noreturn function attribute and ret asm instruction

[Zack Weinberg]

Hans-Peter Nilsson <hp@bitrange.com> writes:

> On 2 Sep 2003, Jim Wilson wrote:
>> The real problem here is that we have no support for asms that change
>> flow of control.  Adding a function attribute to suppress the ret does
>> not fix this.  Letting asms change flow of control would require syntax
>> and semantic changes.  This would be a major change, and it isn't clear
>> if it is a good idea.  It might impede optimization so much that it
>> hurts more than it helps.
>
> Though an attribute on the asm would be simpler syntax-wise,

I dunno.  Attribute handling is one of the hairiest parts of the
syntax.

> Using an asm to terminate a function is common in kernel-like
> code so the request seems reasonable.  (But still not trivial to
> implement, flow-wise.)

It really is trivial at the RTL level - I know, I implemented it.
(You just emit a BARRIER insn right after the ASM_OPERANDS insn.)
It might be harder at the tree level; I'm not sure how noreturn is
being represented in tree-ssa.

zw