[Bug c++/100409] New: C++ FE elides pure throwing call

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100409

            Bug ID: 100409
           Summary: C++ FE elides pure throwing call
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: rguenth at gcc dot gnu.org
  Target Milestone: ---

The C++ FE elides the call to foo() in

int x, y;
int __attribute__((pure,noinline)) foo () { if (x) throw 1; return y; }
void __attribute__((noinline)) foo2 () { foo (); }

despite it throwing an exception.  It does this in finish_expr_stmt
by calling convert_to_void on the following, likely relying on
!TREE_SIDE_EFFECTS

 <call_expr 0x7ffff66dca20
    type <integer_type 0x7ffff65885e8 int public type_6 SI
        size <integer_cst 0x7ffff658b108 constant 32>
        unit-size <integer_cst 0x7ffff658b120 constant 4>
        align:32 warn_if_not_align:0 symtab:0 alias-set -1 canonical-type
0x7ffff65885e8 precision:32 min <integer_cst 0x7ffff658b0c0 -2147483648> max
<integer_cst 0x7ffff658b0d8 2147483647>
        pointer_to_this <pointer_type 0x7ffff65909d8>>

    fn <addr_expr 0x7ffff66c7be0
        type <pointer_type 0x7ffff66e29d8 type <function_type 0x7ffff65989d8>
            unsigned DI
            size <integer_cst 0x7ffff656aeb8 constant 64>
            unit-size <integer_cst 0x7ffff656aed0 constant 8>
            align:64 warn_if_not_align:0 symtab:0 alias-set -1 canonical-type
0x7ffff66e29d8>
        constant
        arg:0 <function_decl 0x7ffff66cde00 foo type <function_type
0x7ffff65989d8>
            addressable used public static uninlinable decl_5 QI t2.c:2:36
align:8 warn_if_not_align:0 context <translation_unit_decl 0x7ffff6578168 t2.c>
            attributes <tree_list 0x7ffff66c8de8
                purpose <identifier_node 0x7ffff66dbe00 noinline
                    normal local bindings <(nil)>>
                chain <tree_list 0x7ffff66c8dc0
                    purpose <identifier_node 0x7ffff65b4200 pure
                        normal local bindings <(nil)>>>> initial <block
0x7ffff66d14e0> result <result_decl 0x7ffff6578e10 D.2349>
            full-name "int foo()"
            struct-function 0x7ffff66de000 chain <var_decl 0x7ffff7ff5c60 y>>>
    t2.c:3:46 start: t2.c:3:42 finish: t2.c:3:47>

it happens here:

  if (expr != error_mark_node && !VOID_TYPE_P (TREE_TYPE (expr)))
    {
...
      expr = build1 (CONVERT_EXPR, void_type_node, expr);
    }
  if (! TREE_SIDE_EFFECTS (expr))
    expr = void_node;
  return expr;