> "Jose E. Marchesi" <jose.march...@oracle.com> writes:
>> Hi Richard.
>> Thanks for looking at this! :)
>>
>>
>>> "Jose E. Marchesi" <jose.march...@oracle.com> writes:
>>>> ping
>>>
>>> I don't know this code very well, and have AFAIR haven't worked
>>> with an assembler that requires external declarations, but since
>>> it's at a second ping :)
>>>
>>>>
>>>>> ping
>>>>>
>>>>>> [Differences from V1:
>>>>>> - Prototype for call_from_call_insn moved before comment block.
>>>>>> - Reuse the `call' flag for SYMBOL_REF_LIBCALL.
>>>>>> - Fallback to check REG_CALL_DECL in non-direct calls.
>>>>>> - New test to check correct behavior for non-direct calls.]
>>>>>>
>>>>>> There are many places in GCC where alternative local sequences are
>>>>>> tried in order to determine what is the cheapest or best alternative
>>>>>> to use in the current target. When any of these sequences involve a
>>>>>> libcall, the current implementation of emit_library_call_value_1
>>>>>> introduce a side-effect consisting on emitting an external declaration
>>>>>> for the funcall (such as __divdi3) which is thus emitted even if the
>>>>>> sequence that does the libcall is not retained.
>>>>>>
>>>>>> This is problematic in targets such as BPF, because the kernel loader
>>>>>> chokes on the spurious symbol __divdi3 and makes the resulting BPF
>>>>>> object unloadable. Note that BPF objects are not linked before being
>>>>>> loaded.
>>>>>>
>>>>>> This patch changes emit_library_call_value_1 to mark the target
>>>>>> SYMBOL_REF as a libcall. Then, the emission of the external
>>>>>> declaration is done in the first loop of final.cc:shorten_branches.
>>>>>> This happens only if the corresponding sequence has been kept.
>>>>>>
>>>>>> Regtested in x86_64-linux-gnu.
>>>>>> Tested with host x86_64-linux-gnu with target bpf-unknown-none.
>>>
>>> I'm not sure that shorten_branches is a natural place to do this.
>>> It isn't something that would normally emit asm text.
>>
>> Well, that was the approach suggested by another reviewer (Jakub) once
>> my initial approach (in the V1) got rejected. He explicitly suggested
>> to use shorten_branches.
>>
>>> Would it be OK to emit the declaration at the same point as for decls,
>>> which IIUC is process_pending_assemble_externals? If so, how about
>>> making assemble_external_libcall add the symbol to a list when
>>> !SYMBOL_REF_USED, instead of calling targetm.asm_out.external_libcall
>>> directly? assemble_external_libcall could then also call get_identifier
>>> on the name (perhaps after calling strip_name_encoding -- can't
>>> remember whether assemble_external_libcall sees the encoded or
>>> unencoded name).
>>>
>>> All being well, the call to get_identifier should cause
>>> assemble_name_resolve to record when the name is used, via
>>> TREE_SYMBOL_REFERENCED. Then process_pending_assemble_externals could
>>> go through the list of libcalls recorded by assemble_external_libcall
>>> and check whether TREE_SYMBOL_REFERENCED is set on the get_identifier.
>>>
>>> Not super elegant, but it seems to fit within the existing scheme.
>>> And I don't there should be any problem with using get_identifier
>>> for libcalls, since it isn't valid to use libcall names for other
>>> types of symbol.
>>
>> This sounds way more complicated to me than the approach in V2, which
>> seems to work and is thus a clear improvement compared to the current
>> situation in the trunk. The approach in V2 may be ugly, but it is
>> simple and easy to understand. Is the proposed more convoluted
>> alternative really worth the extra complexity, given it is "not super
>> elegant"?
>
> Is it really that much more convoluted? I was thinking of something
> like the attached, which seems a bit shorter than V2, and does seem
> to fix the bpf tests.
o_O
Ok I clearly misunderstood what you was proposing. This is way simpler!
How does the magic of TREE_SYMBOL_REFERENCED work? How is it set to
`true' only if the RTL containing the call is retained in the final
chain?
> I think most (all?) libcalls already have an associated decl due to
> optabs-libfuncs.cc, so an alternative to get_identifier would be to
> set the SYMBOL_REF_DECL. Using get_identiifer seems a bit more
> lightweight though.
>
> Richard
>
> diff --git a/gcc/varasm.cc b/gcc/varasm.cc
> index b0eff17b8b5..073e3eb2579 100644
> --- a/gcc/varasm.cc
> +++ b/gcc/varasm.cc
> @@ -2461,6 +2461,10 @@ contains_pointers_p (tree type)
> it all the way to final. See PR 17982 for further discussion. */
> static GTY(()) tree pending_assemble_externals;
>
> +/* A similar list of pending libcall symbols. We only want to declare
> + symbols that are actually used in the final assembly. */
> +static GTY(()) rtx pending_libcall_symbols;
> +
> #ifdef ASM_OUTPUT_EXTERNAL
> /* Some targets delay some output to final using TARGET_ASM_FILE_END.
> As a result, assemble_external can be called after the list of externals
> @@ -2516,12 +2520,20 @@ void
> process_pending_assemble_externals (void)
> {
> #ifdef ASM_OUTPUT_EXTERNAL
> - tree list;
> - for (list = pending_assemble_externals; list; list = TREE_CHAIN (list))
> + for (tree list = pending_assemble_externals; list; list = TREE_CHAIN
> (list))
> assemble_external_real (TREE_VALUE (list));
>
> + for (rtx list = pending_libcall_symbols; list; list = XEXP (list, 1))
> + {
> + rtx symbol = XEXP (list, 0);
> + tree id = get_identifier (XSTR (symbol, 0));
> + if (TREE_SYMBOL_REFERENCED (id))
> + targetm.asm_out.external_libcall (symbol);
> + }
> +
> pending_assemble_externals = 0;
> pending_assemble_externals_processed = true;
> + pending_libcall_symbols = NULL_RTX;
> delete pending_assemble_externals_set;
> #endif
> }
> @@ -2594,8 +2606,11 @@ assemble_external_libcall (rtx fun)
> /* Declare library function name external when first used, if nec. */
> if (! SYMBOL_REF_USED (fun))
> {
> + gcc_assert (!pending_assemble_externals_processed);
> SYMBOL_REF_USED (fun) = 1;
> - targetm.asm_out.external_libcall (fun);
> + get_identifier (XSTR (fun, 0));
> + pending_libcall_symbols = gen_rtx_EXPR_LIST (VOIDmode, fun,
> + pending_libcall_symbols);
> }
> }
