On 11/29/18 8:34 AM, Martin Sebor wrote: > > GCC does disallow decreasing the function alignment -- but only > for functions that were already declared with a more restrictive > one. Like this: > > __attribute__ ((aligned (4))) void f (void); > > __attribute__ ((aligned (2))) void f (void); > > warning: ignoring attribute ‘aligned (2)’ because it conflicts with > attribute ‘aligned (4)’ > > It doesn't warn about going from the default (say 16 on i86) > to something smaller, and it honors that lower alignment up > to the supported minimum. It's probably worth clarifying in > the manual. Let me propose something. Please do. Thanks.
> >>>> >>>> Whether or not to warn in general if the alignment attribute is smaller >>>> than the default may be subject to debate. I guess it depends on the >>>> general intent that we'd find in real world codes. >>> >>> I would expect real world code to care about achieving at least >>> the specified alignment. >> If we only allow increasing, yes. At which point what do the values 0 >> or 1 realistically mean? > > If we only allowed increasing then both 0 and 1 would be > meaningless. > >> >> If we allow decreasing, then the user may be asking for a smaller >> alignment to achieve better code density. > > Yes, that's definitely possible (I just opened pr88231 - > aligned functions laid down inefficiently, as I noticed this > doesn't work as well as it could and arguably should). But they > can't get it if the target doesn't support it. In which case I > think adding a new warning to point it out might be useful. At > the same time, users wanting maximum density across all their > targets shouldn't have to worry about what the minimum alignment > is on each of them and hardwire different constants into > the attribute. I think they should be able to specify 1 and > have GCC round it up as necessary, with no warning. So I'd make > the new warning off by default. That seems like a better UI than forcing the user to know what their target actually supports. So, yea I think I'm in agreement with where you're going. jeff
