[clang][ExprConst] Handle 0 type size in builtin_memcpy etc.

[tbaeder]

Summary

Fixes https://github.com/llvm/llvm-project/issues/44176

[tbaeder]

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[shafik]

clang/lib/AST/ExprConstant.cpp

9548

I think we should issue a diagnostic, we don't have any indication that this is failing, let alone because the destination object size is zero.

[tbaeder]

clang/lib/AST/ExprConstant.cpp

9548

You mean in addition to the one we are emitting?

[tbaeder]

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[tbaeder]

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[tbaeder]

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[tbaeder]

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[tbaeder]

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[tbaeder]

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[aaron.ballman]

clang/test/Sema/builtin-memcpy.c

5–9

The only other test I'd like to see is one like:

constexpr int b() {
  struct {      } a[10];
  __builtin_memcpy(&a[2], a, 2); // UB here should be caught, right?
  return 0;
}

static_assert(b() == 0, "");

(if that's follow-up work, that's fine too, just leave a test with a FIXME comment.)

[tbaeder]

clang/test/Sema/builtin-memcpy.c

5–9

Nope, clang accepts that and GCC rejects the function when calling.

[aaron.ballman]

LGTM with a fixme comment added.

clang/test/Sema/builtin-memcpy.c

5–9

Amusingly, Clang and EDG (GNU mode) accept and GCC rejects. GCC is correct, so it's worth adding a FIXME to the case below so we know this isn't intentional behavior.

clang/lib/AST/ExprConstant.cpp

case Builtin::BI__builtin_wmemmove: {
     }
     if (!T.isTriviallyCopyableType(Info.Ctx)) {
       Info.FFDiag(E, diag::note_constexpr_memcpy_nontrivial) << Move << T;
       return false;
     }
 
     // Figure out how many T's we're copying.
     uint64_t TSize = Info.Ctx.getTypeSizeInChars(T).getQuantity();
+    if (TSize == 0)
+      return false;
     if (!WChar) {
       uint64_t Remainder;
       llvm::APInt OrigN = N;
       llvm::APInt::udivrem(OrigN, TSize, N, Remainder);
       if (Remainder) {
         Info.FFDiag(E, diag::note_constexpr_memcpy_unsupported)
             << Move << WChar << 0 << T << toString(OrigN, 10, /*Signed*/false)
             << (unsigned)TSize;

clang/test/Sema/builtin-memcpy.c

+// RUN: %clang_cc1 %s -fsyntax-only -verify=c
+// RUN: %clang_cc1 -x c++ %s -fsyntax-only -verify=cxx
+
+// cxx-no-diagnostics
+
+
+/// Zero-sized structs should not crash.
+int b() {
+  struct {      } a[10];
+  __builtin_memcpy(&a[2], a, 2); // c-warning {{buffer has size 0, but size argument is 2}}
+  return 0;
+}
+
+#ifdef __cplusplus
+// FIXME: This is UB and GCC correctly diagnoses it. Clang should do the same.
+constexpr int b2() {
+  struct {      } a[10];
+  __builtin_memcpy(&a[2], a, 2);
+  return 0;
+}
+static_assert(b2() == 0, "");
+#endif