On Thu, Mar 18, 2010 at 12:23 PM, Alain Ketterlin
<alain.ketter...@gmail.com> wrote:
>
> I've reported here recently about gcc producing conditional branches
> with static outcome. I've finally managed to produce a simple
> self-contained example. Here it is:
>
> int f()
> {
> static int LSW=-1;
> double x=0.987654321;
> unsigned char ix = *((char *)&x);
> if(ix==0xdd || ix==0x3f)
> LSW = 1;
> else
> LSW = 0;
> return 0;
> }
>
> Compiling this with "gcc-4.5 -O3 -c f.c" (20100304 snapshot) on my
> x86-64 system produces the following code:
>
> 0: b8 b8 ff ff ff mov $0xffffffb8,%eax
> 5: 3c dd cmp $0xdd,%al
> 7: 0f 94 c0 sete %al
> ...
>
> Ok, not a branch, but still useless code. gcc-4.4 does the same, and
> gcc-4.3 is slightly less clever (it moves the whole immediate 8 bytes
> into %rax and does two cmp+je).
>
> I first thought the type-cast was responsible. But if you replace the
> assignment to LSW with a simple return, gcc correctly optimizes the
> whole function away.
>
> Can anybody confirm this? Is it worth a bug report?
I can confirm this and yes, it is worth a bugreport. If you have
cases that do not involve floating-point constants accessed as
integer values that would be even more interesting (because
likely more common).
Thanks,
Richard.
> -- Alain.
>
