Re: [avr-gcc-list] Optimisation mixes up registers in inline assembly

[David Brown]

On 24/07/2019 21:41, Michael Kwasnicki wrote:
> So the inputs to inline assembly are by value and not by reference, as I 
> thought. Right?

Correct.  The inputs are used to initialise the registers.

> The correlation between my named registers and those C variables was 
> pure coincidence.

No, it is pure optimisation, not coincidence.  And of course you don't 
necessarily get registers for variables in C - certainly not for const 
values.

> And by making the named registers both - input and output - it creates a 
> reference to the original C variable.

Yes, roughly.

The big question here is why you are using assembly at all.  Why not 
just write in C?

void test2(void) {
    uint8_t size = 42;
    const uint8_t start = _BV(WGM02) |  _BV(CS00);
    const uint8_t stop = 0x00;
    TCCR0A = _BV(COM0B1) | _BV(WGM01) | _BV(WGM00);
    OCR0A = 9;                              // TOP
    const uint8_t zero = 1;                 // 0 < i <= TOP
    const uint8_t one = 7;                        // 0 < i <= TOP

    OCR0B = one;
    TCCR0B = start;

    while (size--) {
        for (uint8_t bits = 0; bits < 8; bits++) {
            OCR0B = one;
        }
        OCR0B = zero;
    }
    TCCR0B = stop;
}

It is simpler, easier to understand, much easier to get right, and gives 
code that is marginally smaller and faster than your hand-written 
assembly.  (It is even better with -O2, which is the optimisation level 
I recommend as usually giving smaller and faster code than -Os.)

mvh.,

David



> Regards,
>
> Michael K.
>
>
>
> > Am 24.07.2019 um 19:17 schrieb Joseph C. Sible <address@hidden 
> > <mailto:address@hidden>>:
> >
> > Since you intend to modify the inputs, you need to declare them as 
> > outputs as well (with the + constraint). If you don't do this, GCC is 
> > allowed to assume you don't write to them, and can share any that have 
> > the same values.
> >
> > Joseph C. Sible
> >
> >
> > On Wed, Jul 24, 2019, 13:04 Michael Kwasnicki <address@hidden 
> > <mailto:address@hidden>> wrote:
> >
> >     Hello dear list members,
> >
> >     I am new to this list because I need someone to discuss a
> >     potential compiler bug.
> >
> >
> >     I try to mix inline assembly in C code. In the assembly I use
> >     passed in variables which are named:
> >
> >     _https://godbolt.org/z/w8wuJy_
> >     _
> >     _
> >     The register mapping is:
> >     [rZero] -> R20
> >     [rOne] -> R21
> >     [rStart] -> R18
> >     [rStop] -> R19
> >     [rSize] -> R22
> >     [bits] -> R19
> >
> >     If I look at the output assembly I can see the compiler did some
> >     optimisation.
> >     R19 is shared between [bits] and [rStop].
> >     In this case this is okay as the bit count reaches zero at the end
> >     and thus [bits] and [rStop] have the same value then.
> >
> >
> >     But when I change the initial value of `bitcount` in C to 7 all
> >     hell breaks loose.
> >
> >     https://godbolt.org/z/F7rrB_
> >
> >     The register mapping is:
> >     [rZero] -> R20
> >     [rOne] -> R18
> >     [rStart] -> R19
> >     [rStop] -> R21
> >     [rSize] -> R22
> >     [bits] -> R18
> >
> >     This time the compiler optimisation corrupted the program.
> >     R18 is shared between [rOne] and [bits]. [rOne] was supposed to be
> >     a constant that is cached in a register. But [bits] gets decremented.
> >     Thus the value passed to OCR0B varies and is not constant any more
> >     changing the logic of the inline assembly.
> >
> >     I am not experienced with inline assembly so I might have missed
> >     something but my general expectation for assembly is that if I
> >     name things differently, they are different things (registers in
> >     this case).
> >
> >     The issue does not show up with -O0 but anything above.
> >     Locally I am running avr-gcc (GCC) 9.1.0. Not sure what the exact
> >     version at godbolt is.
> >
> >     Cheers,
> >
> >     Michael
> >     _______________________________________________
> >     AVR-GCC-list mailing list
> >     address@hidden <mailto:address@hidden>
> >     https://lists.nongnu.org/mailman/listinfo/avr-gcc-list
>
>
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