checkasm: Improve 32-bit parameter clobbering on x86-64

The previous method of doing parameter clobbering was hacky and unreliable, but this approach works much better. The same could probably be done for AArch64 as well.

Also fix one bug that was detected because of this improvement.

This relies on _Generic(), which is part of C11. AFAIK pretty much all reasonably modern compilers support this, however MSVC doesn't enable it unless compiled with /std:c11. We already use some C11 features (stdatomic.h and anonymous structs/unions), so maybe we should just embrace C11?

changed milestone to %1.1.0

added bug tests x86 labels

requested review from @mstorsjo

FWIW I think the form used here, to just check if we happen to have _Generic available even if we're in C99 mode and conditionally use it then, is fine - it doesn't disrupt builds in other configurations, etc, and should be plenty, as long as most compilers used in the CI actually do end up enabling it.

The same could probably be done for AArch64 as well.

Yeah, I guess so. On the other hand, I haven't ever seen an aarch64 compiler call functions by locally pushing registers on the stack, as opposed to just write them into a preallocated stack area, so there's not much rush either (other than to keep things consistent - and reduce the risk of later surprises of course).

I guess one upside of the previous approach, is that it also takes care of clobbering more than 32 bit for smaller arguments, if the ABI allows more bits of a parameter register to be undefined.

resolved all threads

added 3 commits

• 6fefa6a5 - checkasm: Improve 32-bit parameter clobbering on x86-64
• 0b0b5fbf - checkasm: Move printf format string to .rodata on x86
• 58c856b7 - build: strip() the result of cc.get_define()

Compare with previous version

resolved all threads

approved this merge request

LGTM, this looks reasonable to me. I didn't check the asm changes in detail (in particular not the change to move the string constant to rodata), but I didn't see anything I'd object to at least.

merged