[cfe-users] fetch variable names through debugInfo attached to load instructors

[David Blaikie via cfe-users]

On Tue, May 25, 2021 at 1:13 PM Bella V <bellavistaghome at gmail.com> wrote:

> Thanks! It works.
>
> One sub-question: How do we analyze the member dereferenced struct
> variables as dbg.declares only has the declaration info as inside the
> function the struct variable types could have members dereferenced as well.
>
> Example:
>
> https://godbolt.org/z/zaeYn9do7
>
> dbg.declares have use_var info, i would like to fetch address.id as well.
>

You'd have to do some sort of alias analysis, I guess - seeing that the
load is from a pointer derived from the pointer that the alloca/dbg.declare
describes - analyzing what offset from that is loaded, using the DI
metadata to figure out which field that offset corresponds to.

In general optimizations shouldn't try to do this - optimizations should be
based on the IR alone, without attempting to be aware of the source
constructs used to create them.
For source analysis - the IR isn't a great tool for that, for the sort of
reasons you're seeing here, and the analysis should probably be written in
terms of Clang's AST instead.

- Dave


>
>
> On Thu, May 20, 2021 at 12:25 PM David Blaikie <dblaikie at gmail.com> wrote:
>
>> You'd have to analyze the dbg.declares and track that they refer to the
>> same thing as the geps/loads you're interested in - from the dbg.declares
>> (& dbg.values) you can follow those to find the variables they refer to.
>>
>> On Thu, May 20, 2021 at 12:18 PM Bella V via cfe-users <
>> cfe-users at lists.llvm.org> wrote:
>>
>>> Hello All,
>>>
>>> I read the documentation and I was able to fetch the variable names and
>>> source locations attached to metadata for call instructions
>>> (@llvm.dbg.declare) using dyn_cast<DbgVariableIntrinsic>. I wanted to check
>>> if we could fetch the variable names in the load/gep instructors using
>>> debugInfo. I want to collect the use variable object details within the
>>> function.
>>>
>>>
>>>
>>> I could only fetch the source locations (not the variable names) using
>>> !dbg attached to the load instructions. Any pointers if that is allowed.
>>>
>>>
>>> *C code:*
>>>
>>> int foo (bar_t *b, int len)
>>>
>>> {
>>>
>>>     if (b == (bar_t *)NULL) {
>>>
>>>         return 0;
>>>
>>>     }
>>>
>>>     return -1;
>>>
>>> }
>>>
>>>
>>>
>>> *IR optimised code through LTO:*
>>>
>>> ; Function Attrs: noinline nounwind optnone uwtable
>>>
>>> define i32 @foo(%struct.bar_t*, i32) #0 !dbg !10 {
>>>
>>>   %3 = load i64, i64* getelementptr inbounds ([1 x i64], [1 x i64]
>>>
>>>   %4 = add i64 %3, 1
>>>
>>>   %5 = alloca i32, align 4
>>>
>>>   %6 = alloca % struct.bar_t*, align 8
>>>
>>>   %7 = alloca i32, align 4
>>>
>>>   store % struct.bar_t* %0, % struct.bar_t** %6, align 8
>>>
>>>   call void @llvm.dbg.declare(metadata % struct.bar_t** %6, metadata
>>> !11, metadata !DIExpression()), !dbg !12
>>>
>>>   store i32 %1, i32* %7, align 4
>>>
>>>   call void @llvm.dbg.declare(metadata i32* %7, metadata !12, metadata
>>> !DIExpression()), !dbg !14
>>>
>>>   %8 = load % struct.bar_t*, % struct.bar_t** %6, align 8, !dbg !15
>>>
>>>   %9 = icmp eq % struct.bar_t* %8, null, !dbg !16
>>>
>>>   br i1 %9, label %10, label %11, !dbg !17
>>>
>>>
>>>
>>> Many Thanks.
>>> _______________________________________________
>>> cfe-users mailing list
>>> cfe-users at lists.llvm.org
>>> https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-users
>>>
>>
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