> On 9 Mar 2026, at 10:20, Jan Beulich <[email protected]> wrote: > > On 09.03.2026 10:49, Edwin Torok wrote: >>> On 9 Mar 2026, at 07:47, Jan Beulich <[email protected]> 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.
It avoids the duplicate symbol error with version of Clang that I have, so I have sent a V4 that uses -Og and the updated comments. Was a bit worried this might not work with old GCCs, but AFAICT Og is supported since 4.8, and the minimum is 5.1. Best regards, —Edwin > >>> 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
