Re: Representation clauses and side-efects on STM32F411 ravenscar runtime

[Simon Clubley <clubley@remove_me.eisner.decus.org-Earth.UFP>]

On 2015-08-15, Frédéric Praca <frederic.praca@free.fr> wrote:
>
> Hello Simon,
> assembly has never been my favourite language and I only studied 68000 
> assembly a long time ago :)
> I don't know what you really mean by "load and/or store by byte 
> references".
> So, here's what I get by using objdump 

[snip]

>    USART2.BRR.DIV_Fraction := 16#c#;
>  8000418:	f44f 4388 	mov.w	r3, #17408	; 0x4400
>  800041c:	f2c4 0300 	movt	r3, #16384	; 0x4000
>  8000420:	7a1a      	ldrb	r2, [r3, #8]
                                ^^^^
>  8000422:	f04f 010c 	mov.w	r1, #12
>  8000426:	f361 0203 	bfi	r2, r1, #0, #4
>  800042a:	721a      	strb	r2, [r3, #8]
                                ^^^^

This violates the hardware level restrictions and it's not the first
time I've come across this.

The same thing can happen with C as well when you try to use bitfields
instead of bitmasks; it's a general gcc code generation issue for ARM.

There are a couple of AIs created as a result of submissions by myself
which you might find interesting:

http://www.ada-auth.org/cgi-bin/cvsweb.cgi/ai12s/ai12-0127-1.txt?rev=1.1
http://www.ada-auth.org/cgi-bin/cvsweb.cgi/ai12s/ai12-0128-1.txt?rev=1.1

They contain proposed ideas about how to tackle this problem in a future
version of Ada.

>
> From what I read in the ARM assembly manual, this seems to be ok from an 
> assembly point of view so I think this can come from the hardware 
> restrictions. Am I right ?
>

Yes you are.

Simon.

-- 
Simon Clubley, clubley@remove_me.eisner.decus.org-Earth.UFP
Microsoft: Bringing you 1980s technology to a 21st century world