Hi!
The following patch adds support for relocation of the PCH blob on PCH
restore if we don't manage to get the preferred map slot for it.
The GTY stuff knows where all the pointers are, after all it relocates
it once during PCH save from the addresses where it was initially allocated
to addresses in the preferred map slot.
But, if we were to do it solely using GTY info upon PCH restore, we'd need
another set of GTY functions, which I think would make it less maintainable
and I think it would also be more costly at PCH restore time. Those
functions would need to call something to add bias to pointers that haven't
been marked yet and make sure not to add bias to any pointer twice.
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.
The discovery of the pointers in the blob that need relocation is done
in the relocate_ptrs hook which does the pointer relocation during PCH save.
Unfortunately, I had to make one change to the gengtype stuff due to the
nested_ptr feature of GTY, which some libcpp headers and stringpool.c use.
The relocate_ptrs hook had 2 arguments, pointer to the pointer and a cookie.
When relocate_ptrs is done, in most cases it is called solely on the
subfields of the current object, so e.g.
if ((void *)(x) == this_obj)
op (&((*x).u.fld[0].rt_rtx), cookie);
so relocate_ptrs can assert that ptr_p is within the
state->ptrs[state->ptrs_i]->obj ..
state->ptrs[state->ptrs_i]->obj+state->ptrs[state->ptrs_i]->size-sizeof(void*)
range and compute from that the address in the blob which will need
relocation (state->ptrs[state->ptrs_i]->new_addr is the new address
given to it and ptr_p-state->ptrs[state->ptrs_i]->obj is the relative
offset. Unfortunately, for nested_ptr gengtype emits something like:
{
union tree_node * x0 =
((*x).val.node.node) ? HT_IDENT_TO_GCC_IDENT (HT_NODE
(((*x).val.node.node))) : NULL;
if ((void *)(x) == this_obj)
op (&(x0), cookie);
(*x).val.node.node = (x0) ? CPP_HASHNODE (GCC_IDENT_TO_HT_IDENT
((x0))) : NULL;
}
so relocate_ptrs is called with an address of some temporary variable and
so doesn't know where the pointer will finally be.
So, I've added another argument to relocate_ptrs (and to
gt_pointer_operator). For the most common case I pass NULL as the new middle
argument to that function, first one remains pointer to the pointer that
needs adjustment and last the cookie. The NULL seems to be cheap to compute
and short in the gt*.[ch] files and stands for ptr_p is an address within
the this_obj's range, remember its address. For the nested_ptr case, the
new middle argument contains actual address of the pointer that might need
to be relocated, so instead of the above
op (&(x0), &((*x).val.node.node), cookie);
in there. And finally, e.g. for the reorder case I need a way to tell
restore_ptrs to ignore a particular address for the relocation purposes
and only treat it the old way. I've used for that the case when
the first and second arguments are equal.
In order to enable support for mapping PCH as fallback at different
addresses than the preferred ones, a small change is needed to the
host pch_use_address hooks. One change I've done to all of them is
the change of the type of the first argument from void * to void *&,
such that the actual address can be told to the callers (or shall I
instead use void **?), but another change that still needs to be done
in them if they want the relocation is actually not fail if they couldn't
get a preferred address, but instead modify what the first argument
refers to. I've done that only for host-linux.c and Iain is testing
similar change for host-darwin.c. Didn't change hpux, netbsd, openbsd,
solaris, mingw32 or the fallbacks because I can't test those.
Bootstrapped/regtested on x86_64-linux and i686-linux (both as is and with
incremental
--- gcc/config/host-linux.c.jj 2021-12-06 22:22:42.007777367 +0100
+++ gcc/config/host-linux.c 2021-12-07 00:21:53.052674040 +0100
@@ -191,6 +191,8 @@ linux_gt_pch_use_address (void *&base, s
if (size == 0)
return -1;
+base = (char *) base + ((size + 8191) & (size_t) -4096);
+
/* Try to map the file with MAP_PRIVATE. */
addr = mmap (base, size, PROT_READ | PROT_WRITE, MAP_PRIVATE, fd, offset);
which forces all PCH restores to be relocated. An earlier version of the
patch has been also regrest with base = (char *) base + 16384; in that spot,
so both relocation to a non-overlapping spot and to an overlapping spot have
been tested.
2021-12-07 Jakub Jelinek <ja...@redhat.com>
PR pch/71934
* coretypes.h (gt_pointer_operator): Use 3 pointer arguments instead
of two.
* gengtype.c (struct walk_type_data): Add in_nested_ptr argument.
(walk_type): Temporarily set d->in_nested_ptr around nested_ptr
handling.
(write_types_local_user_process_field): Pass a new middle pointer
to gt_pointer_operator op calls, if d->in_nested_ptr pass there
address of d->prev_val[2], otherwise NULL.
(write_types_local_process_field): Likewise.
* ggc-common.c (relocate_ptrs): Add real_ptr_p argument. If equal
to ptr_p, do nothing, otherwise if NULL remember ptr_p's
or if non-NULL real_ptr_p's corresponding new address in
reloc_addrs_vec.
(reloc_addrs_vec): New variable.
(compare_ptr, read_uleb128, write_uleb128): New functions.
(gt_pch_save): When iterating over objects through relocate_ptrs,
save current i into state.ptrs_i. Sort reloc_addrs_vec and emit
it as uleb128 of differences between pointer addresses into the
PCH file.
(gt_pch_restore): Allow restoring of PCH to a different address
than the preferred one, in that case adjust global pointers by bias
and also adjust by bias addresses read from the relocation table
as uleb128 differences. Otherwise fseek over it. Perform
gt_pch_restore_stringpool only after adjusting callbacks and for
callback adjustments also take into account the bias.
(default_gt_pch_use_address): Change type of first argument from
void * to void *&.
(mmap_gt_pch_use_address): Likewise.
* ggc-tests.c (gt_pch_nx): Pass NULL as new middle argument to op.
* hash-map.h (hash_map::pch_nx_helper): Likewise.
(gt_pch_nx): Likewise.
* hash-set.h (gt_pch_nx): Likewise.
* hash-table.h (gt_pch_nx): Likewise.
* hash-traits.h (ggc_remove::pch_nx): Likewise.
* hosthooks-def.h (default_gt_pch_use_address): Change type of first
argument from void * to void *&.
(mmap_gt_pch_use_address): Likewise.
* hosthooks.h (struct host_hooks): Change type of first argument of
gt_pch_use_address hook from void * to void *&.
* machmode.h (gt_pch_nx): Expect a callback with 3 pointers instead of
two in the middle argument.
* poly-int.h (gt_pch_nx): Likewise.
* stringpool.c (gt_pch_nx): Pass NULL as new middle argument to op.
* tree-cfg.c (gt_pch_nx): Likewise, except for LOCATION_BLOCK pass
the same &(block) twice.
* value-range.h (gt_pch_nx): Pass NULL as new middle argument to op.
* vec.h (gt_pch_nx): Likewise.
* wide-int.h (gt_pch_nx): Likewise.
* config/host-darwin.c (darwin_gt_pch_use_address): Change type of
first argument from void * to void *&.
* config/host-darwin.h (darwin_gt_pch_use_address): Likewise.
* config/host-hpux.c (hpux_gt_pch_use_address): Likewise.
* config/host-linux.c (linux_gt_pch_use_address): Likewise. If
it couldn't succeed to mmap at the preferred location, set base
to the actual one. Update addr in the manual reading loop instead of
base.
* config/host-netbsd.c (netbsd_gt_pch_use_address): Change type of
first argument from void * to void *&.
* config/host-openbsd.c (openbsd_gt_pch_use_address): Likewise.
* config/host-solaris.c (sol_gt_pch_use_address): Likewise.
* config/i386/host-mingw32.c (mingw32_gt_pch_use_address): Likewise.
* config/rs6000/rs6000-gen-builtins.c (write_init_file): Pass NULL
as new middle argument to op in the generated code.
* doc/gty.texi: Adjust samples for the addition of middle pointer
to gt_pointer_operator callback.
gcc/ada/
* gcc-interface/decl.c (gt_pch_nx): Pass NULL as new middle argument
to op.
gcc/c-family/
* c-pch.c (c_common_no_more_pch): Pass a temporary void * var
with NULL value instead of NULL to host_hooks.gt_pch_use_address.
gcc/c/
* c-decl.c (resort_field_decl_cmp): Pass the same pointer twice
to resort_data.new_value.
gcc/cp/
* module.cc (nop): Add another void * argument.
* name-lookup.c (resort_member_name_cmp): Pass the same pointer twice
to resort_data.new_value.
