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[clang][Interp] Zero-init remaining string literal elements #66862

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Merged
merged 1 commit into from
Sep 30, 2023
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[clang][Interp] Zero-init remaining string literal elements #66862

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21 changes: 19 additions & 2 deletions clang/lib/AST/Interp/ByteCodeExprGen.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -858,8 +858,8 @@ bool ByteCodeExprGen<Emitter>::VisitStringLiteral(const StringLiteral *E) {

// If the initializer string is too long, a diagnostic has already been
// emitted. Read only the array length from the string literal.
unsigned N =
std::min(unsigned(CAT->getSize().getZExtValue()), E->getLength());
unsigned ArraySize = CAT->getSize().getZExtValue();
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So getZExtValue() returns uint64_t yet we are using unsigned. I know this is already an existing problem that we make varying assumptions in different places but I am wondering what the long-term plan is for this this. Forgive me if I have asked this question in another context and I just forgot.

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It's a bit tricky since StringLiteral::getLength() returns unsigned again, so we can't use the extra bytes anyway and need to cast to unsigned for the std::min().

unsigned N = std::min(ArraySize, E->getLength());
size_t CharWidth = E->getCharByteWidth();

for (unsigned I = 0; I != N; ++I) {
Expand All @@ -878,6 +878,23 @@ bool ByteCodeExprGen<Emitter>::VisitStringLiteral(const StringLiteral *E) {
llvm_unreachable("unsupported character width");
}
}

// Fill up the rest of the char array with NUL bytes.
for (unsigned I = N; I != ArraySize; ++I) {
if (CharWidth == 1) {
this->emitConstSint8(0, E);
this->emitInitElemSint8(I, E);
} else if (CharWidth == 2) {
this->emitConstUint16(0, E);
this->emitInitElemUint16(I, E);
} else if (CharWidth == 4) {
this->emitConstUint32(0, E);
this->emitInitElemUint32(I, E);
} else {
llvm_unreachable("unsupported character width");
}
}

return true;
}

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21 changes: 21 additions & 0 deletions clang/test/AST/Interp/arrays.cpp
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Expand Up @@ -371,3 +371,24 @@ namespace ArrayInitLoop {
// expected-note {{15 == 6}}
#endif
}

namespace StringZeroFill {
struct A {
char c[6];
};
constexpr A a = { "abc" };
static_assert(a.c[0] == 'a', "");
static_assert(a.c[1] == 'b', "");
static_assert(a.c[2] == 'c', "");
static_assert(a.c[3] == '\0', "");
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Shouldn't we just test all the bytes? It is small enough.

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Should we also test wide string literals etc?

static_assert(a.c[4] == '\0', "");
static_assert(a.c[5] == '\0', "");

constexpr char b[6] = "foo";
static_assert(b[0] == 'f', "");
static_assert(b[1] == 'o', "");
static_assert(b[2] == 'o', "");
static_assert(b[3] == '\0', "");
static_assert(b[4] == '\0', "");
static_assert(b[5] == '\0', "");
}