RE: [PATCH] tcg/optimize: Add fallthrough annotations

[Chenqun (kuhn)]

> -----Original Message-----
> From: Richard Henderson [mailto:richard.henderson@linaro.org]
> Sent: Friday, October 30, 2020 4:07 AM
> To: Thomas Huth <thuth@redhat.com>; Richard Henderson <rth@twiddle.net>;
> qemu-devel@nongnu.org
> Cc: Chenqun (kuhn) <kuhn.chenqun@huawei.com>
> Subject: Re: [PATCH] tcg/optimize: Add fallthrough annotations
> 
> On 10/29/20 5:28 AM, Thomas Huth wrote:
> > To be able to compile this file with -Werror=implicit-fallthrough, we
> > need to add some fallthrough annotations to the case statements that
> > might fall through. Unfortunately, the typical "/* fallthrough */"
> > comments do not work here as expected since some case labels are
> > wrapped in macros and the compiler fails to match the comments in this
> > case. But using __attribute__((fallthrough)) seems to work fine, so
> > let's use that instead.
> 
> Why would the macro matter?  It expands to two case statements with
> nothing in between them.
> 
> This sounds like a compiler bug that should be reported.
> 
Hi all,
  I have queried the GCC options description about the Wimplicit-fallthrough 
and verified it.
The value of Wimplicit-fallthrough ranges from 0 to 5. 
The value 0 is to ignore all warnings, which is certainly not what we need.
If the value is set to 1 or 2, most fall through on the QEMU can take effect.
   Eg：/* FALLTHRU */、/* fallthru */、/* fall-through */、/* FALLTHOUGH */、/* fall 
through */、/* fallthrough */..

When the value ranges from 3 to 5, more fallthrough comments become invalid as 
the value increases.

So, I agree with Philippe's suggestion to add a QEMU_FALLTHROUGH to unify this 
compiler property.

Thanks,
Chen Qun

Additional gcc information is as follows:
https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html
-Wimplicit-fallthrough is the same as -Wimplicit-fallthrough=3 and 
-Wno-implicit-fallthrough is the same as -Wimplicit-fallthrough=0.

The option argument n specifies what kind of comments are accepted:
-Wimplicit-fallthrough=0 disables the warning altogether.
-Wimplicit-fallthrough=1 matches .* regular expression, any comment is used as 
fallthrough comment.
-Wimplicit-fallthrough=2 case insensitively matches .*falls?[ 
\t-]*thr(ough|u).* regular expression.
-Wimplicit-fallthrough=3 case sensitively matches one of the following regular 
expressions:
   -fallthrough
   @fallthrough@
   lint -fallthrough[ \t]*
   [ \t.!]*(ELSE,? |INTENTIONAL(LY)? )?
   FALL(S | |-)?THR(OUGH|U)[ \t.!]*(-[^\n\r]*)?
   [ \t.!]*(Else,? |Intentional(ly)? )?
   Fall((s | |-)[Tt]|t)hr(ough|u)[ \t.!]*(-[^\n\r]*)?
   [ \t.!]*([Ee]lse,? |[Ii]ntentional(ly)? )?
   fall(s | |-)?thr(ough|u)[ \t.!]*(-[^\n\r]*)?
-Wimplicit-fallthrough=4 case sensitively matches one of the following regular 
expressions:
    -fallthrough
    @fallthrough@
    lint -fallthrough[ \t]*
    [ \t]*FALLTHR(OUGH|U)[ \t]*
-Wimplicit-fallthrough=5 doesn’t recognize any comments as fallthrough 
comments, only attributes disable the warning.