[Sema] Demote call-site-based 'alignment is a power of two' check for AllocAlignAttr into a warning

Summary:
As @rsmith notes in https://reviews.llvm.org/D73020#inline-672219
while that is certainly UB land, it may not be actually reachable at runtime, e.g.:
```
template<int N> void *make() {
  if ((N & (N-1)) == 0)
    return operator new(N, std::align_val_t(N));
  else
    return operator new(N);
}
void *p = make<7>();
```
and we shouldn't really error-out there.

That being said, i'm not really following the logic here.
Which ones of these cases should remain being an error?

Reviewers: rsmith, erichkeane

Reviewed By: erichkeane

Subscribers: cfe-commits, rsmith

Tags: #clang

Differential Revision: https://reviews.llvm.org/D73996

1 files changed, 4 insertions, 0 deletions

diff --git a/clang/lib/CodeGen/CGCall.cpp b/clang/lib/CodeGen/CGCall.cpp
index 6cc0154..98365db 100644
--- a/clang/lib/CodeGen/CGCall.cpp
+++ b/clang/lib/CodeGen/CGCall.cpp
@@ -3892,6 +3892,10 @@ public:
     const auto *AlignmentCI = dyn_cast<llvm::ConstantInt>(Alignment);
     if (!AlignmentCI)
       return Attrs;
+    // We may legitimately have non-power-of-2 alignment here.
+    // If so, this is UB land, emit it via `@llvm.assume` instead.
+    if (!AlignmentCI->getValue().isPowerOf2())
+      return Attrs;
     llvm::AttributeList NewAttrs = maybeRaiseRetAlignmentAttribute(
         CGF.getLLVMContext(), Attrs,
         llvm::Align(