[clang][UBSan] Sanitization for alignment assumptions.

Summary:
UB isn't nice. It's cool and powerful, but not nice.
Having a way to detect it is nice though.
[[ https://wg21.link/p1007r3 | P1007R3: std::assume_aligned ]] / http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p1007r2.pdf says:
```
We propose to add this functionality via a library function instead of a core language attribute.
...
If the pointer passed in is not aligned to at least N bytes, calling assume_aligned results in undefined behaviour.
```

This differential teaches clang to sanitize all the various variants of this assume-aligned attribute.

Requires D54588 for LLVM IRBuilder changes.
The compiler-rt part is D54590.

Reviewers: ABataev, craig.topper, vsk, rsmith, rnk, #sanitizers, erichkeane, filcab, rjmccall

Reviewed By: rjmccall

Subscribers: chandlerc, ldionne, EricWF, mclow.lists, cfe-commits, bkramer

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D54589

llvm-svn: 351105

1 files changed, 5 insertions, 3 deletions

diff --git a/clang/lib/CodeGen/CGBuiltin.cpp b/clang/lib/CodeGen/CGBuiltin.cpp
index 4c17602314e..a718f2f19aa 100644
--- a/clang/lib/CodeGen/CGBuiltin.cpp
+++ b/clang/lib/CodeGen/CGBuiltin.cpp
@@ -1904,15 +1904,17 @@ RValue CodeGenFunction::EmitBuiltinExpr(const GlobalDecl GD, unsigned BuiltinID,
     return RValue::get(Result);
   }
   case Builtin::BI__builtin_assume_aligned: {
-    Value *PtrValue = EmitScalarExpr(E->getArg(0));
+    const Expr *Ptr = E->getArg(0);
+    Value *PtrValue = EmitScalarExpr(Ptr);
     Value *OffsetValue =
       (E->getNumArgs() > 2) ? EmitScalarExpr(E->getArg(2)) : nullptr;
 
     Value *AlignmentValue = EmitScalarExpr(E->getArg(1));
     ConstantInt *AlignmentCI = cast<ConstantInt>(AlignmentValue);
-    unsigned Alignment = (unsigned) AlignmentCI->getZExtValue();
+    unsigned Alignment = (unsigned)AlignmentCI->getZExtValue();
 
-    EmitAlignmentAssumption(PtrValue, Alignment, OffsetValue);
+    EmitAlignmentAssumption(PtrValue, Ptr, /*The expr loc is sufficient.*/ SourceLocation(),
+                            Alignment, OffsetValue);
     return RValue::get(PtrValue);
   }
   case Builtin::BI__assume: