How to optimize out the duplicated memory load instructions?

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

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

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

`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.

Hi Johannes,

Silly as me. I just figured out how to correctly use ‘alias’ metadata. I should define them in IR like below:

!3 = !{!3}
!4 = !{!4}

!5 = !{!5, !3}
!6 = !{!6, !4}

And then use !5 and !6. The below usage is wrong:

!3 = !{!3}
!4 = !{!4}
Then use !3 and !4 in IR.

BR,
Terry

Hi Johannes,

Sorry for the typo. I meant ‘Silly me’.

BR,
Terry

I'm not sure if the proper use of the metadata yielded the expected result or not.