[PATCH] These builtin functions set errno. Mark them accordingly.

[Chandler Carruth]

On Mon, May 19, 2014 at 10:26 AM, Chad Rosier <mcrosier at codeaurora.org>wrote:

> Hal and I discussed this on IRC and here's the synopsis (mostly in Hal's
> words):
>
> Many of these builtins turn into libc calls, and those do set errno.
> Marking them as readnone is a problem, because we can reorder/eliminate
> writes to errno.  For example, if we assume readnone for something like
> __builtin_sqrt(); write(); perror(); it may be reordered to write();
> __builtin_sqrt(); perror(); and perror may provide junk.  However, if we
> don't mark the builtins as readnone, then they won't get lowered into ISD
> nodes (for those nodes that are relevant), and that means that they won't
> get instruction selected into native instructions on targets where that's
> possible. So the __builtin_sqrt is kind of an escape hatch to force the
> direct lowering behavior even when we can't do it for sqrt because sqrt
> might set errno.  But, as Hal said, this behavior *is* broken, technically.
> And, in fact, it is not hard to create test cases where you can see buggy
> behavior.  Hak's just not sure what the right way of handling it is. Hal
> thinks we really need some kind of target inp!
>  ut, so that we can know ahead of time whether the builtin will *really*
> set errno or not after isel.  Hal thinks there are two solutions possible.
>
> 1. Like with inline asm registers/constraints; each target will provide a
> table describing what builtins will really not set errno.
> 2. Let Clang essentially query TTI for those builtins that are relevant.
> This second method will require a bit of infrastructure work, but is
> probably not too difficult.
>
> I'm not going to push this patch further because it's not the right
> solution.  In theory, I believe it is correct, but it breaks the __buitlin
> escape hatch, which would likely result in serious performance degradations.
>

Crazy idea:

When we lower one of these to a call to a libc function, save and restore
errno around it.

Too crazy?
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