https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67293
Bug ID: 67293
Summary: Very large DW_AT_const_value produced
Product: gcc
Version: 6.0
Status: UNCONFIRMED
Keywords: memory-hog, missed-optimization
Severity: normal
Priority: P3
Component: debug
Assignee: unassigned at gcc dot gnu.org
Reporter: rguenth at gcc dot gnu.org
CC: jakub at gcc dot gnu.org
Target Milestone: ---
There seems to be no limit on the size of DW_AT_const_value attributes we
generate:
static const int a[1000000] = { 1, [999999] = 1 };
int main() {}
with -O0 -g we produce
<1><6b>: Abbrev Number: 7 (DW_TAG_variable)
<6c> DW_AT_name : a
<6e> DW_AT_decl_file : 1
<6f> DW_AT_decl_line : 1
<70> DW_AT_type : <0x7e>
<74> DW_AT_location : 9 byte block: 3 0 0 0 0 0 0 0 0 (DW_OP_addr: 0)
while with -O -g we end up with
<1><6b>: Abbrev Number: 7 (DW_TAG_variable)
<6c> DW_AT_name : a
<6e> DW_AT_decl_file : 1
<6f> DW_AT_decl_line : 1
<70> DW_AT_type : <0x3d0978>
<74> DW_AT_const_value : 4000000 byte block: 1 0 0 0 0 0 0 0 0 0 0 0 ....
....
I think that's hardly useful nor very friendly to consumers.
We also fail to take the opportunity to represent the not optimized out
constant value in the DW_AT_const_value form if it is shorter:
static const int a = 1;
int main() {}
produces
<1><51>: Abbrev Number: 4 (DW_TAG_variable)
<52> DW_AT_name : a
<54> DW_AT_decl_file : 1
<55> DW_AT_decl_line : 1
<56> DW_AT_type : <0x5b>
<5a> DW_AT_const_value : 1
<1><5b>: Abbrev Number: 5 (DW_TAG_const_type)
when optimized out (1 byte for the const value), compared to
<1><51>: Abbrev Number: 4 (DW_TAG_variable)
<52> DW_AT_name : a
<54> DW_AT_decl_file : 1
<55> DW_AT_decl_line : 1
<56> DW_AT_type : <0x64>
<5a> DW_AT_location : 9 byte block: 3 0 0 0 0 0 0 0 0 (DW_OP_addr: 0)
<1><64>: Abbrev Number: 5 (DW_TAG_const_type)
when not optimized out (10 bytes on 64bit archs, 6 bytes on 32bit archs).
I am arriving here because with early LTO debug I'd like to output more
DW_AT_const_value early (before we know whether the decl will go away).
I think the size optimization is "obvious" (maybe to some extend even factor
in relocation cost, thus allow bigger than 10 or 6 byte constants, like
arbitrary non-aggregate constants), a limit on the maximum size should very
likely be controlled by a --param.
