Re: [Qemu-devel] [PATCH 2/2] target-i386: Don't left shift negative constant

[Eduardo Habkost]

On Thu, Oct 01, 2015 at 11:35:52AM +1000, Richard Henderson wrote:
> On 09/30/2015 06:34 AM, Eduardo Habkost wrote:
> >Left shift of negative values is undefined behavior. Detected by clang:
> >   qemu/target-i386/translate.c:2423:26: runtime error:
> >     left shift of negative value -8
> >
> >This changes the code to reverse the sign after the left shift.
> >
> >Signed-off-by: Eduardo Habkost <address@hidden>
> >---
> >  target-i386/translate.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> >diff --git a/target-i386/translate.c b/target-i386/translate.c
> >index 8b35de1..cc59b7e 100644
> >--- a/target-i386/translate.c
> >+++ b/target-i386/translate.c
> >@@ -2420,7 +2420,7 @@ static void gen_pusha(DisasContext *s)
> >  {
> >      int i;
> >      gen_op_movl_A0_reg(R_ESP);
> >-    gen_op_addl_A0_im(-8 << s->dflag);
> >+    gen_op_addl_A0_im(-(8 << s->dflag));
> 
> Better as -8U << s->d_flag?

That's even more confusing to me. I wouldn't want to require other
people to read the C specification to find out how many type conversions
are happening in that statement. (Because I will have to do that, to
find out what's the type of "-8U").

I would prefer an expression that doesn't involve any type conversion.
But you are more familiar with that code, so it's up to you.

-- 
Eduardo