[Bug tree-optimization/66512] New: PRE fails to optimize calls to pure functions in C++, ok in C

[amonakov at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66512

            Bug ID: 66512
           Summary: PRE fails to optimize calls to pure functions in C++,
                    ok in C
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: amonakov at gcc dot gnu.org
  Target Milestone: ---

The following testcase:

int p(void) __attribute__((const));
void g(int);
void f()
{
  for (;;)
    g(p());
}

is optimized when compiled as C:

f:
        pushq   %rbx
        call    p
        movl    %eax, %ebx
.L2:
        movl    %ebx, %edi
        call    g
        jmp     .L2

and not optimized when compiled as C++:

_Z1fv:
        subq    $8, %rsp
.L2:
        call    _Z1pv
        movl    %eax, %edi
        call    _Z1gi
        jmp     .L2

For C the motion is performed by the PRE pass; tree dumps for PRE diverge
after:

@@ -95,76 +95,52 @@ Value numbering .MEM_4 stmt = g (_3);
 Setting value number of .MEM_4 to .MEM_4
 Processing SCC needed 3 iterations
 Value numbers:
[...]
 avail_out[2] := {  }
-exp_gen[3] := { {call_expr<p>} (0003) }
+exp_gen[3] := {  }
 phi_gen[3] := {  }