[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.

This is a second commit, the original one was r351105,
which was mass-reverted in r351159 because 2 compiler-rt tests were failing.

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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@351177 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 43 insertions, 0 deletions

diff --git a/docs/ReleaseNotes.rst b/docs/ReleaseNotes.rst
index 64991343d9..b6a405dbc7 100644
--- a/docs/ReleaseNotes.rst
+++ b/docs/ReleaseNotes.rst
@@ -321,6 +321,49 @@ Undefined Behavior Sanitizer (UBSan)
 * The Implicit Conversion Sanitizer (``-fsanitize=implicit-conversion``) has
   learned to sanitize compound assignment operators.
 
+* ``alignment`` check has learned to sanitize the assume_aligned-like attributes:
+
+  .. code-block:: c++
+
+      typedef char **__attribute__((align_value(1024))) aligned_char;
+      struct ac_struct {
+        aligned_char a;
+      };
+      char **load_from_ac_struct(struct ac_struct *x) {
+        return x->a; // <- check that loaded 'a' is aligned
+      }
+
+      char **passthrough(__attribute__((align_value(1024))) char **x) {
+        return x; // <- check the pointer passed as function argument
+      }
+
+      char **__attribute__((alloc_align(2)))
+      alloc_align(int size, unsigned long alignment);
+
+      char **caller(int size) {
+        return alloc_align(size, 1024); // <- check returned pointer
+      }
+
+      char **__attribute__((assume_aligned(1024))) get_ptr();
+
+      char **caller2() {
+        return get_ptr(); // <- check returned pointer
+      }
+
+      void *caller3(char **x) {
+        return __builtin_assume_aligned(x, 1024);  // <- check returned pointer
+      }
+
+      void *caller4(char **x, unsigned long offset) {
+        return __builtin_assume_aligned(x, 1024, offset);  // <- check returned pointer accounting for the offest
+      }
+
+      void process(char *data, int width) {
+          #pragma omp for simd aligned(data : 1024) // <- aligned clause will be checked.
+          for (int x = 0; x < width; x++)
+          data[x] *= data[x];
+      }
+
 Core Analysis Improvements
 ==========================