[LLVMdev] Debug info for lazy variables triggers SROA assertion

[Duncan P. N. Exon Smith]

+aprantl

> On 2015-May-31, at 22:13, Kai Nacke <kai.nacke at redstar.de> wrote:
> 
> Hi!
> 
> I created a bug report (https://llvm.org/bugs/show_bug.cgi?id=23712) for this failure but then I realized that my approach may be wrong.
> 
> The following D source contains a lazy variable:
> 
> void bar(lazy bool val)
> {
>    val();
> }
> 
> The lazy variable val is translated to a delegate. The signature and the first IR lines are:
> 
> define void @_D7opover23barFLbZv({ i8*, i1 (i8*)* } %val_arg) #0 {
>  %val = alloca { i8*, i1 (i8*)* }, align 8
>  store { i8*, i1 (i8*)* } %val_arg, { i8*, i1 (i8*)* }* %val, !dbg !18
>  call void @llvm.dbg.declare(metadata { i8*, i1 (i8*)* }* %val, metadata !11, metadata !19), !dbg !18
>   ...
> }
> !11 = !DILocalVariable(tag: DW_TAG_arg_variable, name: "val", arg: 0, scope: !6, file: !4, line: 1, type: !12)
> !12 = !DIBasicType(name: "bool", size: 1, align: 8, encoding: DW_ATE_boolean)
> 
> The attached debug info says that val is of type bool (that's the view of the user).
> But it is represented by a structure containing a pointer to a frame and a pointer to a function.
> 
> Optimizing this code triggers an assertion:
> 
> piece is larger than or outside of variable
>  call void @llvm.dbg.value(metadata i8* %val_arg.fca.0.extract, i64 0, metadata !11, metadata !19), !dbg !18
> !11 = !DILocalVariable(tag: DW_TAG_arg_variable, name: "val", arg: 0, scope: !6, file: !4, line: 1, type: !12)
> !19 = !DIExpression(DW_OP_bit_piece, 0, 64)
> piece is larger than or outside of variable
>  call void @llvm.dbg.value(metadata i1 (i8*)* %val_arg.fca.1.extract, i64 0, metadata !11, metadata !20), !dbg !18
> !11 = !DILocalVariable(tag: DW_TAG_arg_variable, name: "val", arg: 0, scope: !6, file: !4, line: 1, type: !12)
> !20 = !DIExpression(DW_OP_bit_piece, 64, 64)
> LLVM ERROR: Broken module found, compilation aborted!
> 
> (SROA.cpp, line 4207)
> 
> As I understand it, the SROA pass splits the structure into 2 pieces and does the same for the debug info. This wont work because there is only one piece, resulting in the assertion failure.
> 
> Should I use another approach, or is this really a bug?
> 
> Regards,
> Kai
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