Re: libgcc: strange optimization

[Georg-Johann Lay <avr at gjlay dot de>]

Richard Guenther schrieb:
> I suggest to amend the documentation for local call-clobbered register
> variables to say that the only valid sequence using them is from a
> non-inlinable function that contains only direct initializations of the
> register variables from constants or parameters.
>
> Richard.

That's completely counterproductive.

If a developer invents asm or local register variables he has a
very good reason for that choice like to meet hard (with hard as
in HARD) real time constraints.  Disabling inlining a function
that uses local register vars would make many places of local
register vars unusable because thre would no more be a way to
write down the exact register usage footprint of a piece of
asm.  Typical use-cases are interfacing to a function that has a
smaller register footprint than an ordinary block-box function or
doing some arithmetic that needs special hard regs for which there
are no fitting constraints. All this will be impossible if inlining
is disabled for such functions; then it is no more possible to
describe such a low-overhead piece of code without calling a black box 
function, clobber all call-clobbered registers, render a maybe 
tail-function into a non tail-call function etc.

Embedded systems like ARM or PowerPC based get more and more
important over the years and GCC should put more attention to
them and their needs; not only to bolide servers/PCs.
This includes systems with hard real time constraints.

Johann