[llvm-dev] DWARFv5 alignment attribute

Victor Leschuk via llvm-dev llvm-dev at lists.llvm.org
Thu Sep 8 01:07:53 PDT 2016

Hello all, I am currently implementing support for DWARFv5 
DW_AT_alignment attribute 
(http://www.dwarfstd.org/ShowIssue.php?issue=140528.1). I am not 100% 
sure which entities we should mark with this attr.

Consider the following C11 code (sample from 

// every object of type struct data will be aligned to 128-byte boundary
struct data {
char x;
_Alignas(128) char cacheline[128]; // over-aligned array of char,
// not array of over-aligned chars

int main(void)
_Alignas(2048) struct data d; // this instance of data is aligned even 
return 0;

What should be marked as DW_AT_alignment? The link above says that the 
attribute can be applied to:

DW_TAG_array_type, DW_TAG_atomic_type, DW_TAG_base_type, 
DW_TAG_class_type, DW_TAG_coarray_type, DW_TAG_const_type, 
DW_TAG_dynamic_type, DW_TAG_enumeration_type, DW_TAG_file_type, 
DW_TAG_interface_type, DW_TAG_packed_type, DW_TAG_pointer_type, 
DW_TAG_pointer_to_member_type, DW_TAG_reference_type, 
DW_TAG_restrict_type, DW_TAG_rvalue_reference_type, DW_TAG_set_type, 
DW_TAG_shared_type, DW_TAG_string_type, DW_TAG_structure_type, 
DW_TAG_subprogram, DW_TAG_subrange_type, DW_TAG_subroutine_type, 
DW_TAG_thrown_type, DW_TAG_typedef, DW_TAG_union_type, DW_TAG_variable

  Here we have

* DW_TAG_array_type
* DW_TAG_subrange_type
* DW_TAG_member
* DW_TAG_structure_type
* DW_TAG_basic_type
* DW_TAG_variable

Currently I mark only variable "d" as aligned, however I am not sure, 
that this is correct? Which of the listed should contain this attribute? 
What do you think?


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