[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, 7 insertions, 3 deletions

diff --git a/clang/lib/CodeGen/CGExprScalar.cpp b/clang/lib/CodeGen/CGExprScalar.cpp
index f53bb33e463..1c14d4c99a2 100644
--- a/clang/lib/CodeGen/CGExprScalar.cpp
+++ b/clang/lib/CodeGen/CGExprScalar.cpp
@@ -258,8 +258,11 @@ public:
           AVAttr = TTy->getDecl()->getAttr<AlignValueAttr>();
       } else {
         // Assumptions for function parameters are emitted at the start of the
-        // function, so there is no need to repeat that here.
-        if (isa<ParmVarDecl>(VD))
+        // function, so there is no need to repeat that here,
+        // unless the alignment-assumption sanitizer is enabled,
+        // then we prefer the assumption over alignment attribute
+        // on IR function param.
+        if (isa<ParmVarDecl>(VD) && !CGF.SanOpts.has(SanitizerKind::Alignment))
           return;
 
         AVAttr = VD->getAttr<AlignValueAttr>();
@@ -276,7 +279,8 @@ public:
 
     Value *AlignmentValue = CGF.EmitScalarExpr(AVAttr->getAlignment());
     llvm::ConstantInt *AlignmentCI = cast<llvm::ConstantInt>(AlignmentValue);
-    CGF.EmitAlignmentAssumption(V, AlignmentCI->getZExtValue());
+    CGF.EmitAlignmentAssumption(V, E, AVAttr->getLocation(),
+                                AlignmentCI->getZExtValue());
   }
 
   /// EmitLoadOfLValue - Given an expression with complex type that represents a