Re: [Qemu-devel] [PATCH for-1.7] target-i386: Fix build by providing stub kvm_arch_get_supported_cpuid()

[Gleb Natapov]

On Tue, Nov 12, 2013 at 02:12:56PM +0100, Paolo Bonzini wrote:
> Il 12/11/2013 13:16, Peter Maydell ha scritto:
> > On 12 November 2013 12:09, Paolo Bonzini <address@hidden> wrote:
> >> Il 12/11/2013 12:07, Peter Maydell ha scritto:
> >>> For the compiler to eliminate this we are relying on:
> >>>  * dead-code elimination of code following a 'break'
> >>>    statement in a case block
> >>>  * constant-folding of "something || 1" to 1
> >>>  * the compiler having done enough reasoning to be
> >>>    sure that env is not NULL
> >>
> >> Yes, it's not trivial, but there are simpler ways to do it.
> >>
> >> For example there is no need to make sure that env is non-NULL, only to
> >> see that "something || 1" is never zero and thus "if (x) y;" is just
> >> "(void)x; y;".  This seems easier to me than DCE after "break" which
> >> clang is able to do.
> > 
> > You seem to be trying to reason about what the compiler
> > might choose to do or how it might be implemented internally.
> 
> I'm not reasoning about that in general (I was in the context of the
> message you quoted).
> 
> I'm saying it's *reasonable* to expect that "-O0" means "reduce compile
> time, make debugging produce expected results, and try (not too hard) to
> not break what works at -O2".  It's a simple QoI argument based on the
> fact that people *will* switch back and forth between -O2 and -O0.  Of
> course not everything can be kept to work, since the compilers do pretty
> surprising optimizations (not counting the ones that break your code of
> course...).  But I think a limited amount of dead code elimination
> *should* be expected because most people are now preferring "if" to
> "#ifdef" for compiling out code.
> 
> If -O0 does not do that, let's move debug builds to -O1.
> 
Why not enable dce with -fdce?

--
                        Gleb.