[LLVMdev] Static ctors in llvm::Attribute

Chandler Carruth chandlerc at google.com
Tue Feb 7 12:54:04 PST 2012

On Tue, Feb 7, 2012 at 10:01 AM, Chris Lattner <clattner at apple.com> wrote:

> Hi Kostya,
> One unexpected piece of fallout in your recent attributes change (r148553)
> was that it introduced a bunch of static constructors into .o files that
> #include Attributes.h, due to stuff like this:
> const Attributes None      (0);     ///< No attributes have been set
> const Attributes ZExt      (1<<0);  ///< Zero extended before/after call
> const Attributes SExt      (1<<1);  ///< Sign extended before/after call
> const Attributes NoReturn  (1<<2);  ///< Mark the function as not returning
> We really don't like static ctors in LLVM, because it is often used as a
> library, and they cause startup-time performance hits and other bad news.
>  I'm surprised we don't have an explicit section about it in the
> CodingStandards, but we do have:
> http://llvm.org/docs/CodingStandards.html#ll_iostream
> ... which talks about the same thing.
> Anyway, as it turns out, LLVM can optimize those static ctors away in some
> cases, but not all (e.g. -O0).  This was found because LLDB builds with
> -Wstatic-constructor and this header change is causing a flood of warnings.
> I can think of two ways to fix this.  One is to replace all of these with
> "get" functions, which would be really really ugly.  A better change would
> be to replace these with enums, eliminating the whole problem.  Are 64-bit
> enums portable enough to be used here?

I know Kostya and others are looking at this, but I have to say: why-oh-why
can't we have constexpr! ;] *This* is the problem those were meant to
solve, and they actually seem to solve it.... ahh well...
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