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SubjectRe: [PATCH v2 4/8] objtool: add undwarf debuginfo generation
On Thu, Jun 29, 2017 at 04:46:18PM +0200, Ingo Molnar wrote:
>
> * Josh Poimboeuf <jpoimboe@redhat.com> wrote:
>
> > > Plus, shouldn't we use __packed for 'struct undwarf' to minimize the
> > > structure's size (to 6 bytes AFAICS?) - or is optimal packing of the main
> > > undwarf array already guaranteed on every platform with this layout?
> >
> > Ah yes, it should definitely be packed (assuming that doesn't affect performance
> > negatively).
>
> So if I count that correctly that should shave another ~1MB off a typical ~4MB
> table size?

Here's what my Fedora kernel looks like *before* the packed change:

$ eu-readelf -S vmlinux |grep undwarf
[15] .undwarf_ip PROGBITS ffffffff81f776d0 011776d0 0012d9d0 0 A 0 0 1
[16] .undwarf PROGBITS ffffffff820a50a0 012a50a0 0025b3a0 0 A 0 0 1

The total undwarf data size is ~3.5MB.

There are 308852 entries of two parallel arrays:

* .undwarf (8 bytes/entry) = 2470816 bytes
* .undwarf_ip (4 bytes/entry) = 1235408 bytes

If we pack undwarf, reducing the size of the .undwarf entries by two
bytes, it will save 308852 * 2 = 617704.

So the savings will be ~600k, and the typical size will be reduced to ~3MB.

--
Josh

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