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

[Teresa Johnson via llvm-dev]

Thanks for the test case! I can reproduce this, and see with the
compiler I saved from just before r278338 that this is indeed a chance
in behavior. Looking at why this changed...

On Fri, Aug 26, 2016 at 1:42 PM, Mehdi Amini <mehdi.amini at apple.com> wrote:
>
>> On Aug 26, 2016, at 1:34 PM, junbuml at codeaurora.org wrote:
>>
>> 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  ;
>
> These are common variables, this is what I mentioned in my first email. Compiling with -fno-commons or defining them with “int GVi32_a = 0;” should solve it.
>
> However r278338 is not supposed to have changed anything on this aspect. I would have expected maybe r279417 playing a role there.
>
> Anyway I don’t have Gold, so I’ll leave Teresa investigate why the change in behavior.
>
> Do you want to try improving global merge to try to handle this case?
>
> —
> Mehdi
>
>
>
>
>
>>
>> __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.
>



-- 
Teresa Johnson | Software Engineer | tejohnson at google.com | 408-460-2413