Re: Optimisation mixes up registers in inline assembly

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 <[email protected]>:
> 
> 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 <[email protected] <mailto:[email protected]>> 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
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