https://bugs.llvm.org/show_bug.cgi?id=33878
Bug ID: 33878
Summary: BasicAA incorrectly assumes different address spaces
don't alias
Product: libraries
Version: trunk
Hardware: All
OS: All
Status: NEW
Keywords: miscompilation
Severity: normal
Priority: P
Component: Global Analyses
Assignee: unassignedb...@nondot.org
Reporter: nunoplo...@sapo.pt
CC: dav...@freebsd.org, dber...@dberlin.org,
gil....@sf.snu.ac.kr, hfin...@anl.gov,
jeehoon.k...@sf.snu.ac.kr, juneyoung....@sf.snu.ac.kr,
llvm-bugs@lists.llvm.org, reg...@cs.utah.edu,
san...@playingwithpointers.com
BasicAA assumes that pointers of different address spaces can't alias. This is
not true is general, though.
in function BasicAAResult::aliasCheck():
if (O1 != O2) {
// Most objects can't alias null.
if ((isa<ConstantPointerNull>(O2) && isKnownNonNull(O1)) ||
(isa<ConstantPointerNull>(O1) && isKnownNonNull(O2)))
return NoAlias;
There are two bugs here:
- if O1 is null, and O2 is non-null, but they are in different address spaces,
then nothing can be concluded (unless we add target-specific hooks to tell us
that information).
- isKnownNonNull assumes alloca produces a non-null result, which is not true
in general for address space != 0:
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;
(...)
}
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