[Bug tree-optimization/94818] New: GCC emits dead bodies of functions whose all calls have been eliminated by optimisations

[Bug tree-optimization/94818] New: GCC emits dead bodies of functions whose all calls have been eliminated by optimisations

[felix.von.s at posteo dot de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94818

            Bug ID: 94818
           Summary: GCC emits dead bodies of functions whose all calls
                    have been eliminated by optimisations
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: felix.von.s at posteo dot de
  Target Milestone: ---
            Target: x86_64-linux-gnu

In this code:

    __attribute__((__warning__("argh")))
    static void foo(void) {
        __asm__ (
            ";;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;"
            ";;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;"
        );
    }

    void bar0(int x) {
        if (__builtin_constant_p(x))
            foo();
    }

    void bar1(int x) {
        if (__builtin_constant_p(x))
            foo();
    }

No calls to foo are emitted in the final output, and GCC knows this, since no
warnings are emitted. However, GCC will still emit a function body for foo,
unless either the __asm__ statement is removed or both conditions are changed
to integer constant expressions, so that the function is eliminated by one of
the inlining passes.

(I decided to demonstrate it this way instead of using
__attribute__((__noinline__)), just to make sure this is not related to
__attribute__((__noinline__)) somehow implying __attribute__((__used__)).)

[Bug ipa/94818] GCC emits dead bodies of functions whose all calls have been eliminated by optimisations

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94818

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2020-04-28
                 CC|                            |marxin at gcc dot gnu.org
          Component|tree-optimization           |ipa
             Status|UNCONFIRMED                 |NEW

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
Confirmed.  ___builtin_constant_p is only resolved after IPA optimization and
at that point unreachable functions can no longer be removed reliably
(foo is output before bar0 __builtin_constant_p is resolved).

We could add another "IPA" sync point before RTL expansion but not sure if
we really want to.

We could also see there's no callers to bar0 and bar1 and resolve
__builtin_constant_p earlier during IPA.

[Bug ipa/94818] GCC emits dead bodies of functions whose all calls have been eliminated by optimisations

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94818

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Depends on|                            |47205, 89139

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Related to PR 47205, PR 89139


Referenced Bugs:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=47205
[Bug 47205] GCC emits optimized out noinline function
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89139
[Bug 89139] GCC emits code for static functions that aren't used by the
optimized code

[Bug ipa/94818] GCC emits dead bodies of functions whose all calls have been eliminated by optimisations

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94818

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|normal                      |enhancement