[llvm-dev] Self-hosting bots noise

[David Blaikie via llvm-dev]

On Mon, Nov 9, 2015 at 10:27 AM, Reid Kleckner <rnk at google.com> wrote:

> On Mon, Nov 9, 2015 at 8:53 AM, James Y Knight via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> On Nov 9, 2015, at 11:42 AM, David Blaikie via llvm-dev <
>> llvm-dev at lists.llvm.org> wrote:
>>
>>
>>
>> On Mon, Nov 9, 2015 at 4:32 AM, Renato Golin <renato.golin at linaro.org>
>> wrote:
>>
>>> Hi David/Galina,
>>>
>>> FYI, I found one big reason why self-hosting bots fail long after the
>>> offending commit is tested: dirty builds.
>>>
>>> In a dirty build (no make clean), when a file X is changed that breaks
>>> file Y, the bot will only accuse the failure when file Y is changed,
>>> too, so that stage1 with modified X will compile file Y, and break.
>>>
>>
>> That sounds like a bug in the build system - do you have any examples of
>> this? We should fix them.
>>
>> The build should be reliable - any case where a clean build produces a
>> different result from a dirty build reduces/removes confidence in
>> incremental builds & hurts developer productivity (just like flakey tests,
>> etc).
>>
>>
>> I think he meant that the /second-stage/ build should always be done as a
>> clean build -- to make sure that all of the source gets rebuilt with the
>> newly-compiled compiler, instead of only rebuilding the modified sources
>> with the newly-built compiler.
>>
>> Build systems typically aren't expected to watch out for the compiler
>> itself changing out from under them.
>>
>
> +1, I know that my self-hosting Windows buildbot does not do a clean
> second stage build, and I've been meaning to fix that for some time.
>
> Renato, I would rather fix this in our buildbot configs. We should just
> make stage2 always be clean, and that will fix my builders as well as yours.
>

Yeah, sorry, a stage2 is always necessarily clean - because there's a new
compiler. ccache isn't even relevant/useful here, is it (I mean it should
be a no-op, if it hashes the compiler itself and all the compiler's
dependencies too - but I don't know if it does that?)? We want to actually
do the real work of compiling the code with the new compiler for
everything. Nothing should be a cache/build hit. (fancy build system like
Bazel could still get a cache hit on the link steps - if the compiler
produced identical object files, etc)
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