Re: [PATCH V2] Disable sched1 in functions that call setjmp

[Richard Biener <richard.guenther@gmail.com>]

> Am 24.12.2022 um 09:11 schrieb Alexander Monakov via Gcc-patches <gcc-patches@gcc.gnu.org>:
> 
> 
>> On Fri, 23 Dec 2022, Qing Zhao wrote:
>> 
>> BTW, Why sched1 is not enabled on x86 by default?
> 
> Register allocation is tricky on x86 due to small number of general-purpose
> registers, and sched1 can make it even more difficult. I think before register
> pressure modeling was added, sched1 could not be enabled because then allocation
> would sometimes fail, and now there's no incentive to enable it, as it is not so
> important for modern x86 CPUs. Perhaps someone else has a more comprehensive
> answer.
> 
>> Another question is:  As discussed in the original bug PR57067:
>> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=57067 The root cause of this
>> issue related to the abnormal control flow edges (from setjmp/longjmp) cannot
>> be represented correctly at RTL stage, shall we fix this root cause instead? 
> 
> You'd need an experienced reviewer to work with you, especially on high-level
> design decisions such as "How ABNORMAL_DISPATCHER should be represented on RTL".
> I'm afraid it's not just a matter of applying a small patch in one place.

For nonlocal goto we Thread the abnormal dispatcher.  Of course by regenerating abnormal edges, not by keeping and modifying them.  We cannot re-generate the (optimal) set of abnormal edges for setjmp so we want to preserve those edges.  But as you say it’s a very non-trivial change.

Richard 

> Alexander