[LLVMdev] alias information on machine instructions

[Florian Brandner]

hi,

i know it took a while, but here is a patch that adds a list of source
values to machine instructions.

i modified the DAGISelEmiter to automatically catch regular
loads/stores. custom instructions and loads/stores rewritten by the
lowering pass are not automatically captured.

during the instruction selection a source value operand is added to the
DAG for patterns matching a load/store.

after the scheduling these additional operands are added to the machine
instruction automatically.

one can add special source values for spills, using the machine
instruction builder.

i'm not to happy about the representation of the source values in the
selection DAG. i would like to use a new SelectionDAG node, which
then distinguishes between reads and writes, and adds additional
information on the memory access. but so far the current scheme works fine.

i've testet all this for our backend only, which is not public. i do not
know how much has to be done to integrate this with the other, e.g., the
x86, targets. does any of the other targets rewrite loads/stores during
lowering?

any comments how to integrate this into llvm, and how to proceed to get
the alias analysis working after the codegen prepare pass?

florian


Florian Brandner wrote:
> hi,
> 
> 
> Florian Brandner wrote:
>> Dan Gohman wrote:
>>> On Wed, May 23, 2007 at 12:23:38AM -0700, Chris Lattner wrote:
>>>> Right.  The original Value*'s are preserved in the DAG, but dropped when 
>>>> MachineInstrs are created.  We could add a machineoperand to capture this 
>>>> Value* if desired.
>>> Another benefit of keeping the original Value*'s (and offsets) around is it
>>> would provide more information that could be used for verbose asm output
>>> and MachineInstr-level dumps, which would be quite nice.
>>>
>> i'll do this and post a patch, unless someone else is already working on
>> this.
>>
> 
> i have extended the DAG instruction selector and scheduler to preserve
> Value*'s. the values are attached to machine instructions as a separate
> operand list (at least for now). for now this looks very good.
> 
> but i found that the alias analysis is not as good as expected. the
> problem seems to be the codegenprepare pass. GEP instructions are
> lowered to integer calculations (e.g. ptrtoint, add, inttoptr). this
> causes the (basic) alias analysis to answer MayAlias for most queries.
> 
> this is also a problem for the DAG combiner, when the
> -combiner-global-alias-analysis switch is given to llc.
> 
> any ideas how to handle this?
> florian
> 
> 
> 
> 
> 
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-- 
Brandner Florian

CD Laboratory - Compilation Techniques for Embedded Processors
Institut für Computersprachen E185/1
Technische Universität Wien

Argentinierstraße 8 / 185
A-1040 Wien, Austria

Tel.: (+431) 58801-58521

E-Mail: brandner at complang.tuwien.ac.at
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