llvm dev - Jun 2019 - Information Loss of Array Type in Function Interface in IR Generated by Clang

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 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
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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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<mailto: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
_______________________________________________
LLVM Developers mailing list
llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>
https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
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