https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92346
--- Comment #4 from wierton <141242068 at smail dot nju.edu.cn> --- (In reply to Andrew Pinski from comment #3) > (In reply to wierton from comment #2) > > This code can reproduce the phenomenon and there are no output constraints. > > I do understand that the declaration %eax in eax is conflict with the inline > > assembly clobber list, but in compiler such as clang, this dependency will > > be detected and clang will extraly generate a move instruction to save the > > eax and the resume it. I think gcc is a mostly widely used compiler, we as > > users expect this compiler becomes better and better, and this feature will > > help improve the usability, so I post it here. Very sorry for previous post. > > And the documentation is clear there that this is expected behavior. > > https://gcc.gnu.org/onlinedocs/gcc-9.2.0/gcc/Local-Register-Variables. > html#Local-Register-Variables > > "The only supported use for this feature is to specify registers for input > and output operands when calling Extended asm (see Extended Asm)." > > "Defining a register variable does not reserve the register. Other than when > invoking the Extended asm, the contents of the specified register are not > guaranteed. For this reason, the following uses are explicitly not > supported. If they appear to work, it is only happenstance, and may stop > working as intended due to (seemingly) unrelated changes in surrounding > code, or even minor changes in the optimization of a future version of gcc:" Thanks very much for your patience to explain to me, I feel very very sorry for taking your time on such meaningless thing. I will be more cautious before any further possible post.
