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

Fri Aug 26 17:34:07 PDT 2016

I have a fix, just running tests.

On Fri, Aug 26, 2016 at 4:03 PM, Teresa Johnson <tejohnson at google.com>
wrote:

>
>
> On Fri, Aug 26, 2016 at 3:07 PM, Mehdi Amini <mehdi.amini at apple.com>
> wrote:
>
>>
>> > 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->get
>> NamedGlobal(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.
>>
>
> Yeah. In fact the original handling added in r278338 (addCommons) didn't
> even compare the size, it always created a new merged common with an
> ArrayType.
>
>
>>
>> How was Gold doing it before r278338?
>>
>
> It was just letting the ModuleLinker handle it (which kept the largest one
> by size), and then it fixed up the alignment later.
>
> Will change to first compare the size.
>
>
>>
>> —
>>
>> 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
>>
>>
>
>
> --
> Teresa Johnson |  Software Engineer |  tejohnson at google.com |
> 408-460-2413
>

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