jasonliu added inline comments.

Comment at: clang/test/CodeGen/ppc32-struct-return.c:53
+// AIX-SVR4: fatal error: error in backend: -msvr4-struct-return not supported 
on AIX
jasonliu wrote:
> sfertile wrote:
> > jasonliu wrote:
> > > If certain front end option is not supported on certain target, I think 
> > > it makes more sense to have a standard diagnostic in the driver 
> > > component, instead of "crash" in the backend. 
> > > i.e. What if we specify this option on a Windows machine? Maybe we should 
> > > pursue the same behavior. 
> > Its not that we don't intend to support the option on AIX, but that support 
> > for the option takes further verification on AIX.  Since there is a 
> > difference how  AIX justifies subregister sized values in its argument 
> > passing, we can't just assume that this option will pack the return values 
> > the same way on AIX and Linux. 
> > 
> > The focus of this patch was originally to enable and verify the proper IR 
> > generation of va-arg/va-lis/va-start, however the scope of the patch has 
> > kept growing. If there are codegen differences in packing the return 
> > register with the svr4-return option enabled it will grow this patch once 
> > again. The fatal error lets us limit the scope of this patch, while not 
> > silently emiting bad codegen if there is a difference in how gcc 
> > initializes the return  registers. If during verification we decide we 
> > don't want to support the option on AIX, then adopting  a standard 
> > diagnostic in the driver component becomes the appropriate behavior. 
> It wasn't clear for me from the code that this is not a permanent thing.  In 
> that case, does it make sense to leave a TODO here and say we need to 
> re-evaluate this in the future to decide if we want to support it or not on 
> AIX?
> On another note, I'm not sure if this is really needed on AIX target though. 
> But I guess we could discuss it down the road. 
Just to avoid ambiguity, I meant I'm not sure if we really need this *option* 
on AIX.

  rG LLVM Github Monorepo



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