Re: [patch]update the documentation for TARGET_ZERO_CALL_USED_REGS hook and add an assertion

[Qing Zhao <qing.zhao@oracle.com>]

FYI. 

I have committed the change to upstream as:

31933f4f788b6cd64cbb7ee42076997f6d0fe212

Qing
> On Mar 31, 2022, at 8:10 AM, Richard Sandiford <richard.sandiford@arm.com> wrote:
> 
> Qing Zhao <qing.zhao@oracle.com> writes:
>> Hi, 
>> 
>> Per our discussion on: https://gcc.gnu.org/pipermail/gcc-patches/2022-March/592002.html
>> 
>> I come up with the following patch to:
>> 
>> 1. Update the documentation for TARGET_ZERO_CALL_USED_REGS hook;
>> 2. Add an assertion in function.cc to make sure the actually zeroed_regs is a subset of all call used regs;
>>   (The reason I didn’t add a new parameter to TARGET_ZERO_CALL_USED_REGS is, I think adding the 
>>    assertion in the common place function.cc is simpler to be implemented).
> 
> Yeah, that's fair.  I guess in theory, passing the parameter would allow
> targets to choose between two versions of zeroing the register, one with
> a temporary and one without.  But that's a purely hypothetical situation
> and we could always add a parameter later if that turns out to be useful.
> 
> Perhaps more realistically, there might be other uses of the hook in
> future that want to zero registers for different reasons, with their
> own rules about which registers can be zeroed.  In other words, the
> hook is providing a general facility that happens to be useful for
> -fzero-call-used-regs.  But again, we can deal with that if it ever
> happens.
> 
> So I agree this is the right call, especially for stage 4.
> 
>> 3. This new assertion identified a bug in i386 implementation. Fix this bug in i386.
>> 
>> This patch is bootstrapped on both x86 and aarch64, no regression.
>> 
>> Okay for commit?
> 
> OK for the non-x86 bits.
> 
> Thanks,
> Richard
> 
>> thanks.
>> 
>> Qing
>> 
>> ===================================
>> From 2e5bc1b25a707c6a17afbf03da2a8bec5b03454d Mon Sep 17 00:00:00 2001
>> From: Qing Zhao <qing.zhao@oracle.com>
>> Date: Fri, 18 Mar 2022 20:49:56 +0000
>> Subject: [PATCH] Add an assertion: the zeroed_hardregs set is a subset of all
>> call used regs.
>> 
>> We should make sure that the hard register set that is actually cleared by
>> the target hook zero_call_used_regs should be a subset of all call used
>> registers.
>> 
>> At the same time, update documentation for the target hook
>> TARGET_ZERO_CALL_USED_REGS.
>> 
>> This new assertion identified a bug in the i386 implemenation, which
>> incorrectly set the zeroed_hardregs for stack registers. Fixed this bug
>> in i386 implementation.
>> 
>> gcc/ChangeLog:
>> 
>> 2022-03-21  Qing Zhao  <qing.zhao@oracle.com>
>> 
>> 	* config/i386/i386.cc (zero_all_st_registers): Return the value of
>> 	num_of_st.
>> 	(ix86_zero_call_used_regs): Update zeroed_hardregs set according to
>> 	the return value of zero_all_st_registers.
>> 	* doc/tm.texi: Update the documentation of TARGET_ZERO_CALL_USED_REGS.
>> 	* function.cc (gen_call_used_regs_seq): Add an assertion.
>> 	* target.def: Update the documentation of TARGET_ZERO_CALL_USED_REGS.
>> ---
>> gcc/config/i386/i386.cc | 27 ++++++++++++++++++---------
>> gcc/doc/tm.texi         |  7 +++++++
>> gcc/function.cc         | 22 ++++++++++++++++++----
>> gcc/target.def          |  7 +++++++
>> 4 files changed, 50 insertions(+), 13 deletions(-)
>> 
>> diff --git a/gcc/config/i386/i386.cc b/gcc/config/i386/i386.cc
>> index 5a561966eb44..d84047a4bc1b 100644
>> --- a/gcc/config/i386/i386.cc
>> +++ b/gcc/config/i386/i386.cc
>> @@ -3753,16 +3753,17 @@ zero_all_vector_registers (HARD_REG_SET need_zeroed_hardregs)
>>    needs to be cleared, the whole stack should be cleared.  However,
>>    x87 stack registers that hold the return value should be excluded.
>>    x87 returns in the top (two for complex values) register, so
>> -   num_of_st should be 7/6 when x87 returns, otherwise it will be 8.  */
>> +   num_of_st should be 7/6 when x87 returns, otherwise it will be 8.
>> +   return the value of num_of_st.  */
>> 
>> 
>> -static bool
>> +static int
>> zero_all_st_registers (HARD_REG_SET need_zeroed_hardregs)
>> {
>> 
>>   /* If the FPU is disabled, no need to zero all st registers.  */
>>   if (! (TARGET_80387 || TARGET_FLOAT_RETURNS_IN_80387))
>> -    return false;
>> +    return 0;
>> 
>>   unsigned int num_of_st = 0;
>>   for (unsigned int regno = 0; regno < FIRST_PSEUDO_REGISTER; regno++)
>> @@ -3774,7 +3775,7 @@ zero_all_st_registers (HARD_REG_SET need_zeroed_hardregs)
>>       }
>> 
>>   if (num_of_st == 0)
>> -    return false;
>> +    return 0;
>> 
>>   bool return_with_x87 = false;
>>   return_with_x87 = (crtl->return_rtx
>> @@ -3802,7 +3803,7 @@ zero_all_st_registers (HARD_REG_SET need_zeroed_hardregs)
>>       insn = emit_insn (gen_rtx_SET (st_reg, st_reg));
>>       add_reg_note (insn, REG_DEAD, st_reg);
>>     }
>> -  return true;
>> +  return num_of_st;
>> }
>> 
>> 
>> @@ -3851,7 +3852,7 @@ ix86_zero_call_used_regs (HARD_REG_SET need_zeroed_hardregs)
>> {
>>   HARD_REG_SET zeroed_hardregs;
>>   bool all_sse_zeroed = false;
>> -  bool all_st_zeroed = false;
>> +  int all_st_zeroed_num = 0;
>>   bool all_mm_zeroed = false;
>> 
>>   CLEAR_HARD_REG_SET (zeroed_hardregs);
>> @@ -3881,9 +3882,17 @@ ix86_zero_call_used_regs (HARD_REG_SET need_zeroed_hardregs)
>>   if (!exit_with_mmx_mode)
>>     /* x87 exit mode, we should zero all st registers together.  */
>>     {
>> -      all_st_zeroed = zero_all_st_registers (need_zeroed_hardregs);
>> -      if (all_st_zeroed)
>> -	SET_HARD_REG_BIT (zeroed_hardregs, FIRST_STACK_REG);
>> +      all_st_zeroed_num = zero_all_st_registers (need_zeroed_hardregs);
>> +
>> +      if (all_st_zeroed_num > 0)
>> +	for (unsigned int regno = FIRST_STACK_REG; regno <= LAST_STACK_REG; regno++)
>> +	  /* x87 stack registers that hold the return value should be excluded.
>> +	     x87 returns in the top (two for complex values) register.  */
>> +	  if (all_st_zeroed_num == 8
>> +	      || !((all_st_zeroed_num >= 6 && regno == REGNO (crtl->return_rtx))
>> +		   || (all_st_zeroed_num == 6
>> +		       && (regno == (REGNO (crtl->return_rtx) + 1)))))
>> +	    SET_HARD_REG_BIT (zeroed_hardregs, regno);
>>     }
>>   else
>>     /* MMX exit mode, check whether we can zero all mm registers.  */
>> diff --git a/gcc/doc/tm.texi b/gcc/doc/tm.texi
>> index 2f92d37da8c0..c5006afc00d2 100644
>> --- a/gcc/doc/tm.texi
>> +++ b/gcc/doc/tm.texi
>> @@ -12330,6 +12330,13 @@ This target hook emits instructions to zero the subset of @var{selected_regs}
>> that could conceivably contain values that are useful to an attacker.
>> Return the set of registers that were actually cleared.
>> 
>> +For most targets, the returned set of registers is a subset of
>> +@var{selected_regs}, however, for some of the targets (for example MIPS),
>> +clearing some registers that are in the @var{selected_regs} requires
>> +clearing other call used registers that are not in the @var{selected_regs},
>> +under such situation, the returned set of registers must be a subset of all
>> +call used registers.
>> +
>> The default implementation uses normal move instructions to zero
>> all the registers in @var{selected_regs}.  Define this hook if the
>> target has more efficient ways of zeroing certain registers,
>> diff --git a/gcc/function.cc b/gcc/function.cc
>> index d5ed51a6a663..ad0096a43eff 100644
>> --- a/gcc/function.cc
>> +++ b/gcc/function.cc
>> @@ -5892,7 +5892,9 @@ gen_call_used_regs_seq (rtx_insn *ret, unsigned int zero_regs_type)
>>   df_simulate_one_insn_backwards (bb, ret, live_out);
>> 
>>   HARD_REG_SET selected_hardregs;
>> +  HARD_REG_SET all_call_used_regs;
>>   CLEAR_HARD_REG_SET (selected_hardregs);
>> +  CLEAR_HARD_REG_SET (all_call_used_regs);
>>   for (unsigned int regno = 0; regno < FIRST_PSEUDO_REGISTER; regno++)
>>     {
>>       if (!crtl->abi->clobbers_full_reg_p (regno))
>> @@ -5901,6 +5903,13 @@ gen_call_used_regs_seq (rtx_insn *ret, unsigned int zero_regs_type)
>> 	continue;
>>       if (REGNO_REG_SET_P (live_out, regno))
>> 	continue;
>> +#ifdef LEAF_REG_REMAP
>> +      if (crtl->uses_only_leaf_regs && LEAF_REG_REMAP (regno) < 0)
>> +	continue;
>> +#endif
>> +      /* This is a call used register that is dead at return.  */
>> +      SET_HARD_REG_BIT (all_call_used_regs, regno);
>> +
>>       if (only_gpr
>> 	  && !TEST_HARD_REG_BIT (reg_class_contents[GENERAL_REGS], regno))
>> 	continue;
>> @@ -5908,10 +5917,6 @@ gen_call_used_regs_seq (rtx_insn *ret, unsigned int zero_regs_type)
>> 	continue;
>>       if (only_arg && !FUNCTION_ARG_REGNO_P (regno))
>> 	continue;
>> -#ifdef LEAF_REG_REMAP
>> -      if (crtl->uses_only_leaf_regs && LEAF_REG_REMAP (regno) < 0)
>> -	continue;
>> -#endif
>> 
>>       /* Now this is a register that we might want to zero.  */
>>       SET_HARD_REG_BIT (selected_hardregs, regno);
>> @@ -5925,6 +5930,15 @@ gen_call_used_regs_seq (rtx_insn *ret, unsigned int zero_regs_type)
>>   HARD_REG_SET zeroed_hardregs;
>>   start_sequence ();
>>   zeroed_hardregs = targetm.calls.zero_call_used_regs (selected_hardregs);
>> +
>> +  /* For most targets, the returned set of registers is a subset of
>> +     selected_hardregs, however, for some of the targets (for example MIPS),
>> +     clearing some registers that are in selected_hardregs requires clearing
>> +     other call used registers that are not in the selected_hardregs, under
>> +     such situation, the returned set of registers must be a subset of
>> +     all call used registers.  */
>> +  gcc_assert (hard_reg_set_subset_p (zeroed_hardregs, all_call_used_regs));
>> +
>>   rtx_insn *seq = get_insns ();
>>   end_sequence ();
>>   if (seq)
>> diff --git a/gcc/target.def b/gcc/target.def
>> index 72c2e1ef756c..d85adf36a391 100644
>> --- a/gcc/target.def
>> +++ b/gcc/target.def
>> @@ -5122,6 +5122,13 @@ DEFHOOK
>> that could conceivably contain values that are useful to an attacker.\n\
>> Return the set of registers that were actually cleared.\n\
>> \n\
>> +For most targets, the returned set of registers is a subset of\n\
>> +@var{selected_regs}, however, for some of the targets (for example MIPS),\n\
>> +clearing some registers that are in the @var{selected_regs} requires\n\
>> +clearing other call used registers that are not in the @var{selected_regs},\n\
>> +under such situation, the returned set of registers must be a subset of all\n\
>> +call used registers.\n\
>> +\n\
>> The default implementation uses normal move instructions to zero\n\
>> all the registers in @var{selected_regs}.  Define this hook if the\n\
>> target has more efficient ways of zeroing certain registers,\n\