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

clang/include/clang/Basic/BuiltinsX86.def

@@ -2041,6 +2041,8 @@ TARGET_HEADER_BUILTIN(__emulu, "ULLiUiUi", "nh", "intrin.h", ALL_MS_LANGUAGES, "
 
 TARGET_HEADER_BUILTIN(_AddressOfReturnAddress, "v*", "nh", "intrin.h", ALL_MS_LANGUAGES, "")
 
+TARGET_HEADER_BUILTIN(__stosb, "vUc*Ucz", "nh", "intrin.h", ALL_MS_LANGUAGES, "")
+
 #undef BUILTIN
 #undef TARGET_BUILTIN
 #undef TARGET_HEADER_BUILTIN

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);
+  }
   }
 }
 

clang/lib/Headers/intrin.h

@@ -1024,11 +1024,6 @@ __movsw(unsigned short *__dst, unsigned short const *__src, size_t __n) {
                         : "%edi", "%esi", "%ecx");
 }
 static __inline__ void __DEFAULT_FN_ATTRS
-__stosb(unsigned char *__dst, unsigned char __x, size_t __n) {
-  __asm__("rep stosb" : : "D"(__dst), "a"(__x), "c"(__n)
-                        : "%edi", "%ecx");
-}
-static __inline__ void __DEFAULT_FN_ATTRS
 __stosd(unsigned long *__dst, unsigned long __x, size_t __n) {
   __asm__("rep stosl" : : "D"(__dst), "a"(__x), "c"(__n)
                         : "%edi", "%ecx");

clang/test/CodeGen/ms-intrinsics.c

@@ -14,6 +14,22 @@ typedef __SIZE_TYPE__ size_t;
 
 #include <intrin.h>
 
+#if defined(__i386__) || defined(__x86_64__)
+void test__stosb(unsigned char *Dest, unsigned char Data, size_t Count) {
+  return __stosb(Dest, Data, Count);
+}
+
+// CHECK-I386: define{{.*}}void @test__stosb
+// CHECK-I386:   tail call void @llvm.memset.p0i8.i32(i8* %Dest, i8 %Data, i32 %Count, i32 1, i1 true)
+// CHECK-I386:   ret void
+// CHECK-I386: }
+
+// CHECK-X64: define{{.*}}void @test__stosb
+// CHECK-X64:   tail call void @llvm.memset.p0i8.i64(i8* %Dest, i8 %Data, i64 %Count, i32 1, i1 true)
+// CHECK-X64:   ret void
+// CHECK-X64: }
+#endif
+
 void *test_ReturnAddress() {
   return _ReturnAddress();
 }

clang/test/Headers/ms-intrin.cpp

@@ -38,7 +38,6 @@ void f() {
   __movsd(0, 0, 0);
   __movsw(0, 0, 0);
 
-  __stosb(0, 0, 0);
   __stosd(0, 0, 0);
   __stosw(0, 0, 0);