aaron.ballman added inline comments.
================ Comment at: test/Sema/format-strings-bitfield-promotion.c:1 +// RUN: %clang_cc1 -triple x86_64-unknown-unknown -fsyntax-only -verify %s + ---------------- ebevhan wrote: > ebevhan wrote: > > aaron.ballman wrote: > > > aaron.ballman wrote: > > > > Running your test through GCC looks like the behavior matches here for > > > > C; can you also add a C++ test that demonstrates the behavior does not > > > > change? > > > > > > > > https://godbolt.org/z/zRYDMG > > > Strangely, the above godbolt link dropped the output windows, here's a > > > different link that shows the behavioral differences between C and C++ > > > mode in GCC: https://godbolt.org/z/R3zRHe > > Hmm, I'll have a look at this. > That gcc godbolt is a bit odd. The type of the bitfield expression in the C++ > example is `long` and not `int`, but in Clang, it's clearly being converted. > If I change the example a bit, we get this warning: > ``` > <source>:11:12: warning: format '%d' expects argument of type 'int', but > argument 2 has type 'long int' [-Wformat=] > 11 | printf("%d", bf.a); // expected-warning {{format specifies type > 'long' but the argument has type 'int'}} > | ~^ ~~~~ > | | | > | int long int > ``` > But in Clang, we get a cast to `int`: > ``` > | `-ImplicitCastExpr 0xd190748 <col:17, col:20> 'int' <IntegralCast> > | `-ImplicitCastExpr 0xd190730 <col:17, col:20> 'long' <LValueToRValue> > | `-MemberExpr 0xd190618 <col:17, col:20> 'long' lvalue bitfield .a > 0xd18f790 > | `-DeclRefExpr 0xd1905f8 <col:17> 'struct bitfields':'bitfields' > lvalue Var 0xd18fa18 'bf' 'struct bitfields':'bitfields' > ``` > > So gcc and Clang are doing things differently here. > > The code in `isPromotableBitField` says: > ``` > // FIXME: C does not permit promotion of a 'long : 3' bitfield to int. > // We perform that promotion here to match GCC and C++. > ``` > but clearly gcc isn't doing this in the C++ case. The comments also mention > some things about gcc bugs that Clang does not follow, but that's in > reference to a C DR. C++ disallows the rank conversion from int to long as well. [conv.prom]p1 does not apply because `long int` has a higher rank than `int`, but [conv.prom]p5 allows the promotion if the range of values is identical between the two types. C makes this UB in several ways -- you can't have a bit-field whose type is something other than int, unsigned int, or _Bool (6.7.2.1p5) or promoting from types other than those (6.3.1.1p2), but otherwise matches the C++ behavior in terms of promotion (including the rank conversion). You may have to dig further into what Clang is doing, but I would guess that the diagnostics should be triggered in both C and C++ similarly. Ultimately, I'd like to see tests for cases where `sizeof(int) == sizeof(long)`, `sizeof(int) != sizeof(long)`, and variants for C and C++ of each. CHANGES SINCE LAST ACTION https://reviews.llvm.org/D51211/new/ https://reviews.llvm.org/D51211 _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
