Hi Tamar,
> On 5 Oct 2026, at 13:04, Tamar Christina <[email protected]> wrote:
>
> The fix for PR123457 to fix PCH was only partial. While it added the table
> pointers to GTY so they're tracked as roots, it didn't fix the pointers of the
> nested classes that are used as hashes for the object.
>
> This leads to the PCH tests all still crashing when the toolchain is
> configured
> with -PIE.
>
> There are three pointers being used in function_instance::hash ()
>
> const char *base_name;
> const function_base *base;
> const function_shape *shape;
>
> These points when PIE and PCH will be loaded into different offsets by each
> invocation of the compiler sharing the same PCH file since these pointer point
> into the compiler's executable and are no objects allocated by GGC.
>
> The documentation states that GTY((callback)) can only be used for function
> pointers so can't be used for data pointers. The base class is marked
> GTY((user)) anyway. The easiest fix is dropping the pointers from the
> calculation of the hash, however this weakens the hash function.
>
> Instead this patch registers the pointers with the walkers through
> gt_pch_note_callback and then instead of using the pointers in the hash it
> uses
> the stable ptr diff between an anchor and the pointer.
>
> The idea is that as the executable gets loaded into different addresses, since
> PCH enforced that it must be the exact same version of the binary that the
> offsets are constant. So by storing the ptr - offset we get a stable "hash"
> ID.
>
> The anchor here is chosen to be the function_table itself which is being
> relocated by PCH.
>
> Bootstrapped Regtested on aarch64-none-linux-gnu with and
> wihtout --enable-host-pie and no issues.
>
> Ok for master? and backport to active all branches?
>
Ok. Give a week or two on master for the PCH builds to verify before back
porting.
Thanks,
Kyrill
> Thanks,
> Tamar
>
> gcc/ChangeLog:
>
> PR target/20957
> * config/aarch64/aarch64-acle-builtins.cc (function_instance::hash):
> Use ptr diff for hash instead of ptr.
> (gt_pch_nx): Add the pointers to PCH tracking.
>
> ---
> diff --git a/gcc/config/aarch64/aarch64-acle-builtins.cc
> b/gcc/config/aarch64/aarch64-acle-builtins.cc
> index
> 5a063fb46ebbc6818c6db00781799c0fc1a5c79a..9841ae941685db577421081cb9eb360a21cbcbf6
> 100644
> --- a/gcc/config/aarch64/aarch64-acle-builtins.cc
> +++ b/gcc/config/aarch64/aarch64-acle-builtins.cc
> @@ -456,8 +456,10 @@ function_instance::hash () const
> {
> inchash::hash h;
> /* BASE uniquely determines BASE_NAME, so we don't need to hash both. */
> - h.add_ptr (base);
> - h.add_ptr (shape);
> + /* FUNCTION_TABLE has a fixed address relative to BASE and SHAPE within
> + the compiler binary, even if PIE changes the binary's load address. */
> + h.add_hwi ((uintptr_t) base - (uintptr_t) &function_table);
> + h.add_hwi ((uintptr_t) shape - (uintptr_t) &function_table);
> h.add_int (mode_suffix_id);
> h.add_int (type_suffix_ids[0]);
> h.add_int (type_suffix_ids[1]);
> @@ -4681,8 +4683,14 @@ gt_pch_nx (function_instance *)
> }
>
> inline void
> -gt_pch_nx (function_instance *, gt_pointer_operator, void *)
> -{
> +gt_pch_nx (function_instance *instance, gt_pointer_operator, void *)
> +{
> + /* The instance is the first member of its containing GC object. PCH
> + adjusts these pointers if the compiler has a different PIE load base.
> */
> + static_assert (offsetof (registered_function, instance) == 0);
> + gt_pch_note_callback (&instance->base_name, instance);
> + gt_pch_note_callback (&instance->base, instance);
> + gt_pch_note_callback (&instance->shape, instance);
> }
>
> #include "gt-aarch64-acle-builtins.h"
>
>
> --
> <rb20957.patch>
