[llvm-dev] RFC: module flag for hosted mode

[Duncan P. N. Exon Smith via llvm-dev]

+Eric and Akira (for thoughts on module flags)

> On 2016-Sep-16, at 12:47, Mehdi Amini <mehdi.amini at apple.com> wrote:
> 
>> On Sep 16, 2016, at 12:30 PM, Peter Collingbourne <peter at pcc.me.uk> wrote:
>> 
>> In PR30403 we've been discussing how to encode -ffreestanding when using LTO. This bit is currently dropped during LTO because its only representation is in the TargetLibraryInfo created by clang (http://llvm-cs.pcc.me.uk/tools/clang/lib/CodeGen/BackendUtil.cpp#258).
>> 
>> The proposal is to introduce a module flag that we set in any translation unit compiled in hosted (i.e. -fno-freestanding) mode. At LTO time, if the combined module has this flag (i.e. if any of the inputs have this flag), we compile in hosted mode. This means that if we combine freestanding and hosted modules, the entire resulting module will be compiled in hosted mode.
>> 
>> The justification for this behaviour (per Duncan) is that hosted/freestanding is a property of the linkage environment, and if the standard library is claimed to be available for any one translation unit in the linkage unit, it should be available for every other translation unit in the linkage unit.

(Tangent, but related: I'm also wondering if this logic could/should be used to encode other TLI flags, such as -fveclib=<lib>.  I.e., if one translation unit has -fveclib=Accelerate, does that imply that we can use that for the entire linkage unit?  If there's a conflict, such as -fveclib=SVML in one and -fveclib=Accelerate in another, can we safely pick the first one arbitrarily?)

>> One question that arises is how to handle old modules which were compiled in hosted mode and lack the hosted module flag. With the above scheme, LTO would run in freestanding mode if there are no contemporaneous modules. I think this is probably fine, since (1) I'd normally expect there to be at least one contemporaneous module (i.e. the main program, as opposed to old modules belonging to a prebuilt library) and (2) the loop idiom recognizer has already been run over these modules at compile time, so even if all modules are old there's unlikely to be a huge perf regression.
> 
> As an alternative view: since we didn’t support -ffreestanding in LTO mode before, we should be able to just auto-upgrade these bitcode to the -fnofreestanding version.
> 
> That said, as you mentioned it is probably not the common case and I don’t expect us to hit this in practice.

I completely agree with Mehdi here.  We could auto-upgrade to -fno-freestanding, but it's also not important.