llvm dev - Jan 2019 - DIGlobalVariableExpression doc in langref

Hi,

I don't find DIGlobalVariableExpression in langref. Should it be
documented? Could anybody explain what it is?

https://llvm.org/docs/LangRef.html

In one ll file converted from bitcode, I mostly see something like this.

!8273 = !DIGlobalVariableExpression(var: !8274, expr: !DIExpression())

Only one looks like this.

!8945 = !DIGlobalVariableExpression(var: !8946, expr:
!DIExpression(DW_OP_constu, 44, DW_OP_stack_value))

Why DIExpression is mostly empty?

-- 
Regards,
Peng

Adrian - looks like LangRef didn't get updated when
DIGlobalVariableExpression was introduced - could you add a little info
there?

Peng - yeah, looks like the documentation is a bit out of date, no worries.

DIGlobalVariableExpressions are referenced from LLVM global variables, eg:

@x = dso_local global i32 3, align 4, !dbg !0
!0 = !DIGlobalVariableExpression(var: !1, expr: !DIExpression())

One where there is an empty DIExpression means that the underlying LLVM IR
global describes the whole source level global variable - the value doesn't
need to be modified at all to represent the source variable.

If an expression is involved, that expression (possibly along with the
underlying LLVM IR global variable) is used to compute the value of the
source level global variable. (in the example you have - there may not be
any LLVM IR global variable at all - just the expression computing the
constant value of the source level global variable - but in other cases,
say if a big constant that only holds a few different values is optimized
into a lookup table (rather than storing a big integer, just store 0 or 1,
if it's 0 that means teh value is XXXXXXX, if the value is 1 that means the
value is YYYYYY) it might be a combination, using the LLVM IR global
variable to compute the source level value)

On Sat, Jan 19, 2019 at 6:25 PM Peng Yu via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> Hi,
>
> I don't find DIGlobalVariableExpression in langref. Should it be
> documented? Could anybody explain what it is?
>
> https://llvm.org/docs/LangRef.html
>
> In one ll file converted from bitcode, I mostly see something like this.
>
> !8273 = !DIGlobalVariableExpression(var: !8274, expr: !DIExpression())
>
> Only one looks like this.
>
> !8945 = !DIGlobalVariableExpression(var: !8946, expr:
> !DIExpression(DW_OP_constu, 44, DW_OP_stack_value))
>
> Why DIExpression is mostly empty?
>
> --
> Regards,
> Peng
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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Hi David,
> If an expression is involved, that expression (possibly along with the
underlying LLVM IR global variable) is used to compute the value of the source
level global variable. (in the example you have - there may not be any LLVM IR
global variable at all - just the expression computing the constant value of the
source level global variable - but in other cases, say if a big constant that
only holds a few different values is optimized into a lookup table (rather than
storing a big integer, just store 0 or 1, if it's 0 that means teh value is
XXXXXXX, if the value is 1 that means the value is YYYYYY) it might be a
combination, using the LLVM IR global variable to compute the source level
value)
Do you have an example minimal .c file that can demonstrate this in
action? This will make it easier to understand what you mean.

-- 
Regards,
Peng

> On Jan 20, 2019, at 1:52 PM, David Blaikie <dblaikie at gmail.com>
wrote:
> 
> Adrian - looks like LangRef didn't get updated when
DIGlobalVariableExpression was introduced - could you add a little info there?
Done in r351837.

-- adrian