[PATCH] D14094: Add multikey_qsort to STLExtras.h

[David Blaikie via llvm-commits]

dblaikie added a subscriber: dblaikie.

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Comment at: include/llvm/ADT/STLExtras.h:376
@@ +375,3 @@
+    ++Pos;
+    goto tailcall;
+  }
----------------
grosbach wrote:
> ruiu wrote:
> > grosbach wrote:
> > > Does the compiler not auto-recognize the tail call pattern here? If not, it seems like it should.
> > Unlike Scheme we cannot assume that the tail call optimization is always in place. If a compiler does tail call optimization only in -O2 and not in -O0, this code could cause SEGV only in -O0 for some input. That would be confusing. So I think this is needed.
> I'm not sure I agree. -O0 vs. -O2 (or -Os) builds of the compiler (and Debug vs. Release in general) already have radically different memory usage patterns and algorithmic behavior (various +asserts stuff has n^2 algorithms, e.g.).
> 
> Is this a theoretical concern, or are you seeing crashes on real examples with a debug build of the compiler?
Eh, I think we tend not to write recursive algorithms where there aren't nice/known/low bounds, even if the compiler can optimize them away (same reason we write ++i instead of i++, etc).

That said, I wouldn't write it with goto if I could help it - I'd rather find a more natural iterative expression of the issue. (even just a simple "while (true) / return" thing I'd prefer over a raw goto, but a do/while loop seems plausible here)


http://reviews.llvm.org/D14094