On 12/8/2021 1:00 AM, Iain Sandoe wrote:
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!
I think as the Darwin maintainer, you can just commit this once Jakub
commits his bits.
jeff
