llvm dev - Aug 2016 - Use of array type in globals in LTO

Hi,

Recently, I noticed that less number of global variables are merged in 
global-merge pass and in some global variable, array types are used 
instead of its original type. For example, [4xi8] with align 4 is used 
for a i32  global variable. For me, it seems that such pattern is 
observed after r278338  (Resolution-based LTO API). Can anyone give me 
little bit more insight if such type change was intended? If then why 
such uses of array type is needed?

Thanks,
Jun

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Hi,
> Recently, I noticed that less number of global variables are merged in
global-merge pass and in some global variable, array types are used instead of
its original type. For example, [4xi8] with align 4 is used for a i32  global
variable. For me, it seems that such pattern is observed after r278338 
(Resolution-based LTO API).
Are you sure it is performed in the global merge pass? Can you provide an
example of input IR where you see this now but didn’t before?
Also can you confirm you’re using the gold-linker?
> Can anyone give me little bit more insight if such type change was
intended? If then why such uses of array type is needed?
The transformation I know of is the common variables merging, bug it is not new.
We use an array because we have to merge globals from different size and an
array of bytes is the most straightforward representation for that.

— 
Mehdi

On 2016-08-26 11:32, Mehdi Amini wrote:
> Hi,
> 
>> Recently, I noticed that less number of global variables are merged in 
>> global-merge pass and in some global variable, array types are used 
>> instead of its original type. For example, [4xi8] with align 4 is used 
>> for a i32  global variable. For me, it seems that such pattern is 
>> observed after r278338  (Resolution-based LTO API).
> 
> Are you sure it is performed in the global merge pass? Can you provide
> an example of input IR where you see this now but didn’t before?
> Also can you confirm you’re using the gold-linker?
I used gold linker. In spec2006/perlbench, I observed the less number of 
globals are merged in GlobalMerge.cpp after r278338. The reason is 
because, from the very first pass, several global variables use [4xi8] 
with align 4, instead of its original type i32 after r278338. Current 
GlobalMerge pass doesn't seem to handle such fancy-aligned globals. If 
such type change (e.g., from i32 to [4xi8]) in global variables was 
intended in r278338, I think we should enhance GlobalMerge to handle 
such cases.

> 
>> Can anyone give me little bit more insight if such type change was 
>> intended? If then why such uses of array type is needed?
> 
> The transformation I know of is the common variables merging, bug it
> is not new. We use an array because we have to merge globals from
> different size and an array of bytes is the most straightforward
> representation for that.
> 
> —
> Mehdi
-- 
Qualcomm Datacenter Technologies, Inc. as an affiliate of Qualcomm 
Technologies, Inc.
Qualcomm Technologies, Inc. is a member of the Code Aurora Forum, a 
Linux Foundation Collaborative Project.