[LLVMdev] question about licm

[Linhai]

Hi Ashutosh and Hal,

     Thanks a lot for the explanation! It is clear to me right now.

     Best

                                 Linhai

On 02/11/2015 03:58 AM, Hal Finkel wrote:
> ----- Original Message -----
>> From: "Ashutosh Nema" <Ashutosh.Nema at amd.com>
>> To: "songlh" <songlh at cs.wisc.edu>, llvmdev at cs.uiuc.edu
>> Sent: Wednesday, February 11, 2015 3:20:27 AM
>> Subject: Re: [LLVMdev] question about licm
>>
>> Hi,
>>
>> LICM can only hoist instructions which dominates all loop exit
>> blocks.
>> In this case 'upper[j]' is not dominating exit block as its appearing
>> in second operand of logical AND operator.
>>
>> Prior to hoisting it check for condition in 'isGuaranteedToExecute'
>> and it decide not to hoist it.
>>
>> <File: LICM.cpp>
>> 666 bool LICM::isGuaranteedToExecute(Instruction &Inst) {
>> 667
>> 668   // We have to check to make sure that the instruction dominates
>> all
>> 669   // of the exit blocks.  If it doesn't, then there is a path out
>> of the loop
>> 670   // which does not execute this instruction, so we can't hoist
>> it.
> Yes, and to add to this, the load is not safe to speculatively execute (because, as you mention, j might be larger than 10). Now this is an interesting case because the cases where upper[j] might be an out-of-bounds access are the same as those when lower[j] might be an out-of-bounds access, and lower[j] dominates upper[j]. As a result, we *could* notice this and hoist the load anyway (to after lower[j] in the loop preheader), but we currently have no capability to do this.
>
>   -Hal
>
>> Hope this helps.
>>
>> Regards,
>> Ashutosh
>>
>> -----Original Message-----
>> From: llvmdev-bounces at cs.uiuc.edu
>> [mailto:llvmdev-bounces at cs.uiuc.edu] On Behalf Of songlh
>> Sent: Wednesday, February 11, 2015 10:24 AM
>> To: llvmdev at cs.uiuc.edu
>> Subject: [LLVMdev] question about licm
>>
>> hi,
>>
>>     I applied licm with basicaa on the following codes:
>>
>>     int j = atoi(argc[1]);
>>     int lower[] = {10, 9, 8, 7, 6, 5, 4, 3, 2, 1};
>>     int upper[] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
>>
>>     for(i = lower[j]; a[i] < 100 && i < upper[j]; i ++);
>>
>>     I notice that upper[j] is not hoisted out from the loop. Is this
>>     because j could be larger than 10?
>>
>>     Thanks a lot!
>>
>>     Best,
>>
>>                                                Linhai
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