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[clang] Add comment about misleading alloc_size argument names (#134899)

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Attr.td names the first alloc_size argument "ElemSizeParam" and the
second optional argument "NumElemsParam"; but the semantics of how the
two-argument version is used in practice is the opposite of that.

glibc declares calloc() like this, so the second alloc_size argument is
the element size:
```
extern void *calloc (size_t __nmemb, size_t __size)
__THROW __attribute_malloc__ __attribute_alloc_size__ ((1, 2)) __wur;
```

The Linux kernel declares array allocation functions like
`kmalloc_array_noprof()` the same way.

Add a comment explaining that the names used inside clang are
misleading.
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Jann 2025-04-08 22:18:24 +02:00 committed by GitHub
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7
clang/include/clang/Basic/Attr.td
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@ -1709,6 +1709,13 @@ def EmptyBases : InheritableAttr, TargetSpecificAttr<TargetMicrosoftCXXABI> {
def AllocSize : InheritableAttr {
let Spellings = [GCC<"alloc_size">];
let Subjects = SubjectList<[HasFunctionProto]>;
// The parameter names here are a bit misleading.
// When used with a single argument, the first argument is obviously the
// allocation size; but when used with two arguments, the first argument is
// usually the number of elements, while the second argument is usually the
// element size - the reverse of how they are named here.
// The documentation of both GCC and clang does not describe any semantic
// difference between the first and second argument.
let Args = [ParamIdxArgument<"ElemSizeParam">,
ParamIdxArgument<"NumElemsParam", /*opt*/ 1>];
let TemplateDependent = 1;