>> "Jose E. Marchesi" <[email protected]> writes:
>>> 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 asssemble_external_libcall to defer emitting
>>> declarations of external libcall symbols, by saving the call tree
>>> nodes in a temporary list pending_libcall_symbols and letting
>>> process_pending_assembly_externals to emit them only if they have been
>>> referenced. Solution suggested and sketched by Richard Sandiford.
>>>
>>> Regtested in x86_64-linux-gnu.
>>> Tested with host x86_64-linux-gnu with target bpf-unknown-none.
>>>
>>> gcc/ChangeLog
>>>
>>> * varasm.cc (pending_libcall_symbols): New variable.
>>> (process_pending_assemble_externals): Process
>>> pending_libcall_symbols.
>>> (assemble_external_libcall): Defer emitting external libcall
>>> symbols to process_pending_assemble_externals.
>>>
>>> gcc/testsuite/ChangeLog
>>>
>>> * gcc.target/bpf/divmod-libcall-1.c: New test.
>>> * gcc.target/bpf/divmod-libcall-2.c: Likewise.
>>> * gcc.c-torture/compile/libcall-2.c: Likewise.
>>
>> OK, thanks.
>
> Thank you.
> Pushed.
I installed the following fix, since the built got broken in targets
that do not define ASM_OUTPUT_EXTERNAL.
diff --git a/gcc/varasm.cc b/gcc/varasm.cc
index deb7eab7af9..167aea87091 100644
--- a/gcc/varasm.cc
+++ b/gcc/varasm.cc
@@ -2607,7 +2607,9 @@ assemble_external_libcall (rtx fun)
/* Declare library function name external when first used, if nec. */
if (! SYMBOL_REF_USED (fun))
{
+#ifdef ASM_OUTPUT_EXTERNAL
gcc_assert (!pending_assemble_externals_processed);
+#endif
SYMBOL_REF_USED (fun) = 1;
/* Make sure the libcall symbol is in the symtab so any
reference to it will mark its tree node as referenced, via
--
2.30.2
>
>> Richard
>>
>>> ---
>>> .../gcc.c-torture/compile/libcall-2.c | 8 +++++++
>>> .../gcc.target/bpf/divmod-libcall-1.c | 19 ++++++++++++++++
>>> .../gcc.target/bpf/divmod-libcall-2.c | 16 ++++++++++++++
>>> gcc/varasm.cc | 22 ++++++++++++++++++-
>>> 4 files changed, 64 insertions(+), 1 deletion(-)
>>> create mode 100644 gcc/testsuite/gcc.c-torture/compile/libcall-2.c
>>> create mode 100644 gcc/testsuite/gcc.target/bpf/divmod-libcall-1.c
>>> create mode 100644 gcc/testsuite/gcc.target/bpf/divmod-libcall-2.c
>>>
>>> diff --git a/gcc/testsuite/gcc.c-torture/compile/libcall-2.c
>>> b/gcc/testsuite/gcc.c-torture/compile/libcall-2.c
>>> new file mode 100644
>>> index 00000000000..b33944c83ff
>>> --- /dev/null
>>> +++ b/gcc/testsuite/gcc.c-torture/compile/libcall-2.c
>>> @@ -0,0 +1,8 @@
>>> +/* Make sure that external refences for libcalls are generated even for
>>> + indirect calls. */
>>> +
>>> +/* { dg-do compile } */
>>> +/* { dg-options "-O2 -mcmodel=large" { target x86_64-*-* } } */
>>> +/* { dg-final { scan-assembler "globl\t__divti3" } } */
>>> +
>>> +__int128 a, b; void foo () { a = a / b; }
>>> diff --git a/gcc/testsuite/gcc.target/bpf/divmod-libcall-1.c
>>> b/gcc/testsuite/gcc.target/bpf/divmod-libcall-1.c
>>> new file mode 100644
>>> index 00000000000..7481076602a
>>> --- /dev/null
>>> +++ b/gcc/testsuite/gcc.target/bpf/divmod-libcall-1.c
>>> @@ -0,0 +1,19 @@
>>> +/* This test makes sure that no spurious external symbol declarations are
>>> + emitted for libcalls in tried but eventually not used code sequences.
>>> */
>>> +
>>> +/* { dg-do compile } */
>>> +/* { dg-options "-O2 -mcpu=v3" } */
>>> +/* { dg-final { scan-assembler-not "global\t__divdi3" } } */
>>> +/* { dg-final { scan-assembler-not "global\t__moddi3" } } */
>>> +
>>> +int
>>> +foo (unsigned int len)
>>> +{
>>> + return ((unsigned long)len) * 234 / 5;
>>> +}
>>> +
>>> +int
>>> +bar (unsigned int len)
>>> +{
>>> + return ((unsigned long)len) * 234 % 5;
>>> +}
>>> diff --git a/gcc/testsuite/gcc.target/bpf/divmod-libcall-2.c
>>> b/gcc/testsuite/gcc.target/bpf/divmod-libcall-2.c
>>> new file mode 100644
>>> index 00000000000..792d689395a
>>> --- /dev/null
>>> +++ b/gcc/testsuite/gcc.target/bpf/divmod-libcall-2.c
>>> @@ -0,0 +1,16 @@
>>> +/* { dg-do compile } */
>>> +/* { dg-options "-O2 -mcpu=v3" } */
>>> +/* { dg-final { scan-assembler "global\t__divdi3" } } */
>>> +/* { dg-final { scan-assembler "global\t__moddi3" } } */
>>> +
>>> +int
>>> +foo (unsigned int len)
>>> +{
>>> + return ((long)len) * 234 / 5;
>>> +}
>>> +
>>> +int
>>> +bar (unsigned int len)
>>> +{
>>> + return ((long)len) * 234 % 5;
>>> +}
>>> diff --git a/gcc/varasm.cc b/gcc/varasm.cc
>>> index 6ae35edc5ae..deb7eab7af9 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
>>> @@ -2520,8 +2524,17 @@ process_pending_assemble_externals (void)
>>> for (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 +2607,15 @@ 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);
>>> + /* Make sure the libcall symbol is in the symtab so any
>>> + reference to it will mark its tree node as referenced, via
>>> + assemble_name_resolve. These are eventually emitted, if
>>> + used, in process_pending_assemble_externals. */
>>> + get_identifier (XSTR (fun, 0));
>>> + pending_libcall_symbols = gen_rtx_EXPR_LIST (VOIDmode, fun,
>>> + pending_libcall_symbols);
>>> }
>>> }
