Re: [Qemu-devel] [PATCH fix for 2.1] makefile: Fix tools compile

[Alexey Kardashevskiy]

On 07/01/2014 11:51 AM, Alexey Kardashevskiy wrote:
> The existing test whether "-lm" needs to be included or not is
> insufficient as it reports false negative on Fedora20/ppc64.
> As the result, qemu-nbd/qemu-io/qemu-img tools cannot compile.
> 
> This replaces sin() with log() in the test.
> 
> Signed-off-by: Alexey Kardashevskiy <address@hidden>
> 
> ---
> 
> The bug was triggered by efc6de0d0eb21bbd8fbc90da1faf7dd8ed9f5321
> "block/iscsi: handle BUSY condition" which does not anything wrong :)
> 
> It compiled well on Fedora19 though what is weird.
> 
> Is log() good enough or we need to test for both?
> 
> Thanks.
> 
> 
> ps. this is my test on the system upgraded from fc19 yesterday:
> 
> address@hidden ~]$ cat b.c
> #include <math.h>
> int main(void) { return isnan(log(0.0)); }
> 
> address@hidden ~]$ gcc b.c -o b
> /tmp/ccqp1EI4.o: In function `main':
> b.c:(.text+0x20): undefined reference to `log'
> collect2: error: ld returned 1 exit status
> address@hidden ~]$ gcc b.c -o b -lm
> address@hidden ~]$ cat /etc/issue
> Fedora release 20 (Heisenbug)
> Kernel \r on an \m (\l)
> 
> 
> ---
>  configure | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/configure b/configure
> index 23ecb37..c7a2922 100755
> --- a/configure
> +++ b/configure
> @@ -3453,7 +3453,7 @@ fi
>  # Do we need libm
>  cat > $TMPC << EOF
>  #include <math.h>
> -int main(void) { return isnan(sin(0.0)); }
> +int main(void) { return isnan(log(0.0)); }


This is wrong actually. The problem here that compiler knows how to
optimize constants. sin(0.0) is the one while log(0.0) is not (it is
supposed to throw error or something as it the result is infinity).

So the correct test here could be:
int main(void) { volatile double x = 1; return isnan(sin(x)); }

But I am afraid pretty soon compilers will learn how to optimize this as
well :)

So - what would be the right fix there? Always add "-lm" (on ppc64?)?



>  EOF
>  if compile_prog "" "" ; then
>    :
> 


-- 
Alexey