[PATCH][middle-end][i386][version 3]Add -fzero-call-used-regs=[skip|used-gpr-arg|used-arg|all-arg|used-gpr|all-gpr|used|all]

[Qing Zhao]

> On Oct 20, 2020, at 10:24 AM, Uros Bizjak <ubizjak@gmail.com> wrote:
> 
> On Tue, Oct 20, 2020 at 4:01 PM Qing Zhao <QING.ZHAO@oracle.com <mailto:QING.ZHAO@oracle.com>> wrote:
>> 
>> Hi, Uros,
>> 
>> Thanks a lot for your comments.
>> 
>> On Oct 19, 2020, at 2:30 PM, Uros Bizjak <ubizjak@gmail.com> wrote:
>> 
>> 
>> diff --git a/gcc/config/i386/i386.c b/gcc/config/i386/i386.c
>> index f684954..620114f 100644
>> --- a/gcc/config/i386/i386.c
>> +++ b/gcc/config/i386/i386.c
>> @@ -3551,6 +3551,161 @@ ix86_function_value_regno_p (const unsigned int regno)
>> return false;
>> }
>> 
>> +/* Check whether the register REGNO should be zeroed on X86.
>> +   When ALL_SSE_ZEROED is true, all SSE registers have been zeroed
>> +   together, no need to zero it again.
>> +   Stack registers (st0-st7) and mm0-mm7 are aliased with each other.
>> +   very hard to be zeroed individually, don't zero individual st or
>> +   mm registgers at this time.  */
>> +
>> +static bool
>> +zero_call_used_regno_p (const unsigned int regno,
>> + bool all_sse_zeroed)
>> +{
>> +  return GENERAL_REGNO_P (regno)
>> +  || (!all_sse_zeroed && SSE_REGNO_P (regno))
>> +  || MASK_REGNO_P (regno);
>> +}
>> +
>> +/* Return the machine_mode that is used to zero register REGNO.  */
>> +
>> +static machine_mode
>> +zero_call_used_regno_mode (const unsigned int regno)
>> +{
>> +  /* NB: We only need to zero the lower 32 bits for integer registers
>> +     and the lower 128 bits for vector registers since destination are
>> +     zero-extended to the full register width.  */
>> +  if (GENERAL_REGNO_P (regno))
>> +    return SImode;
>> +  else if (SSE_REGNO_P (regno))
>> +    return V4SFmode;
>> +  else
>> +    return HImode;
>> +}
>> +
>> +/* Generate a rtx to zero all vector registers togetehr if possible,
>> +   otherwise, return NULL.  */
>> +
>> +static rtx
>> +zero_all_vector_registers (HARD_REG_SET need_zeroed_hardregs)
>> +{
>> +  if (!TARGET_AVX)
>> +    return NULL;
>> +
>> +  for (unsigned int regno = 0; regno < FIRST_PSEUDO_REGISTER; regno++)
>> +    if ((IN_RANGE (regno, FIRST_SSE_REG, LAST_SSE_REG)
>> +  || (TARGET_64BIT
>> +      && (REX_SSE_REGNO_P (regno)
>> +  || (TARGET_AVX512F && EXT_REX_SSE_REGNO_P (regno)))))
>> + && !TEST_HARD_REG_BIT (need_zeroed_hardregs, regno))
>> +      return NULL;
>> +
>> +  return gen_avx_vzeroall ();
>> +}
>> +
>> +/* Generate a rtx to zero all st and mm registers togetehr if possible,
>> +   otherwise, return NULL.  */
>> +
>> +static rtx
>> +zero_all_st_mm_registers (HARD_REG_SET need_zeroed_hardregs)
>> +{
>> +  if (!TARGET_MMX)
>> +    return NULL;
>> +
>> +  for (unsigned int regno = 0; regno < FIRST_PSEUDO_REGISTER; regno++)
>> +    if ((STACK_REGNO_P (regno) || MMX_REGNO_P (regno))
>> + && !TEST_HARD_REG_BIT (need_zeroed_hardregs, regno))
>> +      return NULL;
>> +
>> +  return gen_mmx_emms ();
>> 
>> 
>> emms is not clearing any register, it only loads x87FPUTagWord with
>> FFFFH. So I think, the above is useless, as far as register clearing
>> is concerned.
>> 
>> 
>> Thanks for the info.
>> 
>> So, for mm and st registers, should we clear them, and how?
> 
> I don't know.
> 
> Please note that %mm and %st share the same register file, and
> touching %mm registers will block access to %st until emms is emitted.
> You can't just blindly load 0 to %st registers, because the register
> file can be in MMX mode and vice versa. For 32bit targets, function
> can also  return a value in the %mm0.

If data flow determine that %mm0 does not return a value at the return, can we clear all the %st as following:

emms
mov %st0, 0
mov %st1, 0
mov %st2, 0
mov %st3, 0
mov %st4, 0
mov %st5, 0
mov %st6, 0
mov %st7, 0

? 

Thanks.

Qing
>