artifacts?

[efriedma]

isKnownNonZero was added in r124183; isKnownNonNull was originally part of BasicAA, then moved in r151446.

[nlopes]

In D35887#824426, @efriedma wrote:

isKnownNonZero was added in r124183; isKnownNonNull was originally part of BasicAA, then moved in r151446.

Thanks for the pointers, Eli!

lib/Analysis/ValueTracking.cpp

 
 /// Return true if we know that the specified value is never null.
 bool llvm::isKnownNonNull(const Value *V) {
   assert(V->getType()->isPointerTy() && "V must be pointer type");
 
   // Alloca never returns null, malloc might.
   if (isa<AllocaInst>(V)) return true;
 
+  // A GEP inbounds of a ptr of address space 0 can never return null because:
+  // 1) the input argument cannot be null; 2) it cannot overflow.
+  if (const GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(V))
+    return GEP->isInBounds() && GEP->getAddressSpace() == 0;
+
   // A byval, inalloca, or nonnull argument is never null.
   if (const Argument *A = dyn_cast<Argument>(V))
     return A->hasByValOrInAllocaAttr() || A->hasNonNullAttr();
 
   // A global variable in address space 0 is non null unless extern weak
   // or an absolute symbol reference. Other address spaces may have null as a
   // valid address for a global, so we can't assume anything.
   if (const GlobalValue *GV = dyn_cast<GlobalValue>(V))

test/Transforms/FunctionAttrs/nonnull.ll

cont:
   ret i1 %null_check
 
 exc:
   %lp = landingpad { i8*, i32 }
   filter [0 x i8*] zeroinitializer
   unreachable
 }
 
+; CHECK: define nonnull i32* @gep1(
+define i32* @gep1(i32* %p) {
+  %q = getelementptr inbounds i32, i32* %p, i32 1
+  ret i32* %q
+}
+
+; CHECK: define i32 addrspace(3)* @gep2(
+define i32 addrspace(3)* @gep2(i32 addrspace(3)* %p) {
+  %q = getelementptr inbounds i32, i32 addrspace(3)* %p, i32 1
+  ret i32 addrspace(3)* %q
+}