Re: [Qemu-devel] [PATCH] memory: Set notdirty_mem_ops validator

[Eric Blake <eblake@redhat.com>]

On 9/6/19 8:44 AM, Eric Blake wrote:
> On 9/6/19 8:24 AM, Philippe Mathieu-Daudé wrote:
> 
>>>>>  static const MemoryRegionOps notdirty_mem_ops = {
>>>>>      .write = notdirty_mem_write,
>>>>> -    .valid.accepts = notdirty_mem_accepts,
>>>>>      .endianness = DEVICE_NATIVE_ENDIAN,
>>>>>      .valid = {
>>>>>          .min_access_size = 1,
>>>>>          .max_access_size = 8,
>>>>>          .unaligned = false,
>>>>> +        .accepts = notdirty_mem_accepts,
>>>>
>>>> I'm surprised the compiler doesn't emit any warning...
>>>
>>> Same here.

Actually, I just played with -Woverride-init in gcc 9.2.1 (and clang's
comparable -Winitializer-overrides, which we intentionally disable
during configure), and they come pretty close - both compilers DO flag
when an implicit zero-initialization due to partial ={} overrides an
earlier initialization.  But sadly, they also warn when one specific
init of a smaller subobject overrides another earlier specific init of a
larger subobject such as an array range operator.  So
qobject/json-lexer.c and others fail to compile under the existing
warning option, which is why we disable it during configure (clang has
it as part of -Wall; gcc only has it as part of -Wextra which we do not
use).

In researching further, I see:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=24010#c4
which explains why -Woverride-init is NOT part of gcc's -Wall, precisely
because of our range pre-initialization usage.

So I filed:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91688
seeing if the gcc devs would consider splitting into
-Woverride-init=[12], where 1 only flags a larger subobject overriding
an earlier smaller one (would have caught our bug) and 2 flags an
equal-size or smaller subobject overriding an earlier large one (which
we would not use, because we rely on that for range pre-initialization).

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org