[Qemu-devel] Re : Re: [PATCH] sparc32 : Signed integer division overflow

[Qemu-devel] Re : Re: [PATCH] sparc32 : Signed integer division overflow

[Olivier DANET]

> ----- Message d'origine -----
> De : Mark Cave-Ayland
> Envoyés : 18.03.14 01:20
> À : Olivier Danet
> Objet : Re: [Qemu-devel] [PATCH] sparc32 : Signed integer division overflow
> 
> On 12/03/14 21:26, Olivier Danet wrote:
> 
> Hi Olivier,
> 
> > Here is a patch for handling this corner case on SPARC32.
> > SPARC64 division already checks this in helper_sdivx(), some other
> > architectures
> > seem to do the same (for example, target-arm/helper.c: HELPER(sdiv))
> >
> > ===================================================================
> > The integer division 0x8000_0000_0000_0000 / -1 must be handled separately
> > to avoid overflows on the QEMU host.
> >
> > Signed-off-by: Olivier Danet <odanet@caramail.com>
> >
> > -------------------------------------------------------------------
> > diff --git a/target-sparc/helper.c b/target-sparc/helper.c
> > index 57c20af..b6b5937 100644
> > --- a/target-sparc/helper.c
> > +++ b/target-sparc/helper.c
> > @@ -116,14 +116,16 @@ static target_ulong
> > helper_sdiv_common(CPUSPARCState *env, target_ulong a,
> > if (x1 == 0) {
> > cpu_restore_state(env, GETPC());
> > helper_raise_exception(env, TT_DIV_ZERO);
> > - }
> > -
> > - x0 = x0 / x1;
> > - if ((int32_t) x0 != x0) {
> > - x0 = x0 < 0 ? 0x80000000 : 0x7fffffff;
> > + } else if (x1 == -1 && x0 == 0x8000000000000000) {
> > + x0 = 0x7fffffff;
> > overflow = 1;
> 
> Thanks for the patch! I think based upon Peter's recent series that the 
> sign constant would need a ULL suffix in order to function correctly on 
> 32-bit platforms.
> 
> My personal preference would be for (1ULL << 63) unless Peter (CC added) 
> can think of a reason to leave the hex constant in its current form?
> 
> That said, I've tested the patch on a Debian etch Linux image and it 
> works for me.
> 
The constant lacks an "ULL" indeed, sorry.

There are both (1ULL << 63) and 0x8000000000000000[ULL] constants in QEMU code, 
and not a single 9223372036854775808ULL...

At least, with (1ULL << 63), we are not tempted to count the zeros.

Regards
Olivier.

Re: [Qemu-devel] Re : Re: [PATCH] sparc32 : Signed integer division overflow

[Richard Henderson]

On 03/18/2014 04:43 PM, Olivier DANET wrote:
>>> - x0 = x0 < 0 ? 0x80000000 : 0x7fffffff;
>>> > > + } else if (x1 == -1 && x0 == 0x8000000000000000) {
>>> > > + x0 = 0x7fffffff;
>>> > > overflow = 1;
>> > 
>> > Thanks for the patch! I think based upon Peter's recent series that the 
>> > sign constant would need a ULL suffix in order to function correctly on 
>> > 32-bit platforms.
>> > 
>> > My personal preference would be for (1ULL << 63) unless Peter (CC added) 
>> > can think of a reason to leave the hex constant in its current form?
>> > 
>> > That said, I've tested the patch on a Debian etch Linux image and it 
>> > works for me.
>> > 
> The constant lacks an "ULL" indeed, sorry.
> 
> There are both (1ULL << 63) and 0x8000000000000000[ULL] constants in QEMU code, 
> and not a single 9223372036854775808ULL...
> 
> At least, with (1ULL << 63), we are not tempted to count the zeros.

Not to bike-shed this too much, but INT32_MIN and INT64_MIN would be better and
more descriptive for these.  Honestly, we're supposed to be dealing with signed
numbers here, not the unsigned number you're creating above.


r~