Leaf functions and noreturn calls

["Dave Korn" <dave dot korn at artimi dot com>]

    Good morning everyone!


  Here's a simple testcase that illustrates what I'm observing in gcc-3.3.3

-----------------------------<snip>-----------------------------
extern void abort (void) __attribute ((noreturn));

int foo (int a, int b)
{
  if (a > 25)
    abort ();
  return (a + b);
}

int bar (int a, int b)
{
  if (a > 25)
    a++;
  return (a + b);
}
-----------------------------<snip>-----------------------------

  Function bar() is clearly a leaf function, so at stackframe layout time I
get called with current_function_is_leaf == 1, and since it uses no
nonvolatile registers I get a function with no stackframe, no registers saved,
no prologue and nothing but a ret insn in the epilog.

  Function foo() is not regarded as a leaf function, because it calls abort,
and so my prologue generation code believes it has to create a stack frame
just to save the link register.

  However, abort is clearly marked noreturn:-

-----------------------------<snip>-----------------------------
;(call_insn 13 31 14 0x0 (parallel [
;            (call (mem:SI (symbol_ref:SI ("abort")) [0 S4 A32])
;                (const_int 0 [0x0]))
;            (clobber (reg:SI 15 r15))
;        ]) 36 {call} (nil)
;    (expr_list:REG_UNUSED (reg:SI 15 r15)
;        (expr_list:REG_NORETURN (const_int 0 [0x0])
;            (expr_list:REG_EH_REGION (const_int 0 [0x0])
;                (nil))))
;    (nil))
-----------------------------<snip>-----------------------------

and without this call it would clearly be a leaf function, and since this call
is noreturn we could in fact still treat it as a leaf function.

  Is there some complication that I haven't realised why noreturn function
calls can't be disregarded in deciding whether or not a function is leaf?
Taking a look at leaf_function_p, I see that it specifically discounts
sibcalls; why not noreturncalls as well?

    cheers,
      DaveK
-- 
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