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

[Tingyuan LIANG via llvm-dev]

Dear David,

    Thanks for your detailed explanation! I get it!
    ^_^

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 1:51 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

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<mailto: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<mailto:dblaikie at gmail.com>>
Sent: Sunday, June 30, 2019 1:40 PM
To: Tingyuan LIANG
Cc: llvm-dev at lists.llvm.org<mailto: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
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