[llvm-dev] Use of array type in globals in LTO

[via llvm-dev]

On 2016-08-26 12:47, Mehdi Amini wrote:
>> On Aug 26, 2016, at 9:06 AM, junbuml at codeaurora.org wrote:
>> 
>> 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 you submit a reproduction for Gold please?
> We need to understand what changed with the new LTO API.


I compiled below C code for aarch64 in lto using gold 
(--target=aarch64-linux-gnu  -flto -fuse-ld=gold). After r278338, two 
globals, GVi32_a and GVi32_b, are [4 x i8] type in the input IR to 
GlobalMerge. Therefore, GlobalMerge do not even start to handle them 
because as of now it ignores fancy-aligned globals.  Before r278338, 
GVi32_a and GVi32_b seems to be i32 in the input IR to GlobalMerge. Is 
this change in the input IR expected ?

-------------------------
int GVi32_a  ;
int GVi32_b  ;

__attribute__((noinline)) void setGV(int a) {
   GVi32_a = a ;
   GVi32_b = a ;
}

__attribute__((noinline)) int loadGV() {
   return GVi32_a + GVi32_b  ;
}

int main(int argc, char *argv[]){
   setGV(argc);
   return loadGV();
}
-------------------------



> 
> 
> 
> —
> 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.