[llvm-dev] How to optimize out the duplicated memory load instructions?

[Johannes Doerfert via llvm-dev]

`noalias` metadata is more complex than this:

https://llvm.org/docs/LangRef.html#noalias-and-alias-scope-metadata

I doubt the below was accepted by the AliasAnalysis as valid annotations 
and consequently ignored.


On 7/23/20 10:48 AM, Terry Guo wrote:
> Hi Johannes,
>
> Thanks for your help. I tried with something like below and nothing
> changes. Maybe I am doing something wrong?
>
> 246001 check_exce_succ59:                                ; preds =
> %check_exce_succ40
> **246002   %mem_base60 = load i8*, i8** %mem_base_addr_ptr, align 8,
> !alias.scope !3
> 246003   %offset161 = add i32 %call56, 4
> 246004   %12 = sext i32 %offset161 to i64
> 246005   %maddr62 = getelementptr inbounds i8, i8* %mem_base60, i64 %12
> 246006   %data_ptr63 = bitcast i8* %maddr62 to i64*
> **246007   store i64 0, i64* %data_ptr63, align 1, !noalias !3
> **246008   %mem_base66 = load i8*, i8** %mem_base_addr_ptr, align 8,
> !alias.scope !3
> 246009   %offset167 = add i32 %call56, 12
> 246010   %13 = sext i32 %offset167 to i64
> ....
> 382681 !3 = !{!3}
> 382682 !4 = !{!4}
>
> BR,
> Terry
>
> On Thu, Jul 23, 2020 at 11:43 PM Johannes Doerfert <
> johannesdoerfert at gmail.com> wrote:
>
>> Hi Terry,
>>
>>
>> various LLVM passes that run with O3 would do this for you, assuming
>> they could prove correctness.
>>
>> FWIW, the address is the same, so it is (most likely) an aliasing
>> problem. I assume
>>
>>     store i64 0, i64* %data_ptr63, align 8
>>
>> might clobber the loaded location so we do not reuse the first load. You
>> use alias metadata in IR or
>>
>> `noalias` (in IR) [= `restrict` (in C/C++)] attributes for arguments to
>> help the alias analysis.
>>
>>
>> Is this what you were looking for?
>>
>>
>> ~ Johannes
>>
>>
>> On 7/23/20 10:25 AM, Terry Guo via llvm-dev wrote:
>>> Hi there,
>>>
>>> The raw input IR is composed by my tool and the below one is produced by
>>> llvm opt tool with O3 optimization level. I am pretty sure that the two
>>> load instructions( %mem_base60  and  %mem_base66) are referring to the
>> same
>>> memory location. How can I instruct llvm optimization pass to optimize
>> out
>>> the  %mem_base66 and make the subsequent code reuse the  %mem_base60? I
>>> tried with metadata 'alias.scope' and 'noalias' and it doesn't help.
>> Thanks
>>> for any advice.
>>>
>>> 392542 check_exce_succ59:                                ; preds =
>>> %check_exce_succ40
>>> **392543   %mem_base60 = load i8*, i8** %mem_base_addr_ptr, align 8
>>> 392544   %offset161 = add i32 %call56, 4
>>> 392545   %11 = sext i32 %offset161 to i64
>>> 392546   %maddr62 = getelementptr inbounds i8, i8* %mem_base60, i64 %11
>>> 392547   %data_ptr63 = bitcast i8* %maddr62 to i64*
>>> 392548   store i64 0, i64* %data_ptr63, align 8
>>> **392549   %mem_base66 = load i8*, i8** %mem_base_addr_ptr, align 8
>>> 392550   %offset167 = add i32 %call56, 12
>>> 392551   %12 = sext i32 %offset167 to i64
>>> 392552   %maddr68 = getelementptr inbounds i8, i8* %mem_base66, i64 %12
>>> 392553   %data_ptr69 = bitcast i8* %maddr68 to i32*
>>> 392554   store i32 %call, i32* %data_ptr69, align 8
>>>
>>> BR,
>>> Terry
>>>
>>>
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