[llvm-dev] Information Loss of Array Type in Function Interface in IR Generated by Clang

[David Blaikie via llvm-dev]

There isn't any option to do that - because it doesn't model the language
accurately (well, ultimately, one of these days, we'll get rid of pointer
types (everything will just be "pointer" (sort of like C's 'void*') - not
specific to the thing it points to) at which point none of this will ever
be represented in the IR) - the C language ignores the array portion (so
you can pass this function a pointer to a single int, or 3 ints, not
exactly 51 ints) entirely, degrading it to a pointer as far as semantics
are concerned.

On Sat, Jun 29, 2019 at 10:48 PM Tingyuan LIANG <tliang at connect.ust.hk>
wrote:

> Dear David,
>
>     Thanks for your prompt reply!
>     Sure, I can implement a AST visitor to go through the AST to get the
> information but I just wonder whether there is any other way to let Clang
> do so.
>     What I am considering is how to let the generated IR looks like below,
> which some tools realize:
>
> define dso_local i32 @_Z1fPii([51 x i32]* %A, i32 %x) local_unnamed_addr
> #0 !dbg !7 {
> entry:
> ...
> }
>
> Best regards,
> ------------------------------------------
> Tingyuan LIANG
> MPhil Student
> Department of Electronic and Computer Engineering
> The Hong Kong University of Science and Technology
> ------------------------------
> *From:* David Blaikie <dblaikie at gmail.com>
> *Sent:* Sunday, June 30, 2019 1:40 PM
> *To:* Tingyuan LIANG
> *Cc:* llvm-dev at lists.llvm.org
> *Subject:* Re: [llvm-dev] Information Loss of Array Type in Function
> Interface in IR Generated by Clang
>
> LLVM IR doesn't maintain a lot of information present in the source. What
> were you hoping to do with that information? Perhaps you'd be best off
> doing something up in clang/using the AST uinstead of LLVM IR?
>
> On Sat, Jun 29, 2019 at 10:36 PM Tingyuan LIANG via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
> Dear all,
>
>     Hi! Recently, I notice a situation where I cannot infer the size of
> the outermost dimension of array in the function interface.
>     To concretely depict the problem, I show the C source code and the
> generated IR code at the end. The array size of A[] is 51 but this
> information is lost in the generated IR.
>     How can I maintain such information in IR? Should I set some argument
> for Clang so it can do so?
>     The Clang command I used is :
>
>      clang -O1 -emit-llvm -S -g tmp.cc -o tmp.bc
>
>     Thanks in advance for your time and suggestion! ^_^
>
> C source code:
> int f ( int A[51], int x)
> {
>     return A[x];
> }
>
> ===========================
> generated IR:
> ; Function Attrs: norecurse nounwind readonly uwtable
> define dso_local i32 @_Z1fPii(i32* nocapture readonly %A, i32 %x)
> local_unnamed_addr #0 !dbg !7 {
> entry:
>   call void @llvm.dbg.value(metadata i32* %A, metadata !13, metadata
> !DIExpression()), !dbg !15
>   call void @llvm.dbg.value(metadata i32 %x, metadata !14, metadata
> !DIExpression()), !dbg !16
>   %idxprom = sext i32 %x to i64, !dbg !17
>   %arrayidx = getelementptr inbounds i32, i32* %A, i64 %idxprom, !dbg !17
>   %0 = load i32, i32* %arrayidx, align 4, !dbg !17, !tbaa !18
>   ret i32 %0, !dbg !22
> }
>
>
> Best regards,
> ------------------------------------------
> Tingyuan LIANG
> MPhil Student
> Department of Electronic and Computer Engineering
> The Hong Kong University of Science and Technology
> _______________________________________________
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> llvm-dev at lists.llvm.org
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>
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