Fwd: [LLVMbugs] [Bug 623] NEW: request for -malloc2alloca optimization pass

[Chris Lattner]

On Tue, 30 Aug 2005, Vikram S. Adve wrote:

> Rob Bocchino has already written such a pass -- perhaps he can tell you more 
> about how to use it.  It relies on an experimental pointer and escape 
> analysis called DSA (analyze -tddatastructure) so please check results 
> carefully.

I asked Eric to file this enhancement request, because (like many gc'd 
languages) he's seeing a LOT of heap allocations, some of which are very 
simple to clean up.  The goal of this pass is to do the easy 50% of the 
cases that doesn't require escape analysis, so it can be very fast.  As an 
optional second step, something requiring a more heavy-weight analysis 
(e.g. escape analysis using DSA) could be used.

This is in the same spirit as using things like mem2reg and scalarrepl, 
which are just light-weight versions of load/store elimination for simple 
but common cases.

-Chris

> Begin forwarded message:
>
>> From: bugzilla-daemon at cs.uiuc.edu
>> Date: August 30, 2005 6:22:38 AM CDT
>> To: llvmbugs at cs.uiuc.edu
>> Subject: [LLVMbugs] [Bug 623] NEW: request for -malloc2alloca optimization 
>> pass
>> 
>> 
>> http://llvm.cs.uiuc.edu/bugs/show_bug.cgi?id=623
>> 
>>            Summary: request for -malloc2alloca optimization pass
>>            Product: tools
>>            Version: trunk
>>           Platform: PC
>>         OS/Version: Linux
>>             Status: NEW
>>           Severity: enhancement
>>           Priority: P2
>>          Component: opt
>>         AssignedTo: unassignedbugs at nondot.org
>>         ReportedBy: eric at vanrietpaap.nl
>> 
>> 
>> If such an optimization pass would do escape analysis pypy-llvm might be 
>> able to
>> use a lot less malloc. These are currently in pypy-llvm costly becausewe 
>> use the
>> Boehm garbage collector that obviously need to do more that a normal malloc 
>> and
>> free.
>> 
>> Because the actual Boehm malloc is called GC_malloc this would need to be 
>> an
>> alias of malloc. In general it would be nice to be able tell the optimizers
>> about aliases of known functions (could that be useful for simplify- 
>> libcalls
>> translations, just guessing here). Or maybe this is already possible by
>> implementing an internal malloc in the .ll file. Not sure here, will have 
>> to try
>> that later.
>> 
>> [If part two makes sence then this enhancement request should have been 
>> split]
>> 
>> 
>> 
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>
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-Chris

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