[LLVMdev] why are volatile memory accesses ordered?

[Daniel Berlin]

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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