So the inputs to inline assembly are by value and not by reference, as I thought. Right? The correlation between my named registers and those C variables was pure coincidence. And by making the named registers both - input and output - it creates a reference to the original C variable.
Regards, Michael K. > Am 24.07.2019 um 19:17 schrieb Joseph C. Sible <josephcsi...@gmail.com>: > > 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 <avr-...@kwasi-ich.de > <mailto:avr-...@kwasi-ich.de>> 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 <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_ <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 > AVR-GCC-list@nongnu.org <mailto:AVR-GCC-list@nongnu.org> > https://lists.nongnu.org/mailman/listinfo/avr-gcc-list > <https://lists.nongnu.org/mailman/listinfo/avr-gcc-list>
_______________________________________________ AVR-GCC-list mailing list AVR-GCC-list@nongnu.org https://lists.nongnu.org/mailman/listinfo/avr-gcc-list
