[Bug rtl-optimization/55747] New: Extra registers are saved in functions that only call noreturn functions

[Bug rtl-optimization/55747] New: Extra registers are saved in functions that only call noreturn functions

[josh.m.conner at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55747

             Bug #: 55747
           Summary: Extra registers are saved in functions that only call
                    noreturn functions
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: rtl-optimization
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: josh.m.conner@gmail.com


On architectures such as ARM, where a link register is used to save the return
address, this value does not need to be saved in a function that only calls
noreturn functions.

For example, if I build the following source:

  __attribute__((noreturn))
  extern void bar (void);

  int x;

  void foo (void)
  {
    if (x)
      bar ();
  }

Using the options "-O2", the link register is saved:

  stmfd   sp!, {r3, lr}
  ...
  ldmeqfd sp!, {r3, pc}

However, this is unnecessary since the only way the link register cannot be
corrupted since any calls to "bar" will not return.

Note that I am not filing this as an ARM target bug since the issue appears to
be a general problem related to dataflow analysis not tracking the difference
between calls to normal functions and calls to noreturn functions.  At any
rate, I see a similar problem in our custom target as well.

[Bug rtl-optimization/55747] Extra registers are saved in functions that only call noreturn functions

[pinskia at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55747

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> 2012-12-20 01:22:11 UTC ---
Actually noreturns are handled special in the compiler.  I filed this bug over
6 years ago and it was closed as invalid back then.

[Bug rtl-optimization/55747] Extra registers are saved in functions that only call noreturn functions

[hp at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55747

Hans-Peter Nilsson <hp at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2013-01-06
                 CC|                            |hp at gcc dot gnu.org
     Ever Confirmed|0                           |1

--- Comment #2 from Hans-Peter Nilsson <hp at gcc dot gnu.org> 2013-01-06 08:26:11 UTC ---
Similarly observed for cris-elf with -O2, r194929, JFTR.

If this changes, some attribute markup should be added to keep return-addresses
for noreturn functions, for sake of stack-traces.  Also return-addresses need
to be generally kept when -fexceptions and similar are in effect, as noreturn
functions can still throw.

[Bug rtl-optimization/55747] Extra registers are saved in functions that only call noreturn functions

[olegendo at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55747

Oleg Endo <olegendo at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Target|                            |sh*-*-* arm*-*-* cris*-*-*
                 CC|                            |olegendo at gcc dot gnu.org

--- Comment #3 from Oleg Endo <olegendo at gcc dot gnu.org> ---
On SH there's a similar issue.  E.g. compiling

void exit (int __status) __attribute__ ((noreturn));

int test (int a, int b, int c)
{
  if (a == b)
    return 5;

  if (a)
    exit (0);

  return 0;
}

with -O2 -m4 results in:

_test:
        cmp/eq  r5,r4
        bt/s    .L3
        tst     r4,r4
        bf/s    .L10
        mov     #0,r0
        rts
        nop
.L3:
        rts
        mov     #5,r0
.L10:
        mov.l   .L11,r1
        sts.l   pr,@-r15     // push return address register
        jsr     @r1          // jump to subroutine
        mov     #0,r4
.L12:
        .align 2
.L11:
        .long   _exit

The jsr should be a simple jmp and the pr push is not required (as with leaf
function calls).

[Bug rtl-optimization/55747] Extra registers are saved in functions that only call noreturn functions

[rearnsha at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55747

Richard Earnshaw <rearnsha at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|---                         |WONTFIX

--- Comment #4 from Richard Earnshaw <rearnsha at gcc dot gnu.org> ---
Sorry, we're not going to change this.  Removing the saved lr value would break
back-tracing out of functions that called abort().