Implement __stosb intrinsic as a volatile memset

Summary: We need `__stosb` to be an intrinsic, because SecureZeroMemory function uses it without including intrin.h. Implementing it as a volatile memset is not consistent with MSDN specification, but it gives us target-independent IR while keeping the most important properties of `__stosb`.

Reviewers: rnk, hans, thakis, majnemer

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D25334

llvm-svn: 284253

1 files changed, 5 insertions, 0 deletions

diff --git a/clang/lib/CodeGen/CGBuiltin.cpp b/clang/lib/CodeGen/CGBuiltin.cpp
index bfec53ace22..8bb9a465864 100644
--- a/clang/lib/CodeGen/CGBuiltin.cpp
+++ b/clang/lib/CodeGen/CGBuiltin.cpp
@@ -7770,6 +7770,11 @@ Value *CodeGenFunction::EmitX86BuiltinExpr(unsigned BuiltinID,
     Value *F = CGM.getIntrinsic(Intrinsic::addressofreturnaddress);
     return Builder.CreateCall(F);
   }
+  case X86::BI__stosb: {
+    // We treat __stosb as a volatile memset - it may not generate "rep stosb"
+    // instruction, but it will create a memset that won't be optimized away.
+    return Builder.CreateMemSet(Ops[0], Ops[1], Ops[2], 1, true);
+  }
   }
 }