[PATCH][middle-end][i386][Version 4] Add -fzero-call-used-regs=[skip|used-gpr-arg|used-arg|all-arg|used-gpr|all-gpr|used|all]

[Qing Zhao]

>> 
>> +/* Generate insns to zero all st/mm registers together.
>> +   Return true when zeroing instructions are generated.
>> +   Assume the number of st registers that are zeroed is num_of_st,
>> +   we will emit the following sequence to zero them together:
>> +                 fldz;         \
>> +                 fldz;         \
>> +                 ...
>> +                 fldz;         \
>> +                 fstp %%st(0); \
>> +                 fstp %%st(0); \
>> +                 ...
>> +                 fstp %%st(0);
>> +   i.e., num_of_st fldz followed by num_of_st fstp to clear the stack
>> +   mark stack slots empty.  */
>> +
>> +static bool
>> +zero_all_st_mm_registers (HARD_REG_SET need_zeroed_hardregs)
>> +{
>> +  unsigned int num_of_st = 0;
>> +  for (unsigned int regno = 0; regno < FIRST_PSEUDO_REGISTER; regno++)
>> +    if (STACK_REGNO_P (regno)
>> +       && TEST_HARD_REG_BIT (need_zeroed_hardregs, regno)
>> +       /* When the corresponding mm register also need to be cleared too.  */
>> +       && TEST_HARD_REG_BIT (need_zeroed_hardregs,
>> +                             (regno - FIRST_STACK_REG + FIRST_MMX_REG)))
>> +      num_of_st++;
> 
> I don't think the above logic is correct. It should go like this:
> 
> - If the function is returning an MMX register,

How to check on this? Is the following correct?

If (GET_CODE(crtl->return_rtx) == REG 
    && (MMX_REG_P (REGNO (crtl->return_rtx)))

   The function is returning an MMX register.


> then the function
> exits in MMX mode, and MMX registers should be cleared in the same way
> as XMM registers.

When clearing XMM registers, we used V4SFmode, what’s the mode we should use to clearing
mmx registers?

> Otherwise the ABI specifies that the function exits
> in x87 mode and x87 stack should be cleared (but see below).
> 
> - There is no direct mapping of stack registers to hard register
> numbers. If a stack register is used, we don't know where in the stack
> the value remains. So, if _any_ stack register is touched, the whole
> stack should be cleared (value, returning in x87 stack register should
> obviously be excluded).

Then, how to exclude the x87 stack register that returns the function return value when we need to 
Clear the whole stack? 
I am a little confused here? Could you explain a little more details?
> 
> - There is no x87 argument register. 32bit targets use MMX0-3 argument
> registers and return value in the XMM register. Please also note that
> complex values take two stack slots in x87 stack.

You mean the complex return value will be returned in two  x87 registers? 

thanks.

Qing
> 
> Uros.
> 
>> +
>> +  if (num_of_st == 0)