On Sat, Jun 24, 2023, at 11:01 AM, Jeff Law via Gcc-patches wrote: > On 6/21/23 02:14, Wang, Yanzhang wrote: >> Hi Jeff, sorry for the late reply. >> >>> The long branch handling is done at the assembler level. So the clobbering >>> of $ra isn't visible to the compiler. Thus the compiler has to be >>> extremely careful to not hold values in $ra because the assembler may >>> clobber $ra. >> >> If assembler will modify the $ra behavior, it seems the rules we defined in >> the riscv.cc will be ignored. For example, the $ra saving generated by this >> patch may be modified by the assmebler and all others depends on it will be >> wrong. So implementing the long jump in the compiler is better. > Basically correct. The assembler potentially clobbers $ra. That's why > in the long jump patches $ra becomes a fixed register -- the compiler > doesn't know when it's clobbered by the assembler. > > Even if this were done in the compiler, we'd still have to do something > special with $ra. The point at which decisions about register > allocation and such are made is before the point where we know the final > positions of jumps/labels. It's a classic problem in GCC's design.
Do you have a reference for more information on the long jump patches? I'm particularly curious about why $ra was selected as the temporary instead of $t1 like the tail pseudoinstruction uses. -s
