llvm dev - Oct 2012 - [LLVMdev] Redundant Add Operation in Code Generation?

Eli,

  Thanks. So I'm unclear exactly which llvm opt will exhibit copy prop.
behavior?

  It seems to me that codegenprepare is doing a useful thing (for me, since
I'm just using the llvm IR and not going to backend, providing it's
"exposing" and not simply "adding for layout for CodeGen
opts" (or
something similar to this)?

Thanks.

On Wed, Oct 17, 2012 at 1:44 PM, Eli Friedman <eli.friedman at
gmail.com>wrote:
> On Wed, Oct 17, 2012 at 1:22 PM, Ryan Taylor <ryta1203 at gmail.com>
wrote:
> > I'm curious why I am seeing this:
> >
> >   %uglygep18.sum = add i32 %lsr_iv8, %tmp45
> >   %scevgep19 = getelementptr i8* %parBits_017, i32 %uglygep18_sum
> >   %scevgep1920 = bitcast i8* %scevgep19 to i16*
> >   %tmp78 = load i16* %scevgep1920, align 2
> >   %uglygep14.sum = add i32 %lsr_iv8, %tmp45
> >   %scevgep15 = getelementptr i8* %extIn_013, i32 %uglygep14_sum
> >   %scevgep1516 = bitcast i8* %scevgep15 to i16*
> >   %tmp79 = load i16* %scevgep1516, align 2
> >   %conv93.i.i = sext i16 %tmp79 to i32
> >   %uglygep.sum = add i32 %lsr_iv8, %tmp45
> >   %scevgep11 = getelementptr i8* %sysBits_010, i32 %uglygep_sum
> >
> > You can see here that "add i32 %lsr_iv8, %tmp45" is done
multiple times,
> > appearing that there are two redundant add operations that are not
needed
> > yet are generated?
>
> CodeGenPrepare manipulates GEPs in a way that can expose redundancy
> where it wasn't obvious before.
>
> -Eli
>
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Eli,

  Actually, I stil see this issue without codegenprepare being used. I'm
also compiling with -o3.

  So I'm still not sure why I'm seeing this issue?

Thanks.

On Wed, Oct 17, 2012 at 1:54 PM, Ryan Taylor <ryta1203 at gmail.com>
wrote:
> Eli,
>
>   Thanks. So I'm unclear exactly which llvm opt will exhibit copy prop.
> behavior?
>
>   It seems to me that codegenprepare is doing a useful thing (for me,
> since I'm just using the llvm IR and not going to backend, providing
it's
> "exposing" and not simply "adding for layout for CodeGen
opts" (or
> something similar to this)?
>
> Thanks.
>
>
> On Wed, Oct 17, 2012 at 1:44 PM, Eli Friedman <eli.friedman at
gmail.com>wrote:
>
>> On Wed, Oct 17, 2012 at 1:22 PM, Ryan Taylor <ryta1203 at
gmail.com> wrote:
>> > I'm curious why I am seeing this:
>> >
>> >   %uglygep18.sum = add i32 %lsr_iv8, %tmp45
>> >   %scevgep19 = getelementptr i8* %parBits_017, i32 %uglygep18_sum
>> >   %scevgep1920 = bitcast i8* %scevgep19 to i16*
>> >   %tmp78 = load i16* %scevgep1920, align 2
>> >   %uglygep14.sum = add i32 %lsr_iv8, %tmp45
>> >   %scevgep15 = getelementptr i8* %extIn_013, i32 %uglygep14_sum
>> >   %scevgep1516 = bitcast i8* %scevgep15 to i16*
>> >   %tmp79 = load i16* %scevgep1516, align 2
>> >   %conv93.i.i = sext i16 %tmp79 to i32
>> >   %uglygep.sum = add i32 %lsr_iv8, %tmp45
>> >   %scevgep11 = getelementptr i8* %sysBits_010, i32 %uglygep_sum
>> >
>> > You can see here that "add i32 %lsr_iv8, %tmp45" is done
multiple times,
>> > appearing that there are two redundant add operations that are not
>> needed
>> > yet are generated?
>>
>> CodeGenPrepare manipulates GEPs in a way that can expose redundancy
>> where it wasn't obvious before.
>>
>> -Eli
>>
>
>
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On Wed, Oct 17, 2012 at 2:03 PM, Ryan Taylor <ryta1203 at gmail.com>
wrote:
> Eli,
>
>   Actually, I stil see this issue without codegenprepare being used.
I'm
> also compiling with -o3.
>
>   So I'm still not sure why I'm seeing this issue?
Hmm... maybe it's LSR doing it?

Anyway, the general idea is that there are only a few passes which
perform redundant instruction elimination: GVN, EarlyCSE, and
InstCombine (IIRC).

-Eli