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

[Mehdi Amini via llvm-dev]

> On Aug 26, 2016, at 2:06 PM, Teresa Johnson <tejohnson at google.com> wrote:
> 
> Mehdi: I see what is going on:
> 
> +    ArrayType *Ty =
> +        ArrayType::get(Type::getInt8Ty(RegularLTO.Ctx), I.second.Size);
> +    GlobalVariable *OldGV = RegularLTO.CombinedModule->getNamedGlobal(I.first);
> +    if (OldGV && OldGV->getType()->getElementType() == Ty) {
> +      // Don't create a new global if the type is already correct, just make
> +      // sure the alignment is correct.
> 
> We compare the existing type to an ArrayType constructed from the
> recorded merged common size. So
> the original i32 type does not look the same (since it compares
> against [4 x i8]).

Right, but that the code I wrote in r279417 right? I guess we should compare the *size* first, and only do something if the size differs.

How was Gold doing it before r278338?

— 
Mehdi




> 
> Not that familiar with type handling in LLVM, but I guess I need to
> just check if the type size is the same?
> 
> On Fri, Aug 26, 2016 at 1:57 PM, Teresa Johnson <tejohnson at google.com> wrote:
>> 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
> 
> 
> 
> -- 
> Teresa Johnson | Software Engineer | tejohnson at google.com | 408-460-2413