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From: "stefansf at linux dot ibm.com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug rtl-optimization/104814] New: [10/11/12 Regression] ifcvt:
 Deleting live variable in IF-CASE-2
Date: Mon, 07 Mar 2022 09:39:44 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D104814

            Bug ID: 104814
           Summary: [10/11/12 Regression] ifcvt: Deleting live variable in
                    IF-CASE-2
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Keywords: wrong-code
          Severity: normal
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: stefansf at linux dot ibm.com
  Target Milestone: ---
            Target: s390x-*-*

Created attachment 52570
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=3D52570&action=3Dedit
dump ce2

short a =3D 0;
static long b =3D 0;
int c =3D 7;
char d =3D 0;
short *e =3D &a;
long f =3D 0;

unsigned long g (unsigned long h, long j) { return j =3D=3D 0 ? h : h / j; }

int main (void) {
  long k =3D f;
  for (; c; --c) {
    for (int i =3D 0; i < 7; ++i)
      ;
    long m =3D g (f, --b);
    d =3D ((char)m | *e) <=3D 43165;
  }
  if (b !=3D -7)
    __builtin_abort ();
  return 0;
}

Variable b should be decremented in each iteration of the outer loop and th=
us
equal -7 at the end.

After combine a load, decrement, and store insn exists:

(insn 13 12 14 3 (set (reg:DI 62 [ b_lsm.16 ])
        (mem/c:DI (const:DI (plus:DI (symbol_ref:DI ("*.LANCHOR0") [flags
0x182])
                    (const_int 8 [0x8]))) [1 b+0 S8 A64])) 1469 {*movdi_64}
     (nil))


(jump_insn 40 39 46 6 (parallel [
            (set (pc)
                (if_then_else (ne (reg:DI 62 [ b_lsm.16 ])
                        (const_int 1 [0x1]))
                    (label_ref 38)
                    (pc)))
            (set (reg:DI 62 [ b_lsm.16 ])
                (plus:DI (reg:DI 62 [ b_lsm.16 ])
                    (const_int -1 [0xffffffffffffffff])))
            (clobber (scratch:DI))
            (clobber (reg:CC 33 %cc))
        ]) "t.c":8:63 2164 {doloop_di}
     (expr_list:REG_UNUSED (reg:CC 33 %cc)
        (int_list:REG_BR_PROB 536870916 (nil)))
 -> 38)


(insn 48 47 49 7 (set (mem/c:DI (const:DI (plus:DI (symbol_ref:DI
("*.LANCHOR0") [flags 0x182])
                    (const_int 8 [0x8]))) [1 b+0 S8 A64])
        (reg:DI 62 [ b_lsm.16 ])) 1469 {*movdi_64}
     (expr_list:REG_DEAD (reg:DI 62 [ b_lsm.16 ])
        (nil)))

Pass ce2 deletes jump insn 40 including the decrement of variable b:

IF-CASE-2 found, start 6, else 4
deleting insn with uid =3D 40.
deleting block 4
Conversion succeeded on pass 1.

Thus variable b equals 0 in the end.=