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

Fri Aug 26 16:03:27 PDT 2016

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->
> 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.
>

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
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