[PATCH] Global merge on constants

[Quentin Colombet]

Hi Duncan,

Does it mean that the current behavior of global merge is wrong or is this unnamed_addr marker apply only to global constants?

Anyway, thanks for the information.

-Quentin

On Feb 27, 2013, at 12:50 AM, Duncan Sands <baldrick at free.fr> wrote:

> 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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