llvm dev - Nov 2010 - [LLVMdev] Question regarding the alias analysis chaining behaviour

Hi,
   I am using LLVM 2.4 on a Linux RHEL5 machine. I was trying to figure out
how the chaining of the alias analysis passes works in LLVM. Here are the
command I used to test the chaining part.
 1.  ./opt hello_world_1_nest_func.bc -o hello_world_1_nest_func_AA.bc
-no-aa -anders-aa -licm
  Result: Anderson's AA and No Alias Analysis both are called.
  2. ./opt hello_world_1_nest_func.bc -o hello_world_1_nest_func_AA.bc
-basicaa -anders-aa -licm
  Result: Anderson's AA and Basic AA both are called.
  3. ./opt hello_world_1_nest_func.bc -o hello_world_1_nest_func_AA.bc
-basicaa -licm
  Result: Only Basic AA is called. (When I use -no-aa instead of -basicaa,
only the NoAA is called).
  4. ./opt hello_world_1_nest_func.bc -o hello_world_1_nest_func_AA.bc
-basicaa -anders-aa -licm -licm
  Result: First licm gets a mix of both AA passes (basic and anders), second
licm only gets basic.
  5. ./opt hello_world_1_nest_func.bc -o hello_world_1_nest_func_AA.bc
-basicaa -anders-aa -licm -basicaa -anders-aa -licm
  Result: Both licm instances get a mix of Anders and Basic AA.

  Am I to conclude that the Anderson's Alias Analysis is only applied to
LICM? and other passes which require alias analysis use BasicAA or NoAA
depending which on which one is specified on the command line? Is this the
reason I am seeing calls to different AAs when although I specify an AA
which neither Basic nor No AA?

  I read on llvm docs that it is not possible to actually specify one
user-created alias analysis on the command line which is applied to all
subsequent passes on the command line. Does this mean that there is a
constraint on the command line user-created AA... which is that it only
applies to the top-level pass (licm above) specified immediately after the
user-created AA?

  I read in the same llvm doc that the "superclass" (The main Alias
Analysis
pass I assume) decides which response to use when the user-defined AA
overrides the default methods. However, in my current version, I don't see
code for deciding between the two AAs. Is this an artifact of the older
version I am using?

  Thanks for your response.

Rajesh
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dirac wrote:
> Hi,
>     I am using LLVM 2.4 on a Linux RHEL5 machine. I was trying to figure
> out how the chaining of the alias analysis passes works in LLVM. Here
> are the command I used to test the chaining part.
>   1.  ./opt hello_world_1_nest_func.bc -o hello_world_1_nest_func_AA.bc
> -no-aa -anders-aa -licm
>    Result: Anderson's AA and No Alias Analysis both are called.
>    2. ./opt hello_world_1_nest_func.bc -o hello_world_1_nest_func_AA.bc
> -basicaa -anders-aa -licm
>    Result: Anderson's AA and Basic AA both are called.
>    3. ./opt hello_world_1_nest_func.bc -o hello_world_1_nest_func_AA.bc
> -basicaa -licm
>    Result: Only Basic AA is called. (When I use -no-aa instead of
> -basicaa, only the NoAA is called).
>    4. ./opt hello_world_1_nest_func.bc -o hello_world_1_nest_func_AA.bc
> -basicaa -anders-aa -licm -licm
>    Result: First licm gets a mix of both AA passes (basic and anders),
> second licm only gets basic.
>    5. ./opt hello_world_1_nest_func.bc -o hello_world_1_nest_func_AA.bc
> -basicaa -anders-aa -licm -basicaa -anders-aa -licm
>    Result: Both licm instances get a mix of Anders and Basic AA.
>
>    Am I to conclude that the Anderson's Alias Analysis is only applied
> to LICM? and other passes which require alias analysis use BasicAA or
> NoAA depending which on which one is specified on the command line? Is
> this the reason I am seeing calls to different AAs when although I
> specify an AA which neither Basic nor No AA?
>
>    I read on llvm docs that it is not possible to actually specify one
> user-created alias analysis on the command line which is applied to all
> subsequent passes on the command line. Does this mean that there is a
> constraint on the command line user-created AA... which is that it only
> applies to the top-level pass (licm above) specified immediately after
> the user-created AA?
The AA that you specify will be created when you specify it and live 
until that pass is invalidated (not explicitly preserved by a pass that 
runs). The next time a pass requires AA, the pass manager will create 
the default AA (BasicAA) and not the one you put on the command line.

You can view what the pass manager is actually doing with "opt 
-debug-pass=Structure ...".

Nick
>    I read in the same llvm doc that the "superclass" (The main
Alias
> Analysis pass I assume) decides which response to use when the
> user-defined AA overrides the default methods. However, in my current
> version, I don't see code for deciding between the two AAs. Is this an
> artifact of the older version I am using?
>
>    Thanks for your response.
>
> Rajesh
>
>
>
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev

[+llvmdev]

Rajeshwar Vanka wrote:
> -----Original Message-----
> From: Nick Lewycky [mailto:nicholas at mxc.ca]
> Sent: Wednesday, November 24, 2010 3:51 PM
> To: dirac
> Cc: llvmdev at cs.uiuc.edu
> Subject: Re: [LLVMdev] Question regarding the alias analysis chaining
> behaviour
>
> dirac wrote:
>> Hi,
>>      I am using LLVM 2.4 on a Linux RHEL5 machine. I was trying to
figure
>> out how the chaining of the alias analysis passes works in LLVM. Here
>> are the command I used to test the chaining part.
>>    1.  ./opt hello_world_1_nest_func.bc -o
hello_world_1_nest_func_AA.bc
>> -no-aa -anders-aa -licm
>>     Result: Anderson's AA and No Alias Analysis both are called.
>>     2. ./opt hello_world_1_nest_func.bc -o
hello_world_1_nest_func_AA.bc
>> -basicaa -anders-aa -licm
>>     Result: Anderson's AA and Basic AA both are called.
>>     3. ./opt hello_world_1_nest_func.bc -o
hello_world_1_nest_func_AA.bc
>> -basicaa -licm
>>     Result: Only Basic AA is called. (When I use -no-aa instead of
>> -basicaa, only the NoAA is called).
>>     4. ./opt hello_world_1_nest_func.bc -o
hello_world_1_nest_func_AA.bc
>> -basicaa -anders-aa -licm -licm
>>     Result: First licm gets a mix of both AA passes (basic and anders),
>> second licm only gets basic.
>>     5. ./opt hello_world_1_nest_func.bc -o
hello_world_1_nest_func_AA.bc
>> -basicaa -anders-aa -licm -basicaa -anders-aa -licm
>>     Result: Both licm instances get a mix of Anders and Basic AA.
>>
>>     Am I to conclude that the Anderson's Alias Analysis is only
applied
>> to LICM? and other passes which require alias analysis use BasicAA or
>> NoAA depending which on which one is specified on the command line? Is
>> this the reason I am seeing calls to different AAs when although I
>> specify an AA which neither Basic nor No AA?
>>
>>     I read on llvm docs that it is not possible to actually specify one
>> user-created alias analysis on the command line which is applied to all
>> subsequent passes on the command line. Does this mean that there is a
>> constraint on the command line user-created AA... which is that it only
>> applies to the top-level pass (licm above) specified immediately after
>> the user-created AA?
>
>>> The AA that you specify will be created when you specify it and
live
>>> until that pass is invalidated (not explicitly preserved by a pass
that
>>> runs). The next time a pass requires AA, the pass manager will
create
>>> the default AA (BasicAA) and not the one you put on the command
line.
>
>>> You can view what the pass manager is actually doing with "opt
>>> -debug-pass=Structure ...".
>
>>> Nick
>
> So, what you are saying is that if I create an AA, and that is used by a
> pass X, then
> the AA I create will only be alive until that pass remains valid? I am a
> little confused here.
>> From my understanding, an AA will be created only once, and its
information
> is updated using
> Copy/delete/replace Value functions. Does this not apply to an AA pass that
> I create?
The BasicAA pass is created only once because it is an ImmutablePass. I 
should've asked what type of Pass your custom AA is, but this is a 
common problem where someone writes an AA as a FunctionPass (or uses 
Andersen's AA before we removed it from the tree) and it doesn't live 
through the whole execution.

See http://llvm.org/docs/WritingAnLLVMPass.html#passtype and below for a 
list of the passes and their requirements. (Yes, BasicAA really does 
meet the requirements for an immutable pass. It stores no state and the 
copy/delete/replace functions are all no-ops.)

Nick
>
> Thanks.
> Rajesh
>
>>     I read in the same llvm doc that the "superclass" (The
main Alias
>> Analysis pass I assume) decides which response to use when the
>> user-defined AA overrides the default methods. However, in my current
>> version, I don't see code for deciding between the two AAs. Is this
an
>> artifact of the older version I am using?
>>
>>     Thanks for your response.
>>
>> Rajesh
>>
>>
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>
>
>

On Wed, Nov 24, 2010 at 2:50 PM, Nick Lewycky <nicholas at mxc.ca> wrote:
> The AA that you specify will be created when you specify it and live
> until that pass is invalidated (not explicitly preserved by a pass that
> runs). The next time a pass requires AA, the pass manager will create
> the default AA (BasicAA) and not the one you put on the command line.
>
I thought analysis passes just rebuilt their state after they got
invalidated.  Shouldn't that happen with an AA pass as well?  Or is AA
special?