[CodeGen] Provide an appropriate alignment for dynamic allocas

GCC documents __builtin_alloca as aligning the storage to at least
__BIGGEST_ALIGNMENT__.

MSVC documents essentially the same for the x64 ABI:
https://msdn.microsoft.com/en-us/library/x9sx5da1.aspx

The 32-bit ABI follows the same rule: it emits a call to _alloca_probe_16

Differential Revision: https://reviews.llvm.org/D24378

llvm-svn: 285316

1 files changed, 7 insertions, 1 deletions

diff --git a/clang/lib/CodeGen/CGBuiltin.cpp b/clang/lib/CodeGen/CGBuiltin.cpp
index 5bbc9ea5341..c73a6e1b061 100644
--- a/clang/lib/CodeGen/CGBuiltin.cpp
+++ b/clang/lib/CodeGen/CGBuiltin.cpp
@@ -1139,7 +1139,13 @@ RValue CodeGenFunction::EmitBuiltinExpr(const FunctionDecl *FD,
   case Builtin::BI_alloca:
   case Builtin::BI__builtin_alloca: {
     Value *Size = EmitScalarExpr(E->getArg(0));
-    return RValue::get(Builder.CreateAlloca(Builder.getInt8Ty(), Size));
+    const TargetInfo &TI = getContext().getTargetInfo();
+    // The alignment of the alloca should correspond to __BIGGEST_ALIGNMENT__.
+    unsigned SuitableAlignmentInBytes =
+        TI.getSuitableAlign() / TI.getCharWidth();
+    AllocaInst *AI = Builder.CreateAlloca(Builder.getInt8Ty(), Size);
+    AI->setAlignment(SuitableAlignmentInBytes);
+    return RValue::get(AI);
   }
   case Builtin::BIbzero:
   case Builtin::BI__builtin_bzero: {