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Date: Tue, 18 Mar 2014 00:20:55 +0000
From: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
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Subject: Re: [Qemu-devel] [PATCH] sparc32 : Signed integer division overflow
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To: Olivier Danet <odanet@caramail.com>
Cc: Blue Swirl <blauwirbel@gmail.com>, Peter Maydell <peter.maydell@linaro.org>, qemu-devel <qemu-devel@nongnu.org>

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.

> + } else {
> + x0 = x0 / x1;
> + if ((int32_t) x0 != x0) {
> + x0 = x0 < 0 ? 0x80000000 : 0x7fffffff;
> + overflow = 1;
> + }
> }
> -

Looks like a whitespace change accidentally made it into this patch too.

> if (cc) {
> env->cc_dst = x0;
> env->cc_src2 = overflow;
> -------------------------------------------------------------------


ATB,

Mark.