On Mon, Oct 26, 2020 at 3:45 PM Qing Zhao <qing.z...@oracle.com> wrote:
>
>
> +/* 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)))
Yes, but please use
if (MMX_REG_P (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?
It doesn't matter that much, any 8byte vector mode will do (including
DImode). Let's use V4HImode.
> 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?
x87 returns in the top (two for complex values) register, so simply
load 7 zeros (and 7 corresponding pops). This will preserve the return
value but clear the whole remaining stack.
> - 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?
Yes, please see ix86_class_max_nregs. Please note that in case of
complex return value, only 6 zeros should be loaded to avoid
clobbering the complex return value.
Uros.
