From: Heiko Carstens

Sent: 29 August 2023 08:49

On Mon, Aug 28, 2023 at 03:51:00PM -0700, Nick Desaulniers wrote:

quoted

On Mon, Aug 28, 2023 at 05:18:37PM +0000, David Laight wrote:

quoted

From: Heiko Carstens

quoted

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

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