dblaikie added a subscriber: JDevlieghere.
dblaikie added a comment.
In D71451#1793828 <https://reviews.llvm.org/D71451#1793828>, @Jac1494 wrote:
> > Am I reading this right that the data would suggest that enabling this
> > feature /reduces/ the size of debug info sections? That doesn't sound right
> > - can you explain why that would be the case? (perhaps the data is
> > incorrect/it wasn't built with -fstandalone-debug?)
>
> Hi @dblaikie sorry for incorrect data.
>
> Updated result are as per below:
>
> Without "-fstandalone-debug" option :-
> $size -A -d build_debug_withoutstandalone/bin/clang-10
> ...
> ...
> .comment 159 0
> .debug_str 2655675 0
> .debug_loc 941 0
> .debug_abbrev 10761 0
> .debug_info 1526674 0
> .debug_ranges 46672 0
> .debug_line 149807 0
> .note.gnu.gold-version 28 0
> Total 4786206
>
> With "-fstandalone-debug" option :-
> $size -A -d build_debug_withstandalone/bin/clang-10
> ...
> ...
> .comment 159 0
> .debug_str 3997839 0
> .debug_loc 941 0
> .debug_abbrev 12746 0
> .debug_info 2225392 0
> .debug_ranges 46672 0
> .debug_line 153779 0
> .note.gnu.gold-version 28 0
> Total 6833045
Ah, that looks more like what I'd expect - so that's a 42% increase in the
final linked binary size? Yeah, that seems somewhat unlikely to be what anyone
would want to enable, though I could be wrong (@aprantl @JDevlieghere - you
folks use standalone-debug by default, have any opinions on this proposed
change to standalone-debug behavior?)
Could you audit some files & see what sort of things are going in there? It
might be that a more nuanced definition of "used" is needed.
For instance, does this global variable get emitted?
extern int i;
inline int f1() {
return i;
}
void f2() {
}
'i' is used according to the AST, but it's not used even statically by this
translation unit - f1 is never called here. So if this patch currently emits a
declaration of 'i' then it shows one way there's some room for improvement -
specifically I'd motivate the declaration emission by the IR usage itself -
does the LLVM IR end up with a declaration of 'i' in it (for the purposes of
loading/storing to it, etc) then add a declaration. This won't catch all cases
(eg: "extern const int i = 7; int f() { int x[i]; ... }" - there won't be any
use of 'i' in the IR in that case), but would potentially be a workable
tradeoff for now. Catching the "use of a global in a constant context" is part
of a much broader challenge - to get that and some other existing cases like it
right, we'd need some kind of reachability analysis of usage. That analysis
could benefit other areas of debug info too, but is probably a lot more work.
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D71451/new/
https://reviews.llvm.org/D71451
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