Wilco Dijkstra via Gcc-patches <gcc-patches@gcc.gnu.org> writes:
> Hi Richard,
>
>> Just to check: I guess this part is an optimisation, because it
>> means that we can share the GOT entry with other TUs. Is that right?
>> I think it would be worth having a comment either way, whatever the
>> rationale. A couple of other very minor things:
>
> It's just to make the code simpler and more orthogonal - the size of the
> sequence and sharing of GOT/literal is identical in both cases.
Hmm, OK. I guess it makes things more consistent in that sense
(PIC vs. non-PIC). But on the other side it's making things less
internally consistent for non-PIC, since we don't use the GOT for
anything else there. I guess in principle there's a danger that a
custom *-elf linker script might not bother to set up the .got properly,
on the assumption that it shouldn't be needed.
But there's not much in it either way, and I agree consistency with
LLVM is a factor too.
> Here is v2 with comments/spaces fixed up:
>
>
> Use a GOT indirection for extern weak symbols instead of a literal - this is
> the same as
> PIC/PIE and mirrors LLVM behaviour. Ensure PIC/PIE use the same offset
> limits for symbols
> that don't use the GOT.
>
> Passes bootstrap and regress. OK for commit?
>
> ChangeLog:
> 2021-04-27 Wilco Dijkstra <wdijk...@arm.com>
>
> * config/aarch64/aarch64.c (aarch64_classify_symbol): Use GOT for
> extern weak symbols.
> Limit symbol offsets for non-GOT symbols with PIC/PIE.
OK, thanks.
Richard
>
> ---
>
> diff --git a/gcc/config/aarch64/aarch64.c b/gcc/config/aarch64/aarch64.c
> index
> 82957dddbe42a7f907b2960294ac7f8abf7be2ff..641c83b479e76cbcc75b299eb7ae5f634d9db7cd
> 100644
> --- a/gcc/config/aarch64/aarch64.c
> +++ b/gcc/config/aarch64/aarch64.c
> @@ -17854,7 +17854,14 @@ aarch64_classify_symbol (rtx x, HOST_WIDE_INT offset)
>
> switch (aarch64_cmodel)
> {
> + case AARCH64_CMODEL_TINY_PIC:
> case AARCH64_CMODEL_TINY:
> + /* With -fPIC non-local symbols use the GOT. For orthogonality
> + always use the GOT for extern weak symbols. */
> + if ((flag_pic || SYMBOL_REF_WEAK (x))
> + && !aarch64_symbol_binds_local_p (x))
> + return SYMBOL_TINY_GOT;
> +
> /* When we retrieve symbol + offset address, we have to make sure
> the offset does not cause overflow of the final address. But
> we have no way of knowing the address of symbol at compile time
> @@ -17862,42 +17869,30 @@ aarch64_classify_symbol (rtx x, HOST_WIDE_INT
> offset)
> symbol + offset is outside the addressible range of +/-1MB in the
> TINY code model. So we limit the maximum offset to +/-64KB and
> assume the offset to the symbol is not larger than +/-(1MB - 64KB).
> - If offset_within_block_p is true we allow larger offsets.
> - Furthermore force to memory if the symbol is a weak reference to
> - something that doesn't resolve to a symbol in this module. */
> -
> - if (SYMBOL_REF_WEAK (x) && !aarch64_symbol_binds_local_p (x))
> - return SYMBOL_FORCE_TO_MEM;
> + If offset_within_block_p is true we allow larger offsets. */
> if (!(IN_RANGE (offset, -0x10000, 0x10000)
> || offset_within_block_p (x, offset)))
> return SYMBOL_FORCE_TO_MEM;
>
> return SYMBOL_TINY_ABSOLUTE;
>
> +
> + case AARCH64_CMODEL_SMALL_SPIC:
> + case AARCH64_CMODEL_SMALL_PIC:
> case AARCH64_CMODEL_SMALL:
> + if ((flag_pic || SYMBOL_REF_WEAK (x))
> + && !aarch64_symbol_binds_local_p (x))
> + return aarch64_cmodel == AARCH64_CMODEL_SMALL_SPIC
> + ? SYMBOL_SMALL_GOT_28K : SYMBOL_SMALL_GOT_4G;
> +
> /* Same reasoning as the tiny code model, but the offset cap here is
> 1MB, allowing +/-3.9GB for the offset to the symbol. */
> -
> - if (SYMBOL_REF_WEAK (x) && !aarch64_symbol_binds_local_p (x))
> - return SYMBOL_FORCE_TO_MEM;
> if (!(IN_RANGE (offset, -0x100000, 0x100000)
> || offset_within_block_p (x, offset)))
> return SYMBOL_FORCE_TO_MEM;
>
> return SYMBOL_SMALL_ABSOLUTE;
>
> - case AARCH64_CMODEL_TINY_PIC:
> - if (!aarch64_symbol_binds_local_p (x))
> - return SYMBOL_TINY_GOT;
> - return SYMBOL_TINY_ABSOLUTE;
> -
> - case AARCH64_CMODEL_SMALL_SPIC:
> - case AARCH64_CMODEL_SMALL_PIC:
> - if (!aarch64_symbol_binds_local_p (x))
> - return (aarch64_cmodel == AARCH64_CMODEL_SMALL_SPIC
> - ? SYMBOL_SMALL_GOT_28K : SYMBOL_SMALL_GOT_4G);
> - return SYMBOL_SMALL_ABSOLUTE;
> -
> case AARCH64_CMODEL_LARGE:
> /* This is alright even in PIC code as the constant
> pool reference is always PC relative and within
