RE: [PATCH 00/22] add support for Clang LTO

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 



From: Paul E. McKenney
> Sent: 01 July 2020 17:06
...
> > Would an asm statement that uses the same 'register' for input and
> > output but doesn't actually do anything help?
> > It won't generate any code, but the compiler ought to assume that
> > it might change the value - so can't do optimisations that track
> > the value across the call.
> 
> It might replace the volatile load, but there are optimizations that
> apply to the downstream code as well.
> 
> Or are you suggesting periodically pushing the dependent variable
> through this asm?  That might work, but it would be easier and
> more maintainable to just mark the variable.

Marking the variable requires compiler support.
Although what 'volatile register int foo;' means might be interesting.

So I was thinking that in the case mentioned earlier you do:
	ptr += LAUNDER(offset & 1);
to ensure the compiler didn't convert to:
	if (offset & 1) ptr++;
(Which is probably a pessimisation - the reverse is likely better.)

	David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)




[Index of Archives]     [Linux&nblp;USB Development]     [Linux Media]     [Video for Linux]     [Linux Audio Users]     [Yosemite Secrets]     [Linux Kernel]     [Linux SCSI]

  Powered by Linux