[PATCH] D137488: [clang][Interp] Array initialization via string literal

[Tom Honermann via Phabricator via cfe-commits]

tahonermann added inline comments.


================
Comment at: clang/lib/AST/Interp/ByteCodeExprGen.cpp:1098-1099
+
+    unsigned N = SL->getLength();
+    for (size_t I = 0; I != NumElems; ++I) {
+      uint32_t CodePoint = I < N ? SL->getCodeUnit(I) : 0;
----------------
tahonermann wrote:
> Aren't `N` and `NumElems` guaranteed to have the same value here? Both are derived from `SL`. The code seems to be written with the expectation that `NumElems` corresponds to the number of elements to be iniitialized in the target array.
I see the change to now use the minimum of `SL->getLength()` and `CAT->getSize().getZExtValue()`. Based on https://godbolt.org/z/5sTWExTac this looks to be unnecessary. When a string literal is used as an array initializer, it appears that the type of the string literal is adjusted to match the size of the array being initialized. I suggest using only `CAT->getSize().getZExtValue()` and adding a comment that this code depends on that adjustment.


================
Comment at: clang/test/AST/Interp/literals.cpp:354-359
+  constexpr char foo[12] = "abc";
+  static_assert(foo[0] == 'a', "");
+  static_assert(foo[1] == 'b', "");
+  static_assert(foo[2] == 'c', "");
+  static_assert(foo[3] == 0, "");
+  static_assert(foo[11] == 0, "");
----------------
tahonermann wrote:
> aaron.ballman wrote:
> > I'd like to see some tests for the other encodings, as well as a test with embedded null characters in the literal.
> > 
> > Testing a string literal that's longer than the array is something we should think about. That code is ill-formed in C++, so I don't think we can add a test for it yet, but it's only a warning in C.
> I agree with Aaron's requests. Please also extend the test to include a `char` element that would be negative for a `signed` 8-bit `char`. Something like:
>   constexpr char foo[12] = "abc\xff";
>   ...
>   #if defined(__CHAR_UNSIGNED__) || __CHAR_BIT__ > 8
>   static_assert(foo[3] == 255, "");
>   #else
>   static_assert(foo[3] == -1, "");
>   #endif
> 
> A couple of more tests to add:
> - One where the string literal has the same length (including the implicit terminator) as the array; to ensure that the implicit terminator is properly accounted for.
> - One where the target array size is deduced from the string literal; to ensure there are no dependencies on an explicit array size.
These cases all look to have been added now. Thank you!


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  https://reviews.llvm.org/D137488/new/

https://reviews.llvm.org/D137488