Add __warn_memset_zero_len builtin as a workaround for glibc issue

[serge-sans-paille]

Summary

Glibc issue: https://sourceware.org/bugzilla/show_bug.cgi?id=25399

The fix consist in considering the missing function as a builtin lowered to a nop.

See https://reviews.llvm.org/D71082 for the original discussion that led to that patch.

[serge-sans-paille]

@akhuang can you confirm this fixes your issue? I've tested locally and it works fine on my side.

[efriedma]

This is very hacky, but it might be the least-bad alternative. I mean, we could change D71082 so it doesn't allow system headers to define memset, but that seems worse.

Please add a testcase.

[merge_guards_bot]

Unit tests: pass. 61933 tests passed, 0 failed and 783 were skipped.

clang-tidy: unknown.

clang-format: pass.

Build artifacts: diff.json, clang-format.patch, CMakeCache.txt, console-log.txt, test-results.xml

[serge-sans-paille]

Add test case

[serge-sans-paille]

In D72869#1824951, @efriedma wrote:

This is very hacky,

Yeah, I know...

but it might be the least-bad alternative. I mean, we could change D71082 so it doesn't allow system headers to define memset, but that seems worse.

Definitively

Please add a testcase.

Done

[efriedma]

LGTM

[akhuang]

In D72869#1824927, @serge-sans-paille wrote:

@akhuang can you confirm this fixes your issue? I've tested locally and it works fine on my side.

Yes, this fixes the __warn_memset_zero_len issue, thanks!

[merge_guards_bot]

Unit tests: pass. 61934 tests passed, 0 failed and 783 were skipped.

clang-tidy: unknown.

clang-format: fail. Please format your changes with clang-format by running git-clang-format HEAD^ or applying this patch.

Build artifacts: diff.json, clang-format.patch, CMakeCache.txt, console-log.txt, test-results.xml

[joerg]

Can this be restricted to Linux?

[efriedma]

We currently don't have any mechanism for restricting builtins to specific operating systems. I guess we could add one, but this change doesn't seem like a compelling argument to add that capability.

clang/include/clang/Basic/Builtins.def

 BUILTIN(__sync_fetch_and_umin, "UiUiD*Ui", "n")
 BUILTIN(__sync_fetch_and_umax, "UiUiD*Ui", "n")
 
 // Random libc builtins.
 BUILTIN(__builtin_abort, "v", "Fnr")
 BUILTIN(__builtin_index, "c*cC*i", "Fn")
 BUILTIN(__builtin_rindex, "c*cC*i", "Fn")
 
+// ignored glibc builtin, see https://sourceware.org/bugzilla/show_bug.cgi?id=25399
+BUILTIN(__warn_memset_zero_len, "v", "nU")
+
 // Microsoft builtins.  These are only active with -fms-extensions.
 LANGBUILTIN(_alloca,          "v*z", "n", ALL_MS_LANGUAGES)
 LANGBUILTIN(__annotation,     "wC*.","n", ALL_MS_LANGUAGES)
 LANGBUILTIN(__assume,         "vb",  "n", ALL_MS_LANGUAGES)
 LANGBUILTIN(_bittest,                "UcNiC*Ni", "n", ALL_MS_LANGUAGES)
 LANGBUILTIN(_bittestandcomplement,   "UcNi*Ni", "n", ALL_MS_LANGUAGES)
 LANGBUILTIN(_bittestandreset,        "UcNi*Ni", "n", ALL_MS_LANGUAGES)
 LANGBUILTIN(_bittestandset,          "UcNi*Ni", "n", ALL_MS_LANGUAGES)

clang/lib/CodeGen/CGBuiltin.cpp

 
   case Builtin::BI__builtin_signbit:
   case Builtin::BI__builtin_signbitf:
   case Builtin::BI__builtin_signbitl: {
     return RValue::get(
         Builder.CreateZExt(EmitSignBit(*this, EmitScalarExpr(E->getArg(0))),
                            ConvertType(E->getType())));
   }
+  case Builtin::BI__warn_memset_zero_len:
+    return RValue::getIgnored();
   case Builtin::BI__annotation: {
     // Re-encode each wide string to UTF8 and make an MDString.
     SmallVector<Metadata *, 1> Strings;
     for (const Expr *Arg : E->arguments()) {
       const auto *Str = cast<StringLiteral>(Arg->IgnoreParenCasts());
       assert(Str->getCharByteWidth() == 2);
       StringRef WideBytes = Str->getBytes();
       std::string StrUtf8;

clang/test/CodeGen/builtins.c

 // CHECK-LABEL: define void @test_builtin_launder
 void test_builtin_launder(int *p) {
   // CHECK: [[TMP:%.*]] = load i32*,
   // CHECK-NOT: @llvm.launder
   // CHECK: store i32* [[TMP]],
   int *d = __builtin_launder(p);
 }
 
+// __warn_memset_zero_len should be NOP, see https://sourceware.org/bugzilla/show_bug.cgi?id=25399
+// CHECK-LABEL: define void @test___warn_memset_zero_len
+void test___warn_memset_zero_len() {
+  // CHECK-NOT: @__warn_memset_zero_len
+  __warn_memset_zero_len();
+}
+
 // Behavior of __builtin_os_log differs between platforms, so only test on X86
 #ifdef __x86_64__
 
 // CHECK-LABEL: define void @test_builtin_os_log
 // CHECK: (i8* %[[BUF:.*]], i32 %[[I:.*]], i8* %[[DATA:.*]])
 void test_builtin_os_log(void *buf, int i, const char *data) {
   volatile int len;
   // CHECK: %[[BUF_ADDR:.*]] = alloca i8*, align 8