Is there any way to tell clang to generate TBAA metadata without other optimizations?

Question

LLVM contains an alias analysis named "type based alias analysis"(pass source code on github), which utilizes the !tbaa metadata embedded with the instruction. The metadata information is like:

!1 = metadata !{metadata !2, metadata !2, i64 0}
!2 = metadata !{metadata !"int", metadata !3, i64 0}
!3 = metadata !{metadata !"omnipotent char", metadata !4, i64 0}
!4 = metadata !{metadata !"Simple C/C++ TBAA"}

It seems that it requires the frontend like clang to generate the metadata information.

Compiling source code file without passing any optimization arguments(or -O0) cannot generate the above metadata. And till now I only find that I have to pass at least -O1 to clang to get these; the problem is that I don't expect other optimizations/transformations(e.g., instcombine) to happen(the passes used by clang -O1 is similar to the results presented by this answer).

Is there any way to achieve this goal?

I don't know a simple way, but one alternative is to enable -O1 then explicitly disable each option that -O1 enables — technosaurus, Dec 29 '14 at 10:19
@technosaurus you mean within clang? Can you tell how to disable that? — Hongxu Chen, Dec 29 '14 at 10:55
@technosaurus yes, i think that thread should be help. i'm trying using `-mllvm` option but still haven't found the way to enable tbaa. — Hongxu Chen, Dec 29 '14 at 11:25

score 2 · Answer 1 · answered May 18 '20 at 14:15

2

The answer is a bit out of date. I believe -disable-llvm-optzns is deprecated/removed. You can do this with the driver using something like this:

clang -O2 -Xclang -disable-llvm-passes -S -emit-llvm

answered May 18 '20 at 14:15

Nemanja Ivanovic

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score 1 · Accepted Answer · answered Jan 05 '15 at 21:33

1

I don't suggest it for production use, but you can do something like this:

clang -cc1 -O1 -no-struct-path-tbaa -disable-llvm-optzns foo.cpp -emit-llvm -o -

to take a look at the TBAA metadata (remove the -no-struct-path-tbaa if you want to look at that as well).

Otherwise you could modify clang pretty easily to always turn on TBAA metadata.

answered Jan 05 '15 at 21:33

echristo

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10
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Thanks, it works smoothly. I just wanna some experiments on LLVM TBAA for C and don't expect for production use, so that's enough for me. BTW, may I know your opinion on implementation of current LLVM type-based alias analysis? – Hongxu Chen Jan 07 '15 at 01:55
1

It's fine, the struct path stuff is a little interesting. It's not state of the art, but it's OK. – echristo Jan 07 '15 at 21:30

Is there any way to tell clang to generate TBAA metadata without other optimizations?

2 Answers2