Re: [PATCH] expr.cc: avoid unexpected side effects in expand_expr_divmod optimization

["Jose E. Marchesi" <jose.marchesi@oracle.com>]

>> Am 08.12.2022 um 11:56 schrieb Jose E. Marchesi via Gcc-patches <gcc-patches@gcc.gnu.org>:
>> 
>> The expand_expr_divmod function in expr.cc attempts to optimize cases
>> where both arguments of a division/modulus are known to be positive
>> when interpreted as signed.  In these cases, both signed division and
>> unsigned division will raise the same value, and therefore the
>> cheapest option can be used.
>> 
>> In order to determine what is the cheaper option in the current
>> target, expand_expr_divmod actually expands both a signed divmod and
>> an unsigned divmod using local "sequences":
>> 
>>  start_sequence ();
>>  ...
>>  expand_divmod (... signed ...);
>>  ...
>>  end_sequence ();
>> 
>>  start_sequence ();
>>  ...
>>  expand_divmod (... unsigned ...);
>>  ...
>>  end_sequence ();
>> 
>> And then compares the cost of each generated sequence, choosing the
>> best one.  Finally, it emits the selected expanded sequence and
>> returns the rtx with the result.
>> 
>> This approach has a caveat.  Some targets do not provide instructions
>> for division/modulus instructions.  In the case of BPF, it provides
>> unsigned division/modulus, but not signed division/modulus.
>> 
>> In these cases, the expand_divmod tries can contain calls to funcalls.
>> For example, in BPF:
>> 
>>  start_sequence ();
>>  ...
>>  expand_divmod (... signed ...); -> This generates funcall to __divdi3
>>  ...
>>  end_sequence ();
>> 
>>  start_sequence ();
>>  ...
>>  expand_divmod (... unsigned ...); -> This generates direct `div' insn.
>>  ...
>>  end_sequence ();
>> 
>> The problem is that when a funcall is expanded, an accompanying global
>> symbol definition is written in the output stream:
>> 
>>  .global __divdi3
>> 
>> And this symbol definition remains in the compiled assembly file, even
>> if the sequence using the direct `div' instruction above is used.
>> 
>> This is particularly bad in BPF, because the kernel bpf loader chokes
>> on the spurious symbol __divdi3 and makes the resulting BPF object
>> unloadable (note that BPF objects are not linked before processed by
>> the kernel.)
>> 
>> In order to fix this, this patch modifies expand_expr_divmod in the
>> following way:
>> 
>> - When trying each sequence (signed, unsigned) the expand_divmod calls
>>  are told to _not_ use libcalls if everything else fails.  This is
>>  done by passing OPTAB_WIDEN as the `methods' argument.  (Before it
>>  was using the default value OPTAB_LIB_WIDEN.)
>> 
>> - If any of the tried expanded sequences contain a funcall, then the
>>  optimization is not attempted.
>
> How do libcalls appear in iff you specify OPTABS_WIDEN only?  Doesn’t
> that allow to simplify this and also use the sequence without a
> libcall?

If you pass OPTABS_WIDEN only then libcalls are not an option and (as
far as I can tell) expand_divmod returns NULL if a libcall is the only
possibility.

> Richard 
>
>> 
>> A couple of BPF tests are also added to make sure this doesn't break
>> at any point in the future.
>> 
>> Tested in bpf-unknown-none and x86_64-linux-gnu.
>> Regtested in x86_64-linux-gnu.  No regressions.
>> 
>> gcc/ChangeLog
>> 
>>    * expr.cc (expand_expr_divmod): Avoid side-effects of trying
>>    sequences involving funcalls in optimization.
>> 
>> gcc/testsuite/ChangeLog:
>> 
>>    * gcc.target/bpf/divmod-funcall-1.c: New test.
>>    * gcc.target/bpf/divmod-funcall-2.c: Likewise.
>> ---
>> gcc/expr.cc                                   | 44 +++++++++++--------
>> .../gcc.target/bpf/divmod-funcall-1.c         |  8 ++++
>> .../gcc.target/bpf/divmod-funcall-2.c         |  8 ++++
>> 3 files changed, 41 insertions(+), 19 deletions(-)
>> create mode 100644 gcc/testsuite/gcc.target/bpf/divmod-funcall-1.c
>> create mode 100644 gcc/testsuite/gcc.target/bpf/divmod-funcall-2.c
>> 
>> diff --git a/gcc/expr.cc b/gcc/expr.cc
>> index d9407432ea5..4d4be5d7bda 100644
>> --- a/gcc/expr.cc
>> +++ b/gcc/expr.cc
>> @@ -9168,32 +9168,38 @@ expand_expr_divmod (tree_code code, machine_mode mode, tree treeop0,
>>       do_pending_stack_adjust ();
>>       start_sequence ();
>>       rtx uns_ret = expand_divmod (mod_p, code, mode, treeop0, treeop1,
>> -                   op0, op1, target, 1);
>> +                   op0, op1, target, 1, OPTAB_WIDEN);
>>       rtx_insn *uns_insns = get_insns ();
>>       end_sequence ();
>>       start_sequence ();
>>       rtx sgn_ret = expand_divmod (mod_p, code, mode, treeop0, treeop1,
>> -                   op0, op1, target, 0);
>> +                   op0, op1, target, 0, OPTAB_WIDEN);
>>       rtx_insn *sgn_insns = get_insns ();
>>       end_sequence ();
>> -      unsigned uns_cost = seq_cost (uns_insns, speed_p);
>> -      unsigned sgn_cost = seq_cost (sgn_insns, speed_p);
>> 
>> -      /* If costs are the same then use as tie breaker the other other
>> -     factor.  */
>> -      if (uns_cost == sgn_cost)
>> -    {
>> -      uns_cost = seq_cost (uns_insns, !speed_p);
>> -      sgn_cost = seq_cost (sgn_insns, !speed_p);
>> -    }
>> -
>> -      if (uns_cost < sgn_cost || (uns_cost == sgn_cost && unsignedp))
>> -    {
>> -      emit_insn (uns_insns);
>> -      return uns_ret;
>> -    }
>> -      emit_insn (sgn_insns);
>> -      return sgn_ret;
>> +      /* Do not try to optimize if any of the sequences tried above
>> +         resulted in a funcall.  */
>> +      if (uns_ret && sgn_ret)
>> +        {
>> +          unsigned uns_cost = seq_cost (uns_insns, speed_p);
>> +          unsigned sgn_cost = seq_cost (sgn_insns, speed_p);
>> +
>> +          /* If costs are the same then use as tie breaker the other
>> +             other factor.  */
>> +          if (uns_cost == sgn_cost)
>> +            {
>> +              uns_cost = seq_cost (uns_insns, !speed_p);
>> +              sgn_cost = seq_cost (sgn_insns, !speed_p);
>> +            }
>> +
>> +          if (uns_cost < sgn_cost || (uns_cost == sgn_cost && unsignedp))
>> +            {
>> +              emit_insn (uns_insns);
>> +              return uns_ret;
>> +            }
>> +          emit_insn (sgn_insns);
>> +          return sgn_ret;
>> +        }
>>     }
>>   return expand_divmod (mod_p, code, mode, treeop0, treeop1,
>>            op0, op1, target, unsignedp);
>> diff --git a/gcc/testsuite/gcc.target/bpf/divmod-funcall-1.c b/gcc/testsuite/gcc.target/bpf/divmod-funcall-1.c
>> new file mode 100644
>> index 00000000000..dffb1506f06
>> --- /dev/null
>> +++ b/gcc/testsuite/gcc.target/bpf/divmod-funcall-1.c
>> @@ -0,0 +1,8 @@
>> +/* { dg-do compile } */
>> +/* { dg-final { scan-assembler-not "__divdi3" } } */
>> +
>> +int
>> +foo (unsigned int len)
>> +{
>> +  return ((unsigned long)len) * 234 / 5;
>> +}
>> diff --git a/gcc/testsuite/gcc.target/bpf/divmod-funcall-2.c b/gcc/testsuite/gcc.target/bpf/divmod-funcall-2.c
>> new file mode 100644
>> index 00000000000..41e8e40c35c
>> --- /dev/null
>> +++ b/gcc/testsuite/gcc.target/bpf/divmod-funcall-2.c
>> @@ -0,0 +1,8 @@
>> +/* { dg-do compile } */
>> +/* { dg-final { scan-assembler-not "__moddi3" } } */
>> +
>> +int
>> +foo (unsigned int len)
>> +{
>> +  return ((unsigned long)len) * 234 % 5;
>> +}
>> -- 
>> 2.30.2
>>