Michael137 wrote:
> > My understanding was that the DIExpression parameter to
> > DIGlobalVariableExpression was empty for global variables with locations.
> > So the patch just encodes the constant into that expression if it's
> > otherwise empty.
>
> I think in theory it can be non-empty (see what happens under a merge globals
> optimization... I'm not sure of an example of when this optimization fires,
> or what the debug info we emit looks like - in theory it should look like a
> global with a non-empty expression that describes offsetting the pointer
> forward to reach the global inside the merged global) & so then you'd have a
> hard time telling whether the expression is meant to be used in addition to
> the location, or as part of evaluating the location.
>
> We don't really have a mechanism for encoding a variable in two locations (or
> a constant and a location) at the same time, I think. We could invent a
> special opcode to use in the expression to communicate this, or define some
> special handling if there's two separate expressions providing a location (or
> a location and a constant in this case) for the same variable (say that
> they're both valid, and emit them both if we can).
>
> @adrian-prantl thoughts?
Thanks for the pointers. The closest I could get to triggering a GlobalMerge of
const static data-members was for following code:
```
struct Foo {
static const int a;
static const int b;
};
const int Foo::a = 100;
const int Foo::b = 200;
```
But only if I mark the constants as `internal` in the IR as follows (I attached
the whole file):
```
%struct.Foo = type { i8 }
@a = internal constant i32 100, align 4, !dbg !0
@b = internal constant i32 200, align 4, !dbg !5
define void @use1() {
%x = load i32, ptr @a, align 4
%y = load i32, ptr @b, align 4
ret void
}
...
```
Then run `opt` as:
```
./bin/opt -global-merge -global-merge-max-offset=100 -global-merge-on-const -S
merge.ll
```
That changes the IR into:
```
...
@_MergedGlobals = private constant <{ i32, i32 }> <{ i32 100, i32 200 }>, align
4, !dbg !0, !dbg !13
@a = internal alias i32, ptr @_MergedGlobals
@b = internal alias i32, getelementptr inbounds (<{ i32, i32 }>, ptr
@_MergedGlobals, i32 0, i32 1)
...
!1 = distinct !DIGlobalVariable(name: "a", linkageName: "_ZN3Foo1aE", scope:
!2, file: !3, line: 6, type: !7, isLocal: false, isDefinition: true,
declaration: !12)
...
!5 = !DIGlobalVariableExpression(var: !6, expr: !DIExpression())
!6 = distinct !DIGlobalVariable(name: "b", linkageName: "_ZN3Foo1bE", scope:
!2, file: !3, line: 7, type: !7, isLocal: false, isDefinition: true,
declaration: !9)
...
!13 = !DIGlobalVariableExpression(var: !6, expr:
!DIExpression(DW_OP_plus_uconst, 4))
```
https://github.com/llvm/llvm-project/pull/72730
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