llvm dev - Oct 2015 - How to create global symbol from record offset

I would like to create something like this (x86_64)

	.section	__DATA,__data
	.align	4
_a:
	.long	18

	.globl	_b
_b:
	.long	48


If you like to notice, there is no alignment between _a and _b.
_b is basically offseting into a record structure.


----

When I use two discrete structs, I get .aligns and I also don't trust 
the tools to keep the two globals together.


%struct.a = type { i32 }
%struct.b = type { i32 }

@a = global %struct.a { i32 1 }, align 4
@b = global %struct.b { i32 848 }, align 4


----

What I don't want is to introduce a global pointer like this

%struct.c = type { %struct.a, %struct.b }
%struct.a = type { i32 }
%struct.b = type { i32 }

@c = internal global %struct.c { %struct.a { i32 18 }, %struct.b { i32 
48 } }, align 4
@p_b = global %struct.b* bitcast (i32* getelementptr inbounds 
(%struct.c* @c, i64 0, i32 1, i32 0) to %struct.b*), align 8


Thanks for any hints how to do this.

Ciao
    Nat!

Would a global alias be sufficient for you?  E.g.

--
%struct.a = type { i32 }
%struct.b = type { i32 }
%struct.c = type { %struct.a, %struct.b }

@c = internal global %struct.c { %struct.a { i32 18 }, %struct.b { i32 48 } },
align 4

@a = alias %struct.a, %struct.a* getelementptr inbounds (%struct.c, %struct.c*
@c, i64 0, i32 0)
@b = alias %struct.b, %struct.b* getelementptr inbounds (%struct.c, %struct.c*
@c, i64 0, i32 1)
--

Gives

--
        .data
        .align  4                       # @c
c:
        .long   18                      # 0x12
        .long   48                      # 0x30


        .globl  a
a = c
        .globl  b
b = c+4
--

Where @a and @b can be referenced from code without introducing the extra level
of indirection:

--
declare void @bar(%struct.a *, %struct.b *)

define void @foo() {
  call void @bar(%struct.a* @a, %struct.b* @b)
  ret void
}

--

foo:                                    # @foo
.Ltmp0:
.seh_proc foo
# BB#0:
        subq    $40, %rsp
.Ltmp1:
        .seh_stackalloc 40
.Ltmp2:
        .seh_endprologue
        leaq    a(%rip), %rcx
        leaq    b(%rip), %rdx
        callq   bar
--



-----Original Message-----
From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On Behalf Of Nat! via
llvm-dev
Sent: Tuesday, October 27, 2015 7:39 AM
To: llvm-dev <llvm-dev at lists.llvm.org>
Subject: [llvm-dev] How to create global symbol from record offset

I would like to create something like this (x86_64)

	.section	__DATA,__data
	.align	4
_a:
	.long	18

	.globl	_b
_b:
	.long	48


If you like to notice, there is no alignment between _a and _b.
_b is basically offseting into a record structure.


----

When I use two discrete structs, I get .aligns and I also don't trust the
tools to keep the two globals together.


%struct.a = type { i32 }
%struct.b = type { i32 }

@a = global %struct.a { i32 1 }, align 4
@b = global %struct.b { i32 848 }, align 4


----

What I don't want is to introduce a global pointer like this

%struct.c = type { %struct.a, %struct.b }
%struct.a = type { i32 }
%struct.b = type { i32 }

@c = internal global %struct.c { %struct.a { i32 18 }, %struct.b { i32 
48 } }, align 4
@p_b = global %struct.b* bitcast (i32* getelementptr inbounds 
(%struct.c* @c, i64 0, i32 1, i32 0) to %struct.b*), align 8


Thanks for any hints how to do this.

Ciao
    Nat!



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I have a problem getting this implemented, the following small test 
program invariably crashes with.

Assertion failed: (isa<X>(Val) && "cast<Ty>() argument
of incompatible
type!"), function cast

Help would be greatly appreciated.
---
#include "llvm/IR/LLVMContext.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/Constants.h"


int main(int argc, char const *argv[])
{
    llvm::LLVMContext &Context = llvm::getGlobalContext();

    llvm::Constant *Fields[3];
    Fields[0] = llvm::ConstantInt::get(Context, llvm::APInt( 32, 18, true));
    Fields[1] = llvm::ConstantInt::get(Context, llvm::APInt( 32, 48, true));
    Fields[2] = llvm::ConstantInt::get(Context, llvm::APInt( 32, 1848, 
true));

    llvm::Type * FieldTypes[3];
    FieldTypes[0] = llvm::Type::getInt32Ty( Context);
    FieldTypes[1] = llvm::Type::getInt32Ty( Context);
    FieldTypes[2] = llvm::Type::getInt32Ty( Context);

    llvm::StructType *RecordType;
    RecordType = llvm::StructType::create(FieldTypes,"foo");

    llvm::Constant  *Foo;
    Foo = llvm::ConstantStruct::get( RecordType, Fields);

    llvm::Constant *Two;
    Two = llvm::ConstantInt::get( llvm::Type::getInt32Ty( Context), 2);

    llvm::Constant  *C;
    C = llvm::ConstantExpr::getGetElementPtr( RecordType, Foo, Two);

    return( 0);
}
---
clang -g -std=c++14 `llvm-config --cflags` -c foo.cpp
clang++ foo.o `llvm-config --cxxflags --ldflags --libs core 
--system-libs` -o foo