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Re: [RFC] New pragma exec_charset
On Fri, Oct 20, 2017 at 9:53 AM, Jakub Jelinek <jakub@redhat.com> wrote:
> On Fri, Oct 20, 2017 at 09:48:38AM +0200, Richard Biener wrote:
>> How does it work semantically to have different exec charsets? That is,
>> if "strings" flow from a region with one -fexec-charset setting to a region
>> with another one is that undefined behavior? Do we now require
>> external function declarations to be in the proper region (declared under
>> the appropriate exec charset flag)? This would mean that passing
>> the exec charset in effect as additional argument isn't a possibility.
>>
>> Or do we have to treat -fexec-charset similar to -frounding-math, that is,
>> we can't ever _interpret_ any string in the compiler? [unless -fexec-charset
>> is the same everywhere]
>>
>> I think the -frounding-math route is probably the easiest (and wisest
>> given the quite low test coverage we'll get) route. Thus, add a -fmixed-charset
>> flag and reject any exec-charset attribute/pragma if that flag is not set?
>> With LTO we could always add this and/or merge -fexec-charset flags
>> appropriately,
>> injecting -fmixed-charset in case TUs use different settings.
>
> It wouldn't have to be an option, simply mark in cfun all functions that
> have more than one exec charset and give up on all optimizations/warnings
> that require to read the characters and merge that unknown exec_charset
> flag during inlining etc. Though, that might still not be enough, e.g.
> the whole function might have one exec charset, but a global const char []
> variable might have another one and during optimization we might be looking
> at that. So perhaps it would need to be a per-TU flag merged during LTO.
There's also IPA flow of strings between functions so unless mixing
exec charsets
invokes undefined behavior I can't see how a per-function flag would help.
But yes, if we can reliably detect whether multiple exec charsets are
used in a TU
we can make this a flag that doesn't have to be set by the user. But that means
the pragma probably _always_ forces that flag given we have that
forced pre-included
file on some targest and the pragma token would occur after that...
Richard.
> Jakub