From: Heiko Carstens > Sent: 29 August 2023 08:49 > > On Mon, Aug 28, 2023 at 03:51:00PM -0700, Nick Desaulniers wrote: > > On Mon, Aug 28, 2023 at 05:18:37PM +0000, David Laight wrote: > > > From: Heiko Carstens > > > > Sent: 28 August 2023 16:32 > > > > if (strlen(uid.vduit) > 0) > > > > > > Does the compiler know enough to optimise that brain-dead test? > > > > > > > For the purposes of skipping diagnostics, no; clang performs semantic > > analysis BEFORE optimization (which is handled by LLVM). As such, clang > > will produce diagnostics on dead code. > > > > Partly because LLVM isn't very ergonomic at emitting diagnostics from > > the backend, partly because Clang code owner and developers don't want > > clang to emit diagnostics dependent on optimization level. > > > > I disagree with my compatriots, and you can read more thoughts here: > > https://discourse.llvm.org/t/rfc-improving-clangs-middle-and-back-end- > diagnostics/69261?u=nickdesaulniers > > Maybe I misunderstand what you write above, however clang (latest+greatest) > does indeed optimize the strlen() away and generates code which only tests > if uid.vduit[0] is zero or not. > > Unlike gcc, which does not optimize this away and which uses the strlen() > inline assembly provided via string.h... And, if -ffreestanding is set (as in some kernel builds), the compiler can't assume what strlen() does. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
