Re: [PATCH v3 1/1] tools/tests/x86_emulator: avoid duplicate symbol error with clang: use -O0

[Jan Beulich <jbeulich@xxxxxxxx>]

On 09.03.2026 10:49, Edwin Torok wrote:
>> On 9 Mar 2026, at 07:47, Jan Beulich <jbeulich@xxxxxxxx> wrote:
>> There's
>> one other question though: Isn't -O0 the default? Where would any other
>> optimization setting come from in HOSTCFLAGS? Ah, I see ./Config.mk does
>> this, for an unclear to me reason.
> 
> One possible reason I’d enable optimisations (even for non-performance 
> critical code) is to get better warnings.
> I looked this up, and I see it is actually documented in the GCC manual:
> "The effectiveness of some warnings depends on optimizations also being 
> enabled.
> For example, -Wsuggest-final-types is more effective with link-time 
> optimization. 
> Some other warnings may not be issued at all unless optimization is enabled.
> While optimization in general improves the efficacy of warnings about control 
> and data-flow problems,
> in some cases it may also cause false positives.”
> 
> In fact warnings rely on optimisations even more than I thought, didn’t know 
> about link time optimisations having an effect
> (from the example that warning is only relevant for C++ for now)
> 
> I can see why in general a compiler would decide to implement it that way (if 
> you run an analysis to get more accurate
> information for showing warnings then you might as well use it to emit better 
> code), but I wish that wasn’t the case. 
> E.g. you may want to use a low optimisation level to get better debugging, 
> without giving up on the better warnings.
> The only way to do that currently is to build it twice (or rely on a CI that 
> builds with different flags).
> 
> If you want to change it I’d suggest setting it at least to -Og, which is 
> recommended over O0:
> "In contrast to -O0, this enables -fvar-tracking-assignments and 
> -fvar-tracking which handle debug information in the prologue and epilogue of 
> functions better than -O0."

And did you check that -Og works for the purposes here? I'd indeed prefer to
use that, if we can.

>> Perhaps that would want mentioning
>> here then as well.
> 
> How about:
> 
> # The default HOSTCFLAGS from $(XEN_ROOT)/Config.mk would set
> # a non-zero optimisation level
> 
> I’d avoid mentioning -O2, in case that changes.

Of course.

Jan