[Qemu-devel] [PATCH V2] build: remove compile warning

[Qemu-devel] [PATCH V2] build: remove compile warning

[Wenchao Xia]

This patch simply remove "variable may be used uninitialized" warning.

Signed-off-by: Wenchao Xia <xiawenc@linux.vnet.ibm.com>
---
V2: Address Stefan and Peter's comments, use 0 in send_msg() instead of
initialize mhHeader.

 libcacard/vscclient.c |    3 +--
 util/iov.c            |    2 +-
 2 files changed, 2 insertions(+), 3 deletions(-)

diff --git a/libcacard/vscclient.c b/libcacard/vscclient.c
index ac23647..7fbf1da 100644
--- a/libcacard/vscclient.c
+++ b/libcacard/vscclient.c
@@ -641,7 +641,6 @@ main(
     GIOChannel *channel_stdin;
     char *qemu_host;
     char *qemu_port;
-    VSCMsgHeader mhHeader;
 
     VCardEmulOptions *command_line_options = NULL;
 
@@ -750,7 +749,7 @@ main(
         .magic = VSCARD_MAGIC,
         .capabilities = {0}
     };
-    send_msg(VSC_Init, mhHeader.reader_id, &init, sizeof(init));
+    send_msg(VSC_Init, 0, &init, sizeof(init));
 
     g_main_loop_run(loop);
     g_main_loop_unref(loop);
diff --git a/util/iov.c b/util/iov.c
index cc6e837..b91cfb9 100644
--- a/util/iov.c
+++ b/util/iov.c
@@ -146,7 +146,7 @@ ssize_t iov_send_recv(int sockfd, struct iovec *iov, unsigned iov_cnt,
 {
     ssize_t total = 0;
     ssize_t ret;
-    size_t orig_len, tail;
+    size_t orig_len = 0, tail;
     unsigned niov;
 
     while (bytes > 0) {
-- 
1.7.1

Re: [Qemu-devel] [PATCH V2] build: remove compile warning

[Stefan Hajnoczi]

On Fri, Jun 07, 2013 at 06:02:09PM +0800, Wenchao Xia wrote:
> This patch simply remove "variable may be used uninitialized" warning.
> 
> Signed-off-by: Wenchao Xia <xiawenc@linux.vnet.ibm.com>
> ---
> V2: Address Stefan and Peter's comments, use 0 in send_msg() instead of
> initialize mhHeader.
> 
>  libcacard/vscclient.c |    3 +--
>  util/iov.c            |    2 +-
>  2 files changed, 2 insertions(+), 3 deletions(-)

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

Re: [Qemu-devel] [PATCH V2] build: remove compile warning

[Markus Armbruster]

Wenchao Xia <xiawenc@linux.vnet.ibm.com> writes:

> This patch simply remove "variable may be used uninitialized" warning.
>
> Signed-off-by: Wenchao Xia <xiawenc@linux.vnet.ibm.com>
> ---
> V2: Address Stefan and Peter's comments, use 0 in send_msg() instead of
> initialize mhHeader.
>
>  libcacard/vscclient.c |    3 +--
>  util/iov.c            |    2 +-
>  2 files changed, 2 insertions(+), 3 deletions(-)
>
> diff --git a/libcacard/vscclient.c b/libcacard/vscclient.c
> index ac23647..7fbf1da 100644
> --- a/libcacard/vscclient.c
> +++ b/libcacard/vscclient.c
> @@ -641,7 +641,6 @@ main(
>      GIOChannel *channel_stdin;
>      char *qemu_host;
>      char *qemu_port;
> -    VSCMsgHeader mhHeader;
>  
>      VCardEmulOptions *command_line_options = NULL;
>  
> @@ -750,7 +749,7 @@ main(
>          .magic = VSCARD_MAGIC,
>          .capabilities = {0}
>      };
> -    send_msg(VSC_Init, mhHeader.reader_id, &init, sizeof(init));
> +    send_msg(VSC_Init, 0, &init, sizeof(init));
>  
>      g_main_loop_run(loop);
>      g_main_loop_unref(loop);

This one's actually a bit more than just a warning suppression.  The
uninitialized value gets passed to send_msg(), which prints it if
verbose > 10.

Makes no sense to me.  Comes from commit 2ac85b9; cc'ing its author for
advice.

> diff --git a/util/iov.c b/util/iov.c
> index cc6e837..b91cfb9 100644
> --- a/util/iov.c
> +++ b/util/iov.c
> @@ -146,7 +146,7 @@ ssize_t iov_send_recv(int sockfd, struct iovec *iov, unsigned iov_cnt,
>  {
>      ssize_t total = 0;
>      ssize_t ret;
> -    size_t orig_len, tail;
> +    size_t orig_len = 0, tail;
>      unsigned niov;
>  
>      while (bytes > 0) {

Here are the uses of orig_len:

        if (tail) {
            /* second, fixup the last element, and remember the original
             * length */
            assert(niov < iov_cnt);
            assert(iov[niov].iov_len > tail);
            orig_len = iov[niov].iov_len;
            iov[niov++].iov_len = tail;
        }

        ret = do_send_recv(sockfd, iov, niov, do_send);

        /* Undo the changes above before checking for errors */
        if (tail) {
            iov[niov-1].iov_len = orig_len;
        }


gcc is too stupid to understand the control flow.  The initialization
shuts it up.

Personally, I dislike "shut up" initializations, because when you
mistakenly adds a new uninitialized use, you get the arbitrary "shut up"
value without warning.

Possible alternative:

        if (tail) {
            /* second, fixup the last element, and remember the original
             * length */
            assert(niov < iov_cnt);
            assert(iov[niov].iov_len > tail);
            orig_len = iov[niov].iov_len;
            iov[niov++].iov_len = tail;
            ret = do_send_recv(sockfd, iov, niov, do_send);
            /* Undo the changes above before checking for errors */
            iov[niov-1].iov_len = orig_len;
        } else {
            ret = do_send_recv(sockfd, iov, niov, do_send);
        }

Re: [Qemu-devel] [PATCH V2] build: remove compile warning

[Wenchao Xia]

   I insist to remove compile warning since I want gcc check my code with
strict rule.

> Wenchao Xia <xiawenc@linux.vnet.ibm.com> writes:
>
>> This patch simply remove "variable may be used uninitialized" warning.
>>
>> Signed-off-by: Wenchao Xia <xiawenc@linux.vnet.ibm.com>
>> ---
>> V2: Address Stefan and Peter's comments, use 0 in send_msg() instead of
>> initialize mhHeader.
>>
>>   libcacard/vscclient.c |    3 +--
>>   util/iov.c            |    2 +-
>>   2 files changed, 2 insertions(+), 3 deletions(-)
>>

>
>> diff --git a/util/iov.c b/util/iov.c
>> index cc6e837..b91cfb9 100644
>> --- a/util/iov.c
>> +++ b/util/iov.c
>> @@ -146,7 +146,7 @@ ssize_t iov_send_recv(int sockfd, struct iovec *iov, unsigned iov_cnt,
>>   {
>>       ssize_t total = 0;
>>       ssize_t ret;
>> -    size_t orig_len, tail;
>> +    size_t orig_len = 0, tail;
>>       unsigned niov;
>>
>>       while (bytes > 0) {
>
> Here are the uses of orig_len:
>
>          if (tail) {
>              /* second, fixup the last element, and remember the original
>               * length */
>              assert(niov < iov_cnt);
>              assert(iov[niov].iov_len > tail);
>              orig_len = iov[niov].iov_len;
>              iov[niov++].iov_len = tail;
>          }
>
>          ret = do_send_recv(sockfd, iov, niov, do_send);
>
>          /* Undo the changes above before checking for errors */
>          if (tail) {
>              iov[niov-1].iov_len = orig_len;
>          }
>
>
> gcc is too stupid to understand the control flow.  The initialization
> shuts it up.
>
> Personally, I dislike "shut up" initializations, because when you
> mistakenly adds a new uninitialized use, you get the arbitrary "shut up"
> value without warning.
>
> Possible alternative:
>
>          if (tail) {
>              /* second, fixup the last element, and remember the original
>               * length */
>              assert(niov < iov_cnt);
>              assert(iov[niov].iov_len > tail);
>              orig_len = iov[niov].iov_len;
>              iov[niov++].iov_len = tail;
>              ret = do_send_recv(sockfd, iov, niov, do_send);
>              /* Undo the changes above before checking for errors */
>              iov[niov-1].iov_len = orig_len;
>          } else {
>              ret = do_send_recv(sockfd, iov, niov, do_send);
>          }
>
   OK to work, but duplicated a line. I think it is not bad to give
default value as zero, even there will be new use later.

-- 
Best Regards

Wenchao Xia

Re: [Qemu-devel] [PATCH V2] build: remove compile warning

[Paolo Bonzini]

Il 08/06/2013 04:04, Wenchao Xia ha scritto:
>   I insist to remove compile warning since I want gcc check my code with
> strict rule.

What is your version of GCC?

Paolo

Re: [Qemu-devel] [PATCH V2] build: remove compile warning

[Paolo Bonzini]

Il 07/06/2013 14:17, Markus Armbruster ha scritto:
>> diff --git a/util/iov.c b/util/iov.c
>> index cc6e837..b91cfb9 100644
>> --- a/util/iov.c
>> +++ b/util/iov.c
>> @@ -146,7 +146,7 @@ ssize_t iov_send_recv(int sockfd, struct iovec *iov, unsigned iov_cnt,
>>  {
>>      ssize_t total = 0;
>>      ssize_t ret;
>> -    size_t orig_len, tail;
>> +    size_t orig_len = 0, tail;
>>      unsigned niov;
>>  
>>      while (bytes > 0) {
> 
> Here are the uses of orig_len:
> 
>         if (tail) {
>             /* second, fixup the last element, and remember the original
>              * length */
>             assert(niov < iov_cnt);
>             assert(iov[niov].iov_len > tail);
>             orig_len = iov[niov].iov_len;
>             iov[niov++].iov_len = tail;
>         }
> 
>         ret = do_send_recv(sockfd, iov, niov, do_send);
> 
>         /* Undo the changes above before checking for errors */
>         if (tail) {
>             iov[niov-1].iov_len = orig_len;
>         }
> 
> 
> gcc is too stupid to understand the control flow.  The initialization
> shuts it up.

Looks like most people's GCC is not that stupid, or I would have broken
build for everyone, right?

Paolo

Re: [Qemu-devel] [PATCH V2] build: remove compile warning

[Wenchao Xia]

于 2013-6-18 18:13, Paolo Bonzini 写道:
> Il 07/06/2013 14:17, Markus Armbruster ha scritto:
>>> diff --git a/util/iov.c b/util/iov.c
>>> index cc6e837..b91cfb9 100644
>>> --- a/util/iov.c
>>> +++ b/util/iov.c
>>> @@ -146,7 +146,7 @@ ssize_t iov_send_recv(int sockfd, struct iovec *iov, unsigned iov_cnt,
>>>   {
>>>       ssize_t total = 0;
>>>       ssize_t ret;
>>> -    size_t orig_len, tail;
>>> +    size_t orig_len = 0, tail;
>>>       unsigned niov;
>>>
>>>       while (bytes > 0) {
>>
>> Here are the uses of orig_len:
>>
>>          if (tail) {
>>              /* second, fixup the last element, and remember the original
>>               * length */
>>              assert(niov < iov_cnt);
>>              assert(iov[niov].iov_len > tail);
>>              orig_len = iov[niov].iov_len;
>>              iov[niov++].iov_len = tail;
>>          }
>>
>>          ret = do_send_recv(sockfd, iov, niov, do_send);
>>
>>          /* Undo the changes above before checking for errors */
>>          if (tail) {
>>              iov[niov-1].iov_len = orig_len;
>>          }
>>
>>
>> gcc is too stupid to understand the control flow.  The initialization
>> shuts it up.
>
> Looks like most people's GCC is not that stupid, or I would have broken
> build for everyone, right?
>
> Paolo
>

my gcc version:
[xiawenc@RH63Wenchao ~]$ gcc -v
Using built-in specs.
Target: x86_64-redhat-linux
Configured with: ../configure --prefix=/usr --mandir=/usr/share/man 
--infodir=/usr/share/info 
--with-bugurl=http://bugzilla.redhat.com/bugzilla --enable-bootstrap 
--enable-shared --enable-threads=posix --enable-checking=release 
--with-system-zlib --enable-__cxa_atexit --disable-libunwind-exceptions 
--enable-gnu-unique-object 
--enable-languages=c,c++,objc,obj-c++,java,fortran,ada 
--enable-java-awt=gtk --disable-dssi 
--with-java-home=/usr/lib/jvm/java-1.5.0-gcj-1.5.0.0/jre 
--enable-libgcj-multifile --enable-java-maintainer-mode 
--with-ecj-jar=/usr/share/java/eclipse-ecj.jar 
--disable-libjava-multilib --with-ppl --with-cloog --with-tune=generic 
--with-arch_32=i686 --build=x86_64-redhat-linux
Thread model: posix
gcc version 4.4.6 20120305 (Red Hat 4.4.6-4) (GCC)


   By default configure, it seems qemu didn't set -Werror to break build.
-- 
Best Regards

Wenchao Xia

Re: [Qemu-devel] [PATCH V2] build: remove compile warning

[Paolo Bonzini]

Il 19/06/2013 08:27, Wenchao Xia ha scritto:
>>> gcc is too stupid to understand the control flow.  The initialization
>>> shuts it up.
>>
>> Looks like most people's GCC is not that stupid, or I would have broken
>> build for everyone, right?
> 
> my gcc version:
> [xiawenc@RH63Wenchao ~]$ gcc -v
> Using built-in specs.
> Target: x86_64-redhat-linux
> Configured with: ../configure --prefix=/usr --mandir=/usr/share/man
> --infodir=/usr/share/info
> --with-bugurl=http://bugzilla.redhat.com/bugzilla --enable-bootstrap
> --enable-shared --enable-threads=posix --enable-checking=release
> --with-system-zlib --enable-__cxa_atexit --disable-libunwind-exceptions
> --enable-gnu-unique-object
> --enable-languages=c,c++,objc,obj-c++,java,fortran,ada
> --enable-java-awt=gtk --disable-dssi
> --with-java-home=/usr/lib/jvm/java-1.5.0-gcj-1.5.0.0/jre
> --enable-libgcj-multifile --enable-java-maintainer-mode
> --with-ecj-jar=/usr/share/java/eclipse-ecj.jar
> --disable-libjava-multilib --with-ppl --with-cloog --with-tune=generic
> --with-arch_32=i686 --build=x86_64-redhat-linux
> Thread model: posix
> gcc version 4.4.6 20120305 (Red Hat 4.4.6-4) (GCC)
> 
> 
>   By default configure, it seems qemu didn't set -Werror to break build.

It sets -Werror here:

gcc (GCC) 4.7.2 20121109 (Red Hat 4.7.2-8)

So configure is doing the right thing for both old and new-enough
compilers, I don't think there's a reason to change the code.

Paolo

Re: [Qemu-devel] [PATCH V2] build: remove compile warning

[Stefan Weil]

Am 18.06.2013 12:13, schrieb Paolo Bonzini:
> Il 07/06/2013 14:17, Markus Armbruster ha scritto:
>>> diff --git a/util/iov.c b/util/iov.c
>>> index cc6e837..b91cfb9 100644
>>> --- a/util/iov.c
>>> +++ b/util/iov.c
>>> @@ -146,7 +146,7 @@ ssize_t iov_send_recv(int sockfd, struct iovec *iov, unsigned iov_cnt,
>>>  {
>>>      ssize_t total = 0;
>>>      ssize_t ret;
>>> -    size_t orig_len, tail;
>>> +    size_t orig_len = 0, tail;
>>>      unsigned niov;
>>>  
>>>      while (bytes > 0) {
>> Here are the uses of orig_len:
>>
>>         if (tail) {
>>             /* second, fixup the last element, and remember the original
>>              * length */
>>             assert(niov < iov_cnt);
>>             assert(iov[niov].iov_len > tail);
>>             orig_len = iov[niov].iov_len;
>>             iov[niov++].iov_len = tail;
>>         }
>>
>>         ret = do_send_recv(sockfd, iov, niov, do_send);
>>
>>         /* Undo the changes above before checking for errors */
>>         if (tail) {
>>             iov[niov-1].iov_len = orig_len;
>>         }
>>
>>
>> gcc is too stupid to understand the control flow.  The initialization
>> shuts it up.
> Looks like most people's GCC is not that stupid, or I would have broken
> build for everyone, right?
>
> Paolo


Hi Paolo,

I get this warning, too, when I run a normal cross compilation with
MinGW-w64:

util/iov.c:190:33: warning: ‘orig_len’ may be used uninitialized in this
function [-Wuninitialized]

My build environment:

Debian wheezy with packages gcc-mingw-w64-i686, gcc-mingw-w64-x86-64
(4.6.3-14+8).

A complete build results in 5 warnings. Here are the other 4 of them:

hw/arm/spitz.c:280:0: warning: "MOD_SHIFT" redefined [enabled by default]
hw/ppc/spapr.c:673:26: warning: cast from pointer to integer of
different size [-Wpointer-to-int-cast]
hw/ppc/spapr_hcall.c:188:1: warning: control reaches end of non-void
function [-Wreturn-type]
hw/ppc/spapr_pci.c:454:1: warning: control reaches end of non-void
function [-Wreturn-type]

I already sent a patch for the MOD_SHIFT issue.

The remaining 3 warnings are also caused by code which makes it difficult
for the compiler to detect that it is correct.

Regards
Stefan

Re: [Qemu-devel] [PATCH V2] build: remove compile warning

[Paolo Bonzini]

Il 22/06/2013 12:03, Stefan Weil ha scritto:
> Am 18.06.2013 12:13, schrieb Paolo Bonzini:
>> Il 07/06/2013 14:17, Markus Armbruster ha scritto:
>>>> diff --git a/util/iov.c b/util/iov.c
>>>> index cc6e837..b91cfb9 100644
>>>> --- a/util/iov.c
>>>> +++ b/util/iov.c
>>>> @@ -146,7 +146,7 @@ ssize_t iov_send_recv(int sockfd, struct iovec *iov, unsigned iov_cnt,
>>>>  {
>>>>      ssize_t total = 0;
>>>>      ssize_t ret;
>>>> -    size_t orig_len, tail;
>>>> +    size_t orig_len = 0, tail;
>>>>      unsigned niov;
>>>>  
>>>>      while (bytes > 0) {
>>> Here are the uses of orig_len:
>>>
>>>         if (tail) {
>>>             /* second, fixup the last element, and remember the original
>>>              * length */
>>>             assert(niov < iov_cnt);
>>>             assert(iov[niov].iov_len > tail);
>>>             orig_len = iov[niov].iov_len;
>>>             iov[niov++].iov_len = tail;
>>>         }
>>>
>>>         ret = do_send_recv(sockfd, iov, niov, do_send);
>>>
>>>         /* Undo the changes above before checking for errors */
>>>         if (tail) {
>>>             iov[niov-1].iov_len = orig_len;
>>>         }
>>>
>>>
> 
> I get this warning, too, when I run a normal cross compilation with
> MinGW-w64:
> 
> util/iov.c:190:33: warning: ‘orig_len’ may be used uninitialized in this
> function [-Wuninitialized]
> 
> My build environment:
> 
> Debian wheezy with packages gcc-mingw-w64-i686, gcc-mingw-w64-x86-64
> (4.6.3-14+8).
> 
> A complete build results in 5 warnings. Here are the other 4 of them:
> 
> hw/arm/spitz.c:280:0: warning: "MOD_SHIFT" redefined [enabled by default]
> hw/ppc/spapr.c:673:26: warning: cast from pointer to integer of
> different size [-Wpointer-to-int-cast]

Isn't this one a Win64 bug?

> hw/ppc/spapr_hcall.c:188:1: warning: control reaches end of non-void
> function [-Wreturn-type]
> hw/ppc/spapr_pci.c:454:1: warning: control reaches end of non-void
> function [-Wreturn-type]

I think you could report this to mingw.  GCC should handle "if (!0)
foo()" just fine if foo is noreturn, perhaps the "assertion failure"
runtime function is not noreturn in mingw.

> I already sent a patch for the MOD_SHIFT issue.
> 
> The remaining 3 warnings are also caused by code which makes it difficult
> for the compiler to detect that it is correct.

Not as hard as this one, though.

Anyway, I would be okay a fix that makes the code easier to follow for
compilers (and doesn't have too much duplication so that humans are also
happy).  But adding "= 0" is the worst, because smart compilers will not
detect a human's mistakes anymore.

Paolo

Re: [Qemu-devel] [PATCH V2] build: remove compile warning

[Stefan Weil]

Am 24.06.2013 16:44, schrieb Paolo Bonzini:
> Il 22/06/2013 12:03, Stefan Weil ha scritto:
>> I get this warning, too, when I run a normal cross compilation with
>> MinGW-w64:
>>
>> util/iov.c:190:33: warning: ‘orig_len’ may be used uninitialized in this
>> function [-Wuninitialized]
>>
>> My build environment:
>>
>> Debian wheezy with packages gcc-mingw-w64-i686, gcc-mingw-w64-x86-64
>> (4.6.3-14+8).
>>
>> A complete build results in 5 warnings. Here are the other 4 of them:
>>
>> hw/arm/spitz.c:280:0: warning: "MOD_SHIFT" redefined [enabled by default]
>> hw/ppc/spapr.c:673:26: warning: cast from pointer to integer of
>> different size [-Wpointer-to-int-cast]
> Isn't this one a Win64 bug?

Yes. http://patchwork.ozlabs.org/patch/252666/ fixes it.

>> hw/ppc/spapr_hcall.c:188:1: warning: control reaches end of non-void
>> function [-Wreturn-type]
>> hw/ppc/spapr_pci.c:454:1: warning: control reaches end of non-void
>> function [-Wreturn-type]
> I think you could report this to mingw.  GCC should handle "if (!0)
> foo()" just fine if foo is noreturn, perhaps the "assertion failure"
> runtime function is not noreturn in mingw.

It's a gcc problem. Removing the assertion manually in the source
code and compiling with NDEBUG (which we do by default)results
in the same compiler warning.

I have sent a small patch series which fixes both warnings and
which hopefully matches your criteria for acceptable code changes :-)

See http://patchwork.ozlabs.org/patch/253937/ and two more.

Regards
Stefan

Re: [Qemu-devel] [PATCH V2] build: remove compile warning

[Paolo Bonzini]

Il 24/06/2013 19:58, Stefan Weil ha scritto:
>>> hw/ppc/spapr_hcall.c:188:1: warning: control reaches end of non-void
>>> function [-Wreturn-type]
>>> hw/ppc/spapr_pci.c:454:1: warning: control reaches end of non-void
>>> function [-Wreturn-type]
>> I think you could report this to mingw.  GCC should handle "if (!0)
>> foo()" just fine if foo is noreturn, perhaps the "assertion failure"
>> runtime function is not noreturn in mingw.
> 
> It's a gcc problem. Removing the assertion manually in the source
> code and compiling with NDEBUG (which we do by default)results
> in the same compiler warning.

Huh?  That seems wrong, assertions are there for a reason...

Paolo

Re: [Qemu-devel] [PATCH V2] build: remove compile warning

[Wenchao Xia]

于 2013-6-24 22:44, Paolo Bonzini 写道:
> Il 22/06/2013 12:03, Stefan Weil ha scritto:
>> Am 18.06.2013 12:13, schrieb Paolo Bonzini:
>>> Il 07/06/2013 14:17, Markus Armbruster ha scritto:
>>>>> diff --git a/util/iov.c b/util/iov.c
>>>>> index cc6e837..b91cfb9 100644
>>>>> --- a/util/iov.c
>>>>> +++ b/util/iov.c
>>>>> @@ -146,7 +146,7 @@ ssize_t iov_send_recv(int sockfd, struct iovec *iov, unsigned iov_cnt,
>>>>>   {
>>>>>       ssize_t total = 0;
>>>>>       ssize_t ret;
>>>>> -    size_t orig_len, tail;
>>>>> +    size_t orig_len = 0, tail;
>>>>>       unsigned niov;
>>>>>
>>>>>       while (bytes > 0) {
>>>> Here are the uses of orig_len:
>>>>
>>>>          if (tail) {
>>>>              /* second, fixup the last element, and remember the original
>>>>               * length */
>>>>              assert(niov < iov_cnt);
>>>>              assert(iov[niov].iov_len > tail);
>>>>              orig_len = iov[niov].iov_len;
>>>>              iov[niov++].iov_len = tail;
>>>>          }
>>>>
>>>>          ret = do_send_recv(sockfd, iov, niov, do_send);
>>>>
>>>>          /* Undo the changes above before checking for errors */
>>>>          if (tail) {
>>>>              iov[niov-1].iov_len = orig_len;
>>>>          }
>>>>
>>>>
>>
>> I get this warning, too, when I run a normal cross compilation with
>> MinGW-w64:
>>
>> util/iov.c:190:33: warning: ‘orig_len’ may be used uninitialized in this
>> function [-Wuninitialized]
>>
>> My build environment:
>>
>> Debian wheezy with packages gcc-mingw-w64-i686, gcc-mingw-w64-x86-64
>> (4.6.3-14+8).
>>
>> A complete build results in 5 warnings. Here are the other 4 of them:
>>
>> hw/arm/spitz.c:280:0: warning: "MOD_SHIFT" redefined [enabled by default]
>> hw/ppc/spapr.c:673:26: warning: cast from pointer to integer of
>> different size [-Wpointer-to-int-cast]
>
> Isn't this one a Win64 bug?
>
>> hw/ppc/spapr_hcall.c:188:1: warning: control reaches end of non-void
>> function [-Wreturn-type]
>> hw/ppc/spapr_pci.c:454:1: warning: control reaches end of non-void
>> function [-Wreturn-type]
>
> I think you could report this to mingw.  GCC should handle "if (!0)
> foo()" just fine if foo is noreturn, perhaps the "assertion failure"
> runtime function is not noreturn in mingw.
>
>> I already sent a patch for the MOD_SHIFT issue.
>>
>> The remaining 3 warnings are also caused by code which makes it difficult
>> for the compiler to detect that it is correct.
>
> Not as hard as this one, though.
>
> Anyway, I would be okay a fix that makes the code easier to follow for
> compilers (and doesn't have too much duplication so that humans are also
> happy).  But adding "= 0" is the worst, because smart compilers will not
> detect a human's mistakes anymore.
>
> Paolo
>

Hi Palo,
   There is V3 remove uninitlized warning without adding "= 0", could
u take a look for it?

http://lists.nongnu.org/archive/html/qemu-devel/2013-06/msg02348.html

-- 
Best Regards

Wenchao Xia