[Bug rtl-optimization/124441] The `volatile` keyword will block the optimization of replacing temporary variables in the `expand/TER` RTL pass.

[xingyushuai at hygon dot cn via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=124441

--- Comment #2 from Yushuai Xing <xingyushuai at hygon dot cn> ---
(In reply to Andrew Pinski from comment #1)
> Because that is yhe semantics of volatile

The extracted portion of the expand dump information is as follows::

;;   basic block 2, loop depth 0
;;    pred:       ENTRY
  # DEBUG BEGIN_STMT
  # DEBUG ip => ip_p_14(D)
  # DEBUG BEGIN_STMT
  _1 = *tag_16(D);
  literal_len_17 = _1 >> 2;
  # DEBUG literal_len => literal_len_17
  # DEBUG BEGIN_STMT
  # DEBUG tag_type => _1
  # DEBUG BEGIN_STMT
  # DEBUG BEGIN_STMT
  __asm__("and $3, %k0
        " : "tag_type" "=r" tag_type_19, "=@ccz" is_literal_20 : "0" _1 :
"cc");
  # DEBUG is_literal => is_literal_20
  # DEBUG tag_type => tag_type_19
  # DEBUG BEGIN_STMT
  _2 = *ip_p_14(D);
  _3 = literal_len_17 + 1;
  _4 = (sizetype) _3;
  _5 = _2 + _4;
  _6 ={v} *_5;
  # DEBUG tag_literal => (unsigned int) _6
  # DEBUG BEGIN_STMT
  _7 = (sizetype) tag_type_19;
  _8 = _2 + _7;
  _9 ={v} *_8;
  tag_copy_22 = (unsigned int) _9;
  # DEBUG tag_copy => tag_copy_22
  # DEBUG BEGIN_STMT
  if (is_literal_20 != 0)
    goto <bb 3>; [50.00%]
  else
    goto <bb 4>; [50.00%]
;;    succ:       3
;;                4
....
Replacing Expressions
_3 replace with --> _3 = literal_len_17 + 1;

_4 replace with --> _4 = (sizetype) _3;

_31 replace with --> _31 = literal_len_17 + 2;

_32 replace with --> _32 = (sizetype) _31;

_36 replace with --> _36 = _7 + 1;


In my opinion, variables _5 and _8 can be replaced because they are used to
calculate addresses, while variables _6 and _9 should not be replaced.