[PATCH] D124221: Reimplement `__builtin_dump_struct` in Sema.

[Richard Smith - zygoloid via Phabricator via cfe-commits]

rsmith added inline comments.


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Comment at: clang/lib/Sema/SemaChecking.cpp:433
+
+  llvm::StringRef getFormatSpecifier(QualType T) {
+    if (auto *BT = T->getAs<BuiltinType>()) {
----------------
yihanaa wrote:
> rsmith wrote:
> > rsmith wrote:
> > > aaron.ballman wrote:
> > > > yihanaa wrote:
> > > > > I think this is better maintained in "clang/AST/FormatString.h". For example analyze_printf::PrintfSpecifier can get format specifier, what do you all think about?
> > > > +1 to this suggestion -- my hope is that we could generalize it more then as I notice there are missing specifiers for things like intmax_t, size_t, ptrdiff_t, _Decimal types, etc. Plus, that will hopefully make it easier for __builtin_dump_struct to benefit when new format specifiers are added, such as ones for printing a _BitInt.
> > > I am somewhat uncertain: every one of these is making arbitrary choices about how to format the value, so it's not clear to me that this is general logic rather than something specific to `__builtin_dump_struct`. For example, using `%f` rather than one of the myriad other ways of formatting `double` is somewhat arbitrary. Using `%s` for any `const char*` is *extremely* arbitrary and will be wrong and will cause crashes in some cases, but it may be the pragmatically correct choice for a dumping utility. A general-purpose mechanism would use `%p` for all kinds of pointer.
> > > 
> > > We could perhaps factor out the formatting for cases where there is a clear canonical default formatting, such as for integer types and probably `%p` for pointers, then call that from here with some special-casing, but without a second consumer for that functionality it's really not clear to me what form it should take.
> > I went ahead and did this, mostly to match concurrent changes to the old implementation. There are a few cases where our existing "guess a format specifier" logic does the wrong thing for dumping purposes, which I've explicitly handled -- things like wanting to dump a `char` / `signed char` / `unsigned char` member as a number rather than as a (potentially non-printable or whitespace) character.
>  When I was patching that old implementation, I found that for uint8_t, int8_t, Clang's existing "guess a format specifier" logic would treat the value as an integer, but for unsigned char, signed char, char types, it would Treat it as a character, please look at this example ( https://godbolt.org/z/ooqn4468T ), I guess this existing logic may have made some special judgment.
Yeah. I think in the case where we see some random call to `printf`, `%c` is probably the right thing to guess here, but it doesn't seem appropriate to me to use this in a dumping routine. If we could dump as `'x'` for printable characters and as `'\xAB'` for everything else, that'd be great, but `printf` can't do that itself and I'm not sure we want to be injecting calls to `isprint` or whatever to make that work. Dumping as an integer always seems like it's probably the least-bad option.

Somewhat related: I wonder if we should use `"\"%s\""` instead of simply `"%s"` when dumping a `const char*`. That's not ideal but probably clearer than the current dump output.


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  https://reviews.llvm.org/D124221/new/

https://reviews.llvm.org/D124221