[Bug rtl-optimization/123333] New: Failure to optimize away if-then-else with a compile-time computable condition

[law at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=123333

            Bug ID: 123333
           Summary: Failure to optimize away if-then-else with a
                    compile-time computable condition
           Product: gcc
           Version: 16.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: law at gcc dot gnu.org
  Target Milestone: ---

Bohan Lei submitted a patch to avoid triggering an ICE in a couple RISC-V
bitmanip patterns.  The particular assertion triggered indicates an
if-then-else condition that is compile-time computable as false.  Filing mostly
for tracking purposes.

/* { dg-do link  { target { rv64 } } } */
/* { dg-options "-march=rv64gc_zbb_zbs -mabi=lp64d -flto" } */
/* { dg-skip-if "" { *-*-* } { "-O0" "-Og" "-O1" } } */

struct S {
  int a;
  char b;
  int c;
} s;

const signed char c = -37;
int d;
struct S v1[] = {{0, 8}, 0, 0, -108976}, v2[] = {{}, 0, 0, 2804};
int a;
struct S v3[3];
int *p = &a;

void foo() {
  int a;
  if (a)
    ;
  else if (v1[0].b)
    s.a = 0;
  else
    d = 0;
  if (*p)
    if (v3[1].c)
      if (1 ^ (d & c & v2[1].c & ~v1[1].c | s.a))
        v3[2].c = 0;
}

int main() { foo(); }

The problem is combine fails to optimize this in simplify_if_then_else:

(if_then_else (eq (and:DI (reg:DI 155 [ d ])
            (const_int 2176 [0x880]))
        (const_int 1 [0x1]))
    (label_ref 45)
    (pc))