On Tue, Aug 29, 2023 at 1:32 AM David Laight <David.Laight@xxxxxxxxxx> wrote: > > 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. Exactly. But triple checking if -ffreestanding is being used in arch/s390/ I only see: arch/s390/purgatory/Makefile 26:KBUILD_CFLAGS += -fno-zero-initialized-in-bss -fno-builtin -ffreestanding arch/s390/Makefile 28:KBUILD_CFLAGS_DECOMPRESSOR += -ffreestanding --- So I don't think -ffreestanding is at play here. -- Thanks, ~Nick Desaulniers
