> On 7 Dec 2021, at 14:50, Jakub Jelinek via Gcc-patches <gcc-patches@gcc.gnu.org> wrote:
> 
> On Tue, Dec 07, 2021 at 10:55:07AM +0100, Jakub Jelinek via Gcc-patches wrote:
>> So, this patch instead builds a relocation table (sorted list of addresses
>> in the blob which needs relocation) at PCH save time, stores it in a very
>> compact form into the gch file and upon restore, adjusts pointers in GTY
>> roots (that is right away in the root structures) and the addresses in the
>> relocation table.
>> The cost on stdc++.gch/O2g.gch (previously 85MB large) is about 3% file size
>> growth, there are 2.5 million pointers that need relocation in the gch blob
>> and the relocation table uses uleb128 for address deltas and needs ~1.01 bytes
>> for one address that needs relocation, and about 20% compile time during
>> PCH save (I think it is mainly because of the need to qsort those 2.5
>> million pointers).  On PCH restore, if it doesn't need relocation (the usual
>> case), it is just an extra fread of sizeof (size_t) data and fseek
>> (in my tests real time on vanilla tree for #include <bits/stdc++.h> CU
>> was ~0.175s and with the patch but no relocation ~0.173s), while if it needs
>> relocation it took ~0.193s, i.e. 11.5% slower.
> 
> I'll note that without PCH that
> #include <bits/stdc++.h>
> int i;
> testcase compiles with -O2 -g in ~1.199s, i.e. 6.2 times slower than PCH with
> relocation and 6.9 times than PCH without relocation.

I’ve run tests across the Darwin range, including old and new m32 hosts, and this
seems to be working well.

The attached patch should be applied before (or merged with) the change for
relocation when it is applied - since the operation of the PCH hooks needs some
adjustment on Darwin.

thanks for working on this!
Iain