[LLVMdev] merging globals

[Eli Friedman]

On Thu, Oct 16, 2008 at 12:35 PM, Mike Stump <mrs at apple.com> wrote:
>> It doesn't really change the issue, though; we want the merging to be
>> a front-end option, and we still need a solution which handles
>> variables that gets marked by the optimizer.
>
> I think so.  If we could get C/C++ to just bless merging and then just
> support that and ignore legacy standards and legacy code, we might be
> able to leave it as is.

The only allowance I can think of that's general enough to allow
everything LLVM knows how to do at the moment is "the result of an
equality comparison between two pointers to objects with distinct base
objects is undefined".  (See
http://lists.cs.uiuc.edu/pipermail/llvmdev/2008-October/017769.html
and http://lists.cs.uiuc.edu/pipermail/llvmdev/2008-October/017747.html
for the examples I'm thinking of.)  I strongly doubt we can get away
with that.

Here's a more concrete version of the solution I'm proposing: we add a
new optional marking to constant globals, say "mergeable".  There are
two reasonable semantics: one is that the result of equality
comparisons of a pointer into this global with a pointer into any
other similarly marked global is undefined.  This is actually slightly
more aggressive than what the current standard seems to allow even for
string constants, but it seems reasonable.  The more conservative
definition of the semantics is just to say that the compiler chooses
whether any pair of mergeable globals are distinct objects, which is
roughly how the current C99 standard defines string merging.

This has the following effects on current optimizers: constmerge only
merges globals marked mergeable.  Only mergeable constants are emitted
into mergeable sections in assembly.  If we use the conservative
definition of mergeable, fix any code that assumes distinct globals
don't get merged, like the code that folds equality comparisons
between distinct globals to false.  Optionally, add a new optimization
step which marks constants as mergable when their address isn't taken.

-Eli