Re: [avr-gcc-list] >4.5.1 better than this at register-structure (xmega) access?

[Erik Walthinsen]

On 10/11/2012 12:13 PM, David Kelly wrote:
> Why should they have to know anything about AVR to optimize struct references?
Thus the testcase, it just happens to look like the AVR header.  But I 
think you're half right, half wrong: struct reference optimization is a 
generic problem, but the specific solution ("ldi r30 loaddr,ldi r31 
hiaddr,st Z,rX" or the better "sts addr,rX") is very AVR specific.

> Am suspicious gcc is treating TCC0 as volatile, the address of the struct as 
> volatile, while the desire is for the contents to be volatile. Appears to be 
> defined as a pointer to a struct of volatiles, but perhaps the volatile 
> property is leaking?
That seems possible, and there's probably a decent way to build a 
testcase to confirm that.  I'm kinda doubtful that's what's going on, 
but I don't know much of anything about the gcc internals either way.

> Dereferencing TCC0.CTRLE by taking its address and casting it as a pointer may 
> strip volatile allowing gcc to optimize. It should be able to optimize anyway.
Hmm, I should check into my FS() workaround macro and make sure I'm 
careful about the volatiles, it's possible I could shoot myself in the 
foot with repeated hardware-register references using that...

FWIW, I *have* seen the compiler do the right thing with struct 
references in the past, but it was for either read or write (I don't 
remember which) and *not* the opposite.  I have no idea what version of 
compiler it was unfortunately.