Re: [PATCH] hw/i386: fix short-circuit logic with non-optimizing builds

[Michael S. Tsirkin]

On Sun, Nov 19, 2023 at 02:19:25PM -0600, Dan Hoffman wrote:
> Clang 16.0.6
> 
> I can re-submit with the compiler and version if that helps.

Worth mentioning this and the flags used I think.

> On Sun, Nov 19, 2023 at 2:02 PM Michael S. Tsirkin <mst@redhat.com> wrote:
> >
> > On Sun, Nov 19, 2023 at 11:03:54AM -0600, Dan Hoffman wrote:
> > > On Sun, Nov 19, 2023 at 1:23 AM Michael S. Tsirkin <mst@redhat.com> wrote:
> > > >
> > > > On Sat, Nov 18, 2023 at 10:25:31AM -0800, Daniel Hoffman wrote:
> > > > > `kvm_enabled()` is compiled down to `0` and short-circuit logic is
> > > > > used to remmove references to undefined symbols at the compile stage.
> > > > > Some build configurations with some compilers don't attempt to
> > > > > simplify this logic down in some cases (the pattern appears to be
> > > > > that the literal false must be the first term) and this was causing
> > > > > some builds to emit references to undefined symbols.
> > > > >
> > > > > Signed-off-by: Daniel Hoffman <dhoff749@gmail.com>
> > > >
> > > > Could we add a bit more detail here? Will help make sure
> > > > this does not break again in the future.
> > >
> > > The configuration script was ran as such:  ../configure
> > > --without-default-features --target-list=x86_64-softmmu,i386-softmmu
> > > --enable-debug --enable-tcg-interpreter --enable-debug-tcg
> > > --enable-debug-mutex
> > >
> > > I'm pretty sure the only relevant flags here are
> > > --without-default-features, --target-list including x86_64-softmmu and
> > > --enable-debug
> > >
> > > The only error I see is this: [...]/hw/i386/x86.c:422:(.text+0x1004):
> > > undefined reference to `kvm_hv_vpindex_settable' (the other
> > > kvm_enabled() was moved for the sake of consistency). My compiler is
> > > clang (16.0.6).
> > >
> > > I haven't looked into the heuristics or logic for how the compile-time
> > > short-circuit logic works, but I assumed only the first parameter is
> > > "guaranteed" to be checked for a literal false (guaranteed is in
> > > quotes because that's just how clang works, not because it's a feature
> > > of the language IIRC).
> > >
> > > This pattern relies on somes subtle behavior with the compiler, so my
> > > suggestion going forward would be to not rely on code optimizations
> > > removing undefined references based on short-circuit logic (instead
> > > have some configuration macro defined that disables all relevant
> > > code). I'm a new contributor, so I submitted the minimum to make it
> > > work on my machine.
> > >
> > > If you have any other questions, please let me know.
> > >
> > > Thanks!
> >
> > which compiler is this?
> >
> > --
> > MST
> >