[llvm-dev] Memory Store/Load Optimization Issue (Emulating stack)
Daniel Berlin via llvm-dev
llvm-dev at lists.llvm.org
Wed Feb 10 12:24:01 PST 2016
On Wed, Feb 10, 2016 at 12:18 PM, Paul Peet via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> Thank you for the hint.
>
> I adjusted the code and it works:
>
> The code after replacing inttoptr with getelementptr:
>
> define { i32, i32, i8* } @test(i32 %foo, i32 %bar, i8* %sp) {
> entry:
> ; push foo (On "stack")
> %sp_1 = getelementptr i8, i8* %sp, i32 -4
> %sp_1_ptr = bitcast i8* %sp_1 to i32*
> store i32 %foo, i32* %sp_1_ptr, align 4
>
> ; push bar
> %sp_2 = getelementptr i8, i8* %sp_1, i32 -4
> %sp_2_ptr = bitcast i8* %sp_2 to i32*
> store i32 %bar, i32* %sp_2_ptr, align 4
>
> ; val1 = pop (val1 = bar)
> %sp_3_ptr = bitcast i8* %sp_2 to i32*
> %val1 = load i32, i32* %sp_3_ptr, align 4
> %sp_3 = getelementptr i8, i8* %sp_2, i32 4
>
> ; val2 = pop (val2 = foo)
> %sp_4_ptr = bitcast i8* %sp_3 to i32*
> %val2 = load i32, i32* %sp_4_ptr, align 4
> %sp_4 = getelementptr i8, i8* %sp_3, i32 4
>
> %ret_1 = insertvalue { i32, i32, i8* } undef, i32 %val1, 0
> %ret_2 = insertvalue { i32, i32, i8* } %ret_1, i32 %val2, 1
> %ret_3 = insertvalue { i32, i32, i8* } %ret_2, i8* %sp_4, 2
>
> ret { i32, i32, i8* } %ret_3
> }
>
> After optimization ("opt -instcombine ./code.ll -S")
>
> define { i32, i32, i8* } @test(i32 %foo, i32 %bar, i8* %sp) {
> entry:
> %sp_1 = getelementptr i8, i8* %sp, i64 -4
> %sp_1_ptr = bitcast i8* %sp_1 to i32*
> store i32 %foo, i32* %sp_1_ptr, align 4
> %sp_2 = getelementptr i8, i8* %sp, i64 -8
> %sp_2_ptr = bitcast i8* %sp_2 to i32*
> store i32 %bar, i32* %sp_2_ptr, align 4
> %ret_1 = insertvalue { i32, i32, i8* } undef, i32 %bar, 0
> %ret_2 = insertvalue { i32, i32, i8* } %ret_1, i32 %foo, 1
> %ret_3 = insertvalue { i32, i32, i8* } %ret_2, i8* %sp, 2
> ret { i32, i32, i8* } %ret_3
> }
>
> My only questions are now:
> - How is it that inttoptr cannot provide that specific alias information
> so it can optimize that store/load away ?
>
Because nothing tracks what happens to the ints, and what happens when they
are converted back to pointers and whether it's sane :)
http://llvm.org/docs/GetElementPtr.html#how-is-gep-different-from-ptrtoint-arithmetic-and-inttoptr
- Might it be possible to get inttoptr providing such alias analysis ?
>
It doesn't make a lot of sense to try in most cases.
Most of the cases ptrtoint/inttoptr is useful are those where you want to
do crazy things to the pointer.
> - I came across MemorySSA while browsing though the llvm source. Is it
> possible that one can use MemorySSA to do such optimization without alias
> analysis ?
>
MemorySSA relies on alias analysis to generate the SSA form.
> - Where do I have to look in the source which is doing this kind of
> optimization (Is it instcombine which uses lib/Analysis/Loads.cpp ?)
>
> It's probably a combination of opts. The most likely candidate is -gvn,
but I would look at the pass dumps after each opt
> Regards,
> Paul
>
>
> 2016-02-10 0:26 GMT+01:00 Philip Reames <listmail at philipreames.com>:
>
>> Two points:
>> - Using inttoptr is a mistake here. GEPs are strongly preferred and
>> provide strictly more aliasing information to the optimizer.
>> - The zext is a bit weird. I'm not sure where that came from, but I'd
>> not bother looking into until the preceding point is addressed.
>>
>> In general, you may find these docs useful:
>> http://llvm.org/docs/Frontend/PerformanceTips.html
>>
>> Philip
>>
>>
>>
>> On 02/08/2016 06:54 AM, Paul Peet via llvm-dev wrote:
>>
>> Hello,
>>
>> I am trying to emulate the "stack" as like on x86 when using push/pop so
>> afterwards I can use LLVM's optimizer passes to simplify (reduce junk) the
>> code.
>>
>> The LLVM IR code:
>>
>> define { i32, i32, i32 } @test(i32 %foo, i32 %bar, i32 %sp) {
>> ; push foo (On "stack")
>> %sp_1 = sub i32 %sp, 4
>> %sp_1_ptr = inttoptr i32 %sp_1 to i32*
>> store i32 %foo, i32* %sp_1_ptr, align 4
>>
>> ; push bar
>> %sp_2 = sub i32 %sp_1, 4
>> %sp_2_ptr = inttoptr i32 %sp_2 to i32*
>> store i32 %bar, i32* %sp_2_ptr, align 4
>>
>> ; val1 = pop (val1 = bar)
>> %sp_3_ptr = inttoptr i32 %sp_2 to i32*
>> %val1 = load i32, i32* %sp_3_ptr, align 4
>> %sp_3 = add i32 %sp_2, 4
>>
>> ; val2 = pop (val2 = foo)
>> %sp_4_ptr = inttoptr i32 %sp_3 to i32*
>> %val2 = load i32, i32* %sp_4_ptr, align 4
>> %sp_4 = add i32 %sp_3, 4
>>
>> %ret_1 = insertvalue { i32, i32, i32 } undef, i32 %val1, 0
>> %ret_2 = insertvalue { i32, i32, i32 } %ret_1, i32 %val2, 1
>> %ret_3 = insertvalue { i32, i32, i32 } %ret_2, i32 %sp_4, 2
>>
>> ret { i32, i32, i32 } %ret_3
>> }
>>
>> This code will "push" two values onto the stack and pop them in reverse
>> order so afterwards "foo" and "bar" will be swapped and returned back.
>>
>> After running this through "opt -O2 ./test.ll", I am getting this:
>>
>> define { i32, i32, i32 } @test(i32 %foo, i32 %bar, i32 %sp) #0 {
>> %sp_1 = add i32 %sp, -4
>> %1 = zext i32 %sp_1 to i64
>> %sp_1_ptr = inttoptr i64 %1 to i32*
>> store i32 %foo, i32* %sp_1_ptr, align 4
>> %sp_2 = add i32 %sp, -8
>> %2 = zext i32 %sp_2 to i64
>> %sp_2_ptr = inttoptr i64 %2 to i32*
>> store i32 %bar, i32* %sp_2_ptr, align 4
>> %val2 = load i32, i32* %sp_1_ptr, align 4
>> %ret_1 = insertvalue { i32, i32, i32 } undef, i32 %bar, 0 ; Swapped
>> %ret_2 = insertvalue { i32, i32, i32 } %ret_1, i32 %val2, 1; Not
>> Swapped (Not optimized; Should be %foo)
>> %ret_3 = insertvalue { i32, i32, i32 } %ret_2, i32 %sp, 2
>> ret { i32, i32, i32 } %ret_3
>> }
>>
>> As you can see that the IR has got additional code, eg. zext. But the
>> main problem here is that val2 hasn't been optimized.
>> Could anyone show me some hints what is preventing the second val from
>> being optimized? (My guess would be the zext because I am using %sp as a
>> 32bit pointer although the "target" is 64bit).
>>
>> Regards,
>> Paul
>>
>>
>> _______________________________________________
>> LLVM Developers mailing listllvm-dev at lists.llvm.orghttp://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>>
>>
>
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