[llvm-dev] Optimization issues (Alias Analysis?)

[Daniel Berlin via llvm-dev]

It's not generally possible to know what happens, and it doesn't happen
often enough in cases where people want good performance to optimize.

One could build a constant lattice based tracker and try to do better, but
...


On Wed, Jul 6, 2016 at 5:40 AM, Peet Nick via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Oh, I understand.
> Although I am quite curious why this isn't supported as in C it might be
> very common to cast integer to pointers and back (This is what triggers
> clang to emit inttoptr instructions).
>
> On Tue, Jul 5, 2016 at 8:52 PM, Hal Finkel <hfinkel at anl.gov> wrote:
>
>> Hi Nick,
>>
>> We really don't do AA on inttoptr at all. You should use i8* (or some
>> other appropriate pointer type) and GEPs instead.
>>
>>  -Hal
>>
>> ------------------------------
>>
>> *From: *"Peet Nick via llvm-dev" <llvm-dev at lists.llvm.org>
>> *To: *llvm-dev at lists.llvm.org
>> *Sent: *Monday, July 4, 2016 11:05:11 AM
>> *Subject: *[llvm-dev] Optimization issues (Alias Analysis?)
>>
>>
>> Hey,
>>
>> I am currently working on a VM which is based on LLVM and I would like to
>> use its optimizer, but it somehow it can't detect something very simple (I
>> guess.)
>>
>> This is the LLVM IR:
>>
>> target datalayout = "e-m:e-p:32:32-f64:32:64-f80:32-n8:16:32-S128"
>> target triple = "i386-unknown-linux-gnu"
>>
>> %struct.regs = type { i32, i32, i32 }
>>
>> define void @Test(%struct.regs* noalias nocapture sret, i32, i32, i32)
>> local_unnamed_addr #0 {
>>   %5 = add i32 %3, -4
>>   %6 = inttoptr i32 %5 to i32*
>>   store i32 %2, i32* %6, align 4
>>
>>   %7 = add i32 %3, -8
>>   %8 = inttoptr i32 %7 to i32*
>>   store i32 %1, i32* %8, align 4
>>
>>   %9 = load i32, i32* %6, align 4
>>
>>   %.sroa.0.0..sroa_idx = getelementptr inbounds %struct.regs,
>> %struct.regs* %0, i32 0, i32 0
>>   store i32 %9, i32* %.sroa.0.0..sroa_idx, align 4
>>
>>   %.sroa.4.0..sroa_idx4 = getelementptr inbounds %struct.regs,
>> %struct.regs* %0, i32 0, i32 1
>>   store i32 %1, i32* %.sroa.4.0..sroa_idx4, align 4
>>
>>   %.sroa.7.0..sroa_idx5 = getelementptr inbounds %struct.regs,
>> %struct.regs* %0, i32 0, i32 2
>>   store i32 %3, i32* %.sroa.7.0..sroa_idx5, align 4
>>
>>   ret void
>> }
>>
>> This is compiled from some cpp just to reproduce the issue.
>> The issue here is that the load isn't eliminated and that the third store
>> isn't directly using %2.
>>
>> I thought that this might be an issue with the current BasicAliasAnalysis
>> so I also tried CFL which isn't working either.
>>
>> ~ Nick
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>>
>>
>>
>> --
>> Hal Finkel
>> Assistant Computational Scientist
>> Leadership Computing Facility
>> Argonne National Laboratory
>>
>
>
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