m68k: Simple loop compiles into boundless recursion with -O2

Segher Boessenkool segher@kernel.crashing.org
Wed Jan 13 19:20:15 GMT 2021


On Wed, Jan 13, 2021 at 05:30:35PM +0100, Fredrik Noring wrote:
> Many thanks, Alexander,
> > Please invoke objdump with -dr instead to see the relocations.
> Indeed:
>   1a:	4eb9 0000 0000 	jsr 0 <memset2>
> 			1c: R_68K_32	memset

Yup, always always always use -dr :-)

> > The relocation associated with this instruction should point to memset.
> > Most likely the compiler is optimizing your memset2 function to call
> > the standard function 'memset'.
> > 
> > When implementing memset itself you need to pass -ffreestanding to GCC,
> > which will disable this optimization.
> Yes, I had -nostdlib but -ffreestanding is apparently needed as well. Thanks
> again.

You probably want -ffreestanding anyway for your situation: -nostdlib
does not mean "there is no standard library", it just means "do not link
with it".

For the archives, no one has said it yet: to just disable the
optimisation transforming the loop into a memset, you can use
-fno-tree-loop-distribute-patterns .


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