[PATCH] D24315: [CGP] Be less conservative about tail-duplicating a ret to allow tail calls

[Michael Kuperstein via llvm-commits]

mkuper added inline comments.

================
Comment at: lib/CodeGen/CodeGenPrepare.cpp:2008
@@ -2013,2 +2007,3 @@
   SmallVector<CallInst*, 4> TailCalls;
+  bool AllowDifferingSizes;
   if (PN) {
----------------
iteratee wrote:
> mkuper wrote:
> > iteratee wrote:
> > > It's kind of awkward to pass this as an output parameter and not use it.
> > > Would it be better to make it a pointer argument with a default value of nullptr?
> > I agree it's awkward, I'm just not sure the other way around (having a temporary in the callee, and then setting after a null check) is less awkward, overall. Same goes for other possible solutions (returning a pair, say).
> > 
> > In any case, I don't feel strongly about this, and I don't think the LLVM coding standards say anything concrete about this. So if you significantly prefer a pointer, let me know, and I'll change it.
> How about this:
> Use a local variable in attributesPermitTailCall.
> Use make_scope_exit to set the output parameter on exit only if it isn't null.
That'll work too (I didn't even know we had make_scope_exit!), but it seems like overkill.

Another awkward option that doesn't involve make_scope_exit, as long as we're at it:

```
... bool *ADS = nullptr) {
  bool TempADS;
  bool &AllowDifferingSizes = ADS ? *ADS : TempADS;
...
}
```


https://reviews.llvm.org/D24315