[LLVMdev] Add a new information and preserve it in LLVM

[Daniel Stewart]

"The addPreserved<>() functionality is only there so that the LLVM compiler
can cache the results of analysis passes for efficiency."

 

The addPreserved() function only tells the compiler to cache it? I was under
the impression from the code that it was an indication that a transformation
pass doesn't invalidate whatever analysis pass it says it preserves. If it
is only an indication that the compiler should cache it, how does the
compiler know if a particular analysis pass should be re-run?

 

I'm going through the LegacyPassManager now and there definitely are some
parts I don't understand what they are for.

 

Daniel

 

From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu] On
Behalf Of John Criswell
Sent: Friday, April 04, 2014 6:27 PM
To: Hanbing Li; llvmdev at cs.uiuc.edu
Subject: Re: [LLVMdev] Add a new information and preserve it in LLVM

 

On 4/4/14 3:02 PM, Hanbing Li wrote:

Hello,

 

I am trying to add some thing into LLVM, while I encountered some problems.

 

So my situation is that I have some information W, some transform passes may
change it, after these passes, I need the new W. What I did is to create an
analysis pass similar to scalar-evolution or loopinfo, I can get the
information by using getAnalysis<W>(); and preserve this information W by
using AU.addPreserved<W>();. Then the problem came, for the module pass, the
information can't be preserved. (I found this: To the best of my knowledge,
the LLVM pass manager never preserves a FunctionPass analysis that is
requested by a ModulePass; every time you call getAnalysis for a function,
the FunctionPass is re-run.
http://lists.cs.uiuc.edu/pipermail/llvmdev/2012-March/048139.html) So this
means that I can't update W when some module passes changed it?

 

My questions are:

1, Module pass really can't preserve the W?


If W is a FunctionPass, then I believe you are correct: a ModulePass will
re-run the FunctionPass every time it uses the FunctionPass.

I get the impression that you're using the PassManager infrastructure
improperly, and that is probably why you're not getting the results you
want.

First, the ability to preserve a pass's results with addPreserved<W>() is an
optimization.  A FunctionPass or ModulePass() that analyzes the LLVM IR
should work whether it is run one time or a hundred times.  The
addPreserved<>() functionality is only there so that the LLVM compiler can
cache the results of analysis passes for efficiency.

Second, the way to fix your problem depends on what you want to do.  If you
want pass W to analyze the LLVM IR of a program and cache its results
(because the analysis it does is expensive), then make it a ModulePass.
That way, other ModulePass'es can update and preserve its results.

If you are just using W as a container in which to record information that
transform passes are doing, then you might want to make W an ImmutablePass.
ImmutablePasses are only run once and are never invalidated; other passes
can use them to record information about what they do.  ImmutablePasses, I
think, were not really designed for this purpose, but they'll get the job
done.

Regards,

John Criswell

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