>From their Discord server in reply to a question about whether such a patch would be upstreamed: "I suspect this only works in gcc -O0 because of its AST-level "fold", which clang intentionally doesn't implement. So probably not."
I hope that's enough information to resolve this patch. If any of you need anything else, please let me know. On Tue, Nov 21, 2023 at 12:28 PM Dan Hoffman <dhoff...@gmail.com> wrote: > > I'm writing a patch to clang's constant folding to address this case > (doesn't seem too difficult). I'll either follow up with a link to > some submissions I've made or a bug report on the project describing > the issue. > > > > On Tue, Nov 21, 2023 at 10:15 AM Eric Blake <ebl...@redhat.com> wrote: > > > > On Mon, Nov 20, 2023 at 11:20:52AM +0100, Philippe Mathieu-Daudé wrote: > > > (Cc'ing Eric) > > > > > > On 20/11/23 10:28, Michael S. Tsirkin wrote: > > > > On Sun, Nov 19, 2023 at 07:34:58PM -0600, Dan Hoffman wrote: > > > > > As far as I can tell, yes. Any optimization level above O0 does not > > > > > have this > > > > > issue (on this version of Clang, at least) > > > > > > > > Aha, this is with -O0. That makes sense. > > > > > > But then, why the other cases aren't problematic? > > > > > > $ git grep -E ' (&&|\|\|) !?kvm_enabled' > > > hw/arm/boot.c:1228: assert(!(info->secure_board_setup && > > > kvm_enabled())); > > > > This one's obvious; no kvm_*() calls in the assert. > > > > > hw/i386/microvm.c:270: (mms->rtc == ON_OFF_AUTO_AUTO && > > > !kvm_enabled())) { > > > > Ones like this require reading context to see whether the if() block > > guarded any kvm_*() calls for the linker to elide - but still a fairly > > easy audit. > > > > > > > > > > > > I'm surprised the order of conditions matters for code elision... > > > > > > > > > > > Signed-off-by: Daniel Hoffman <dhoff...@gmail.com> > > > > > > --- > > > > > > hw/i386/x86.c | 15 ++++++++++++--- > > > > > > 1 file changed, 12 insertions(+), 3 deletions(-) > > > > > > > > > > > > diff --git a/hw/i386/x86.c b/hw/i386/x86.c > > > > > > index b3d054889bb..2b6291ad8d5 100644 > > > > > > --- a/hw/i386/x86.c > > > > > > +++ b/hw/i386/x86.c > > > > > > @@ -131,8 +131,12 @@ void x86_cpus_init(X86MachineState > > > > > *x86ms, int > > > > > default_cpu_version) > > > > > > /* > > > > > > * Can we support APIC ID 255 or higher? With KVM, that > > > > > requires > > > > > > * both in-kernel lapic and X2APIC userspace API. > > > > > > + * > > > > > > + * kvm_enabled() must go first to ensure that kvm_* > > > > > references are > > > > > > + * not emitted for the linker to consume (kvm_enabled() is > > > > > > + * a literal `0` in configurations where kvm_* aren't > > > > > defined) > > > > > > */ > > > > > > - if (x86ms->apic_id_limit > 255 && kvm_enabled() && > > > > > > + if (kvm_enabled() && x86ms->apic_id_limit > 255 && > > > > > > (!kvm_irqchip_in_kernel() || !kvm_enable_x2apic())) { > > > > Indeed, if clang -O0 treats 'if (cond1 && 0 && cond2)' differently > > than 'if (0 && cond1 && cond2)' for purposes of eliding the code for > > cond2, that seems like a blatant missed optimization that we should be > > reporting to the clang folks. > > > > While this patch may solve the immediate issue, it does not scale - if > > we ever have two separate guards that can both be independently > > hard-coded to 0 based on configure-time decisions, but which are both > > used as guards in the same expression, it will become impossible to > > avoid a '(cond1 && 0 && cond2)' scenario across all 4 possible > > configurations of those two guards. > > > > I have no objection to the patch, but it would be nice if the commit > > message could point to a clang bug report, if one has been filed. > > > > -- > > Eric Blake, Principal Software Engineer > > Red Hat, Inc. > > Virtualization: qemu.org | libguestfs.org > >