[PATCH] Global merge on constants

[Duncan Sands]

Hi Quentin, you should only be merging them if they are marked with
unnamed_addr (this indicates that no-one is expecting the globals
to have distinct addressses).  As EH type infos shouldn't get
unnamed_addr, that should mean that the EH issue isn't a problem any
more.

Ciao, Duncan.

> You will find enclosed a patch that adds the support of constant variables for
> the generic global merge pass.
> I've also attached a motivating example, test.c.
>
> In that file there are several global variables accessed within the same function.
> If all the variables are non-const, global merge generates one unique variable,
> which translates into one unique load in the assembly file.
>
> clang -arch arm -O3 test.c -o - -S
>
> If some are const (the example is very stupid), global merge does not merge
> anything and one load for each variable is issued, i.e., 3
> clang -arch arm -O3 test.c -o - -S -DWITHCONST
>
> If global merge had merged the constant globals, only 2 loads would have been
> issued. The patch allows to do that.
>
> Although the patch is fairly simple, it was not done before because it seems it
> was breaking the EH processing (see comment in the original code).
> Is it still true?
>
> If yes, how could I reproduce that?
>
> Note: that the patch disables by default the handling of global constants
> because the current heuristic may generate worse code. A tuning may be
> necessary, but it would be done as a second step
>
> -Quentin
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