[LLVMdev] Redundant Add Operation in Code Generation?

[Ryan Taylor]

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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