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Re: [RFC][PR middle-end/59285] builtin-unreachable-6 on ARM

On 11/26/13 15:44, Steven Bosscher wrote:
So we have a block which calls fubar.  The block would have no successors.
And I think we're right back in the same situation.  We're going to have a
BARRIER after that block with no successors and ifcvt is going to muck
things up tripping the checking failure.

In RTL-land the NORETURN flag is already a statement thing via
REG_NORETURN. But I'm not sure if such a note is added for calls
followed originally by a builtin_unreachable. I'm guessing "not".
I'm not aware of any code to do that.

Also note that we could get the same behaviour from a
*<magicaddr>  = magicnum

if we were talking to a special piece of hardware. Though perhaps we wouldn't if-convert the volatile memory reference.

And yet, that's what merge_if_blocks will do if you remove the
BARRIER. You're basically closing the diamond so that the conditional
jump can be optimized away. That's a good thing, of course.
Exactly why I ultimately came up with the hack I posted rather than the hack to cond_exec_find_if_block which was my original fix.

But it seems arbitrary to do it in ifcvt.c and only for cond_exec
targets. Does the condjump survive to assembler output on e.g. x86_64?
Yup.  It lives all the way to assembler output.


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