On Thu, Dec 13, 2018 at 8:48:38 EET Segher Boessenkool wrote: > On Wed, Dec 12, 2018 at 06:26:10PM +0200, Dimitar Dimitrov wrote: > > I expect that if I mark a HW register as "clobber", compiler would save > > its > > contents before executing the asm statement, and after that it would > > restore its contents. This is the GCC behaviour for all but the SP and > > PIC registers. That is why I believe that PR52813 is a valid bug. > > It won't do it for *any* fixed registers. But you do not want to error > or even warn for some fixed registers, for example the "flags" register > on x86 is *always* written to by asm.
Yes, you are correct. > > But you never want to warn for non-fixed registers, and e.g. > PIC_OFFSET_TABLE_REGNUM isn't always a fixed register (when flag_pic is 0 > for example). I could not trace how PIC_OFFSET_TABLE_REGNUM on i386 gets marked as fixed register. I'll dig more through the source. > > > I'm not sure how GCC could recover if SP is clobbered. If SP is clobbered > > in such a way that GCC will not notice (e.g. thread switching), then why > > should GCC know about it in the first place? > > Up until today, GCC has always just ignored it if you claimed to clobber > the stack pointer. My point is that the silent ignoring is confusing to users, as shown by PR52813. How would you suggest me to proceed: - Leave patch as-is. - Revert patch. Update documentation to point that clobber marker for fixed registers is ignored by GCC. Close PR52813 as invalid. - Revert patch. Discuss more broadly and specify behaviour of asm clobber for fixed registers (and SP in particular). Thanks, Dimitar
