> "Jose E. Marchesi" <jose.march...@oracle.com> 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.
> 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);
>> }
>> }
