[LLVMdev] Redundant Add Operation in Code Generation?

[Ryan Taylor]

Ok, thanks.

Even still though I would expect -instcombine (run after lsr) would do this
cleanup?

On Fri, Oct 19, 2012 at 2:41 PM, Andrew Trick <atrick at apple.com> wrote:

>
> On Oct 19, 2012, at 2:34 PM, Ryan Taylor <ryta1203 at gmail.com> wrote:
>
> That solves the issue but it seems odd to me that instcombine doesn't take
> care of it?
>
>
> LSR is part of the backend. It's lowering the IR for a specific target. It
> seems to think those redundant operations are good for reducing register
> pressure, but doesn't actually have much knowledge about register pressure.
> At this point, we won't do any more IR level "cleanup" since that tends to
> undo lowering. The Machine IR passes will do some careful cleanup.
>
> -Andy
>
> So is this just a setup for the backend? If not, seems like if there is a
> possibility that lsr could create these redundant operations, should it not
> clean itself up? Or am I mistaken?
>
> On Fri, Oct 19, 2012 at 1:29 PM, Andrew Trick <atrick at apple.com> wrote:
>
>>
>> On 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?
>>
>>
>> That's LSR, as you can see from the variable names ;) It might think that
>> load(Base + Index) is a legal addressing mode for your target. -disable-lsr
>> might be the right thing for you anyway.
>>
>> Incidentally, MachineCSE could clean this up if it doesn't get folded
>> into the address, but like LSR, it tries hard not to increase register
>> pressure.
>>
>> -Andy
>>
>
>
>
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