Re: [Qemu-devel] [PATCH v2 17/17] target/riscv: convert to TranslatorOps

[Eric Blake <eblake@redhat.com>]

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On 04/10/2018 07:59 AM, Emilio G. Cota wrote:
> On Tue, Apr 10, 2018 at 11:24:37 +1000, Richard Henderson wrote:
>> On 04/07/2018 04:20 AM, Emilio G. Cota wrote:
>>> +        next_page = (ctx->base.pc_first & TARGET_PAGE_MASK) + TARGET_PAGE_SIZE;
>>> +        if (ctx->base.pc_next >= next_page) {
>>
>> This fails for the last page of the address space.
>> Better is
>>
>>   page_start = ctx->base.pc_first & TARGET_PAGE_MASK;
>>   if (ctx->base.pc_next - page_start >= TARGET_PAGE_SIZE) {
> 
> Apart from the variable name change, I fail to see how this (A - B >= C)
> is not equivalent to the above (A => B + C). What am I missing?

Integer overflow.  Adding TARGET_PAGE_SIZE might wrap next_page to 0,
which changes the semantics of the conditional; while performing the
subtraction avoids the case of overflow.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org


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