On 2021-02-19, Jim Wilson wrote:
On Fri, Feb 19, 2021 at 4:36 PM Fangrui Song <i@xxxxxxxxxx> wrote:
How should we make the following code work with both
-fno-asynchronous-unwind-tables and -fasynchronous-unwind-tables?
int main() {
asm(".cfi_undefined %rip");
}
The example is minimalistic. The idea is that if inline asm does stack
push/pop and wants to manually add .cfi_* directives, the current
error behavior is undesired.
I don't think that there is a way currently to make this work.
In general, I'd say that if you are doing something non-trivial you should
not be using an extended asm. You should either use a built-in function or
just write it in assembly language directly. Extended asms really only
work well for simple stuff. If you use a built-in function, then the
compiler should emit any CFI directives for that built-in if necessary. If
writing in assembly language then you can hand write CFI directives for the
entire function, or rely on assembler support if the assembler can figure
it out from the code.
But if you really want to do this in an extended asm, then we would need to
extend the asm syntax so that one can put CFI info in the middle of an
asm. One way to do that might be a special operator for formatting asm
strings that only prints the string if we are emitting unwind info. We
would have to find a free punctuation character that isn't already used by
any existing port, and then assuming backquote is free we could do
int main() {
asm("`.cfi_undefined %rip`");
}
and the backquotes then only print the string if we are emitting unwind
info. Or alternatively make this a % operator like %`. Either way, I
think this would look funny, so might not be the best approach. It would
likely be difficult for people to use. I seriously doubt many users can
write CFI code and get it right.
In GNU as, can we have a .cfi_sections value to ignore subsequent .cfi_* directives?
If such a directive exists, gcc -fno-asynchronous-unwind-tables can emit
that directive, and pass through all the .cfi_* directives, then let GNU as ignore them.
