[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

llvm-svn: 351177

1 files changed, 11 insertions, 7 deletions

diff --git a/clang/lib/CodeGen/CGCall.cpp b/clang/lib/CodeGen/CGCall.cpp
index 455a25434ff..7d494bb1f1c 100644
--- a/clang/lib/CodeGen/CGCall.cpp
+++ b/clang/lib/CodeGen/CGCall.cpp
@@ -2410,7 +2410,10 @@ void CodeGenFunction::EmitFunctionProlog(const CGFunctionInfo &FI,
           if (!AVAttr)
             if (const auto *TOTy = dyn_cast<TypedefType>(OTy))
               AVAttr = TOTy->getDecl()->getAttr<AlignValueAttr>();
-          if (AVAttr) {
+          if (AVAttr && !SanOpts.has(SanitizerKind::Alignment)) {
+            // If alignment-assumption sanitizer is enabled, we do *not* add
+            // alignment attribute here, but emit normal alignment assumption,
+            // so the UBSAN check could function.
             llvm::Value *AlignmentValue =
               EmitScalarExpr(AVAttr->getAlignment());
             llvm::ConstantInt *AlignmentCI =
@@ -4535,13 +4538,14 @@ RValue CodeGenFunction::EmitCall(const CGFunctionInfo &CallInfo,
 
       llvm::Value *Alignment = EmitScalarExpr(AA->getAlignment());
       llvm::ConstantInt *AlignmentCI = cast<llvm::ConstantInt>(Alignment);
-      EmitAlignmentAssumption(Ret.getScalarVal(), AlignmentCI->getZExtValue(),
-                              OffsetValue);
+      EmitAlignmentAssumption(Ret.getScalarVal(), RetTy, Loc, AA->getLocation(),
+                              AlignmentCI->getZExtValue(), OffsetValue);
     } else if (const auto *AA = TargetDecl->getAttr<AllocAlignAttr>()) {
-      llvm::Value *ParamVal =
-          CallArgs[AA->getParamIndex().getLLVMIndex()].getRValue(
-              *this).getScalarVal();
-      EmitAlignmentAssumption(Ret.getScalarVal(), ParamVal);
+      llvm::Value *AlignmentVal = CallArgs[AA->getParamIndex().getLLVMIndex()]
+                                      .getRValue(*this)
+                                      .getScalarVal();
+      EmitAlignmentAssumption(Ret.getScalarVal(), RetTy, Loc, AA->getLocation(),
+                              AlignmentVal);
     }
   }