[Lldb-commits] [PATCH] D27459: Add a more succinct logging syntax

[Pavel Labath via Phabricator via lldb-commits]

labath added inline comments.


================
Comment at: source/Core/Log.cpp:78
+  char *text;
+  vasprintf(&text, format, args);
+  message << text;
----------------
zturner wrote:
> dancol wrote:
> > I usually implement printf-into-std::string by using `vsnprintf` to figure out how many characters we generate, using `std::string::resize` to create a buffer of that many characters (unfortunately, zero-filling them), then `vsnprintf` directly into that buffer. This way, you only need one allocation.
> > 
> > The currant approach involves at least three allocations: first, the string generated by `vasprintf`. Second, the internal `stringstream` buffer. Third, the copy of the buffer that `std::stringstream::str` generates.
> > 
> > It's more expensive that it needs to be.
> To be fair, we should really be deleting these methods long term, and using `formatv`.  This way you often end up with 0 allocations (for short messages), and on top of that, only one underlying format call (as opposed to wasting time calling `vasprintf` twice here).
> The currant approach involves at least three allocations: first, the string generated by vasprintf. Second, the internal stringstream buffer. Third, the copy of the buffer that std::stringstream::str generates.

`str()` returns a reference to the underlying buffer (this is an llvm stream, not std::stringstream), so there is no copy there. Since this is not really performance critical, I'd prefer to keep the code simpler (plus, I think the extra vsnprintf call balances the extra malloc, roughly speaking).

And, same as Zachary, I hope this code will go away eventually.


https://reviews.llvm.org/D27459