Re: [Qemu-devel] [PATCH] tcg: Fix complilation with TCG

[Dr. David Alan Gilbert]

* Richard Henderson (address@hidden) wrote:
> On 11/22/2017 01:03 PM, Peter Maydell wrote:
> > On 22 November 2017 at 12:01, Richard Henderson
> > <address@hidden> wrote:
> >> On 11/22/2017 09:41 AM, Juan Quintela wrote:
> >>> This commit started use tb_unlock() and tlb_set_dirty() on non TCG
> >>> code.  Add the function as stubs.
> >>>
> >>> commit 27266271977c5a30f2f7d493e042be1897827bdd
> >>> Author: Peter Maydell <address@hidden>
> >>> Date:   Mon Nov 20 18:08:27 2017 +0000
> >>>
> >>>     exec.c: Factor out before/after actions for notdirty memory writes
> >>>     ....
> >>>
> >>> Signed-off-by: Juan Quintela <address@hidden>
> >>> ---
> >>>  accel/stubs/tcg-stub.c | 8 ++++++++
> >>>  1 file changed, 8 insertions(+)
> >>
> >> Hmm.  An alternate solution is
> >>
> >>
> >> diff --git a/exec.c b/exec.c
> >> index 03238a3449..ac902da89d 100644
> >> --- a/exec.c
> >> +++ b/exec.c
> >> @@ -2378,6 +2378,7 @@ void memory_notdirty_write_prepare(NotDirtyInfo *ndi,
> >>  /* Called within RCU critical section. */
> >>  void memory_notdirty_write_complete(NotDirtyInfo *ndi)
> >>  {
> >> +    assert(tcg_enabled());
> >>      if (ndi->locked) {
> >>          tb_unlock();
> >>      }
> >>
> >>
> >> so that the rest of the function is compiled away.
> >>
> >> Thus my confusion about how Peter was adding any references to these 
> >> functions
> >> -- he split half the function such that half is now not protected by an 
> >> assert.
> > 
> > Wow, I totally do not expect "assert(tcg_enabled())" to mean
> > "we rely on the compiler to be able to determine that this
> > code is dead" (and in general I'm not a fan of the compilation
> > relying on the compiler determining that code is dead).
> 
> We do this all of the time for more regular tests that are obviously
> compile-time.  I am a really big fan of this, because it makes sure that the
> (usually 32-bit) else branch continues to compile.
> 
> It's just that with --disable-tcg, tcg_enabled() expands to the less-obviously
> constant false.

On windows though assert() can return, see:

https://sourceforge.net/p/mingw-w64/bugs/306/

Dave


> 
> r~
> 
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK