On Mon, May 19, 2014 at 10:26 AM, Chad Rosier <mcros...@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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