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 <pbonzini@xxxxxxxxxx> 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. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
