[LLVMdev] why are volatile memory accesses ordered?

[Daniel Berlin]

No worries, i think i've sent about 3 emails this week to the mailing
list where I discovered my mistake 30 seconds later and felt like an
idiot :)


On Thu, Apr 2, 2015 at 6:20 PM, Sanjoy Das
<sanjoy at playingwithpointers.com> wrote:
> Daniel, you're right.  This is embarrassing -- I started the whole
> discussion based on a typo.
>
> Sorry for the noise!
>
> -- Sanjoy
>
> On Thu, Apr 2, 2015 at 6:18 PM, Daniel Berlin <dberlin at dberlin.org> wrote:
>> Dunno whether it's worthwhile, but we definitely test for it, and if
>> his two non-volatile loads were identical, we'd definitely eliminate
>> them in GVN.
>>
>>
>>
>> On Thu, Apr 2, 2015 at 6:09 PM, Chandler Carruth <chandlerc at google.com> wrote:
>>> Could you explain why you think it is worthwhile to optimize code involving
>>> a volatile access?
>>>
>>> On Thu, Apr 2, 2015 at 6:08 PM Sanjoy Das <sanjoy at playingwithpointers.com>
>>> wrote:
>>>>
>>>> Marking volatile accesses as !unordered seems excessively
>>>> conservative.  For instance, LLVM is not able to optimize
>>>>
>>>> declare void @escape(i32*)
>>>> declare i32 @read_only(i32) readonly
>>>>
>>>> define i32 @f(i1* %c) {
>>>>  entry:
>>>>   %a = alloca i32
>>>>   %b = alloca i32
>>>>   call void @escape(i32* %a)
>>>>   call void @escape(i32* %b)
>>>>
>>>>   %a0 = load i32, i32* %b, align 4
>>>>   %lv = load volatile i32, i32* %b
>>>>   %a1 = load i32, i32* %a, align 4
>>>>
>>>>   %result = add i32 %a0, %a1
>>>>   ret i32 %result
>>>> }
>>>>
>>>> to "%result = %a0 << 1" via -O3.
>>>>
>>>> NB: changing the volatile load to a volatile store triggers the
>>>> optimization (via -instcombine) -- llvm::FindAvailableLoadedValue just
>>>> skips over noalias ordered stores.
>>>>
>>>> -- Sanjoy
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>>>
>>>
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