[LLVMdev] [PATCH] Teaching ScalarEvolution to handle IV=add(zext(trunc(IV)), Step)

[Matthew Curtis]

On 12/21/2012 12:26 PM, Dan Gohman wrote:
> On Thu, Dec 20, 2012 at 8:36 AM, Matthew Curtis <mcurtis at codeaurora.org> wrote:
>> Ok, so I think I've mis-represented what's really happening.
>> Ignore my previous statements concerning %add :)
>>
>> Again, given:
>>
>> 05:  for.body:                                         ; preds = %entry,
>> %for.body
>> 06:    %j.04 = phi i32 [ 0, %entry ], [ %inc, %for.body ]
>> 07:    %result.03 = phi i32 [ 0, %entry ], [ %add, %for.body ]
>> 08:    %conv2 = and i32 %result.03, 255
>> 09:    %add = add nsw i32 %conv2, 3
>> 10:    %inc = add nsw i32 %j.04, 1
>> 11:    %cmp = icmp slt i32 %inc, 8000
>> 12:    br i1 %cmp, label %for.body, label %for.end
>>
>> LLVM executes the following:
>>
>> 01:  createSCEV(%conv2 = and i32 %result.03, 255)
>> 02:    calls getSCEV(%result.03)
>> 03:       returns (3 + (zext i8 {-3,+,3}<%for.body> to i32))
> The problem is that this is not a correct expression for %result.03.
> It isn't safe to use incorrect expressions in ScalarEvolution even if
> it happens to work out in the case you're looking at.
>
> Dan
Any suggestions on how to construct a correct expression? Or otherwise 
teach ScalarEvolution to handle this case?

BTW, just so I understand, the expression for %result.03 is correct for 
n > 0, right?

Matthew C.

-- 
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, hosted by The Linux Foundation