[llvm-dev] LoopStrengthReduce.cpp

[Jonas Paulsson via llvm-dev]

Hi Hal,

yes, it's all about the induction variable. SystemZ has a late pass 
(pre-emit) that looks for MI sequences that can be rewritten to 'branch 
on count'. Currently only about half the number of BRCTs are output 
compared to gcc on the same benchmarks. One reason for this is that when 
a loop gets unrolled, the loop gets a greater increment / decrement than 
1, which makes the late peephole transformation not work.

Since LSR generates formula for induction variables and evaluates them, 
I thought this pass could somehow be made to generate and emit preferred 
forms of induction variables. This seems like a good idea since this 
should generally help all targets with similar h/w loop instructions 
that begins with the trip count in a register and decrements and loops 
until zero.

Could this be done somehow, or is it really so that all targets have to 
have their own passes to do this?

/Jonas


On 2016-03-29 03:50, Hal Finkel wrote:
> Hi Jonas,
>
> Are you talking specifically about the induction variable? You might look at what I did for PowerPC's counter-based loops (lib/Target/PowerPC/PPCCTRLoops.cpp, etc.).
>
>   -Hal
>
> ----- Original Message -----
>> From: "Jonas Paulsson via llvm-dev" <llvm-dev at lists.llvm.org>
>> To: "llvm-dev" <llvm-dev at lists.llvm.org>
>> Sent: Monday, March 28, 2016 8:02:40 AM
>> Subject: [llvm-dev] LoopStrengthReduce.cpp
>>
>> Hi,
>>
>> I am looking for a way to rewrite induction variables to use an
>> addition
>> of -1 whenever possible (and not otherwise unprofitable). This is
>> needed
>> to utilize hardware loop instructions, which are present on SystemZ
>> (branch on count). Later in the backend, an 'add -1; compare w/ 0;
>> jne
>> 0'-sequence can be replaced with a brct instruction.
>>
>> I could not find any way in the LSR pass to generate a formula for
>> this
>> (neither to express the lower target cost). I would like to generate
>> a
>> formula with a step of -1 and make the cost for it to be less than
>> that
>> of a similar formula with another constant.
>>
>> Could someone please give me any clue as to if there is a way to
>> currently do this, or if not, what would be the normal way?
>>
>> thanks,
>>
>> Jonas paulsson
>>
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