[AA] Assert that alias() arguments are pointers

[nikic]

Assert instead of returning NoAlias for non-pointers. This makes sure that people don't confuse alias (working on locations) with getModRefInfo (working on instructions).

[llvmbot]

@llvm/pr-subscribers-llvm-analysis

Author: Nikita Popov (nikic)

Changes

Assert instead of returning NoAlias for non-pointers. This makes sure that people don't confuse alias (working on locations) with getModRefInfo (working on instructions).

---

Full diff: https://github.com/llvm/llvm-project/pull/138242.diff

2 Files Affected:

• (modified) llvm/lib/Analysis/AliasAnalysis.cpp (+3)
• (modified) llvm/lib/Analysis/BasicAliasAnalysis.cpp (-3)

diff --git a/llvm/lib/Analysis/AliasAnalysis.cpp b/llvm/lib/Analysis/AliasAnalysis.cpp
index efabf69b06047..f4946c30de9bc 100644
--- a/llvm/lib/Analysis/AliasAnalysis.cpp
+++ b/llvm/lib/Analysis/AliasAnalysis.cpp
@@ -110,6 +110,9 @@ AliasResult AAResults::alias(const MemoryLocation &LocA,
 AliasResult AAResults::alias(const MemoryLocation &LocA,
                              const MemoryLocation &LocB, AAQueryInfo &AAQI,
                              const Instruction *CtxI) {
+  assert(LocA.Ptr->getType()->isPointerTy() &&
+         LocB.Ptr->getType()->isPointerTy() &&
+         "Can only call alias() on pointers");
   AliasResult Result = AliasResult::MayAlias;
 
   if (EnableAATrace) {
diff --git a/llvm/lib/Analysis/BasicAliasAnalysis.cpp b/llvm/lib/Analysis/BasicAliasAnalysis.cpp
index a46edc0b75f54..2de9bb502baf4 100644
--- a/llvm/lib/Analysis/BasicAliasAnalysis.cpp
+++ b/llvm/lib/Analysis/BasicAliasAnalysis.cpp
@@ -1574,9 +1574,6 @@ AliasResult BasicAAResult::aliasCheck(const Value *V1, LocationSize V1Size,
   if (isValueEqualInPotentialCycles(V1, V2, AAQI))
     return AliasResult::MustAlias;
 
-  if (!V1->getType()->isPointerTy() || !V2->getType()->isPointerTy())
-    return AliasResult::NoAlias; // Scalars cannot alias each other
-
   // Figure out what objects these things are pointing to if we can.
   const Value *O1 = getUnderlyingObject(V1, MaxLookupSearchDepth);
   const Value *O2 = getUnderlyingObject(V2, MaxLookupSearchDepth);

[dtcxzyw]

LGTM.

[fhahn]

LGTM, thanks.

might be good to also update the doc comment, if it isn’t already

[slackito]

Hi, I'm debugging a problem after this commit with one of the Enzyme (https://github.com/EnzymeAD/) tests.

There's some code that doesn't call alias directly:

        if (llvm::isModSet(AA.getModRefInfo(
                inst2, MemoryLocation::getForArgument(callsite_op, i, TLI)))) {
                    ...

the call to getModRefInfo ends up here:

  if (Loc.Ptr) {
    AliasResult AR = alias(MemoryLocation::get(S), Loc, AAQI, S);

and running the test case under lldb, Loc.Ptr is not null (so the check passes) but doesn't have pointer type (so the assert inside alias() fails):

(lldb) p Loc.Ptr
(const llvm::Value *const) 0x00005092ffe57460
(lldb) p Loc.Ptr->getType()->dump()
i32

should the check in getModRefInfo be more strict and also check the type of Loc.Ptr before calling alias()?

[nikic]

@slackito Most likely the code is calling MemoryLocation::getForArgument on a non-pointer argument.

[slackito]

Thanks for the advice! I'll fix it on my end then.