[LLVMdev] Modifying the patterns/ instruction selection phase in LLVM 2.7

[John Criswell]

On 1/20/11 12:27 PM, Venkatraman Govindaraju wrote:

Just out of curiosity, have either of you considered writing an LLVM 
transform that simply replaces these call instructions with inline 
assembly code that does what you want?  If that works, it seems much 
simpler than modifying/enhancing the code generator.

-- John T.

> I have similar requirements for my project. This is what I do.
>
>    1. Add a new intrinsic function to LLVM that corresponds to
> "functions with particular type of function names".
>        The steps to create intrinsic function is documented in
> http://llvm.org/docs/ExtendingLLVM.html#intrinsic.
>    2. Create a lowering pass that lowers "functions with particular
> type" ( eg. p0, p1) to the intrinsic.
>    3. Create a instruction pattern in SparcInstrInfo.td that matches
> the intrinsic.
>        From the arguments to the intrinsic function,  encode the
> "<number>"  and output a "sethi" instruction.
>
> Thanks,
> Venkatraman
>
>
> On Tue, Jan 18, 2011 at 11:18 PM, Raghu Prabhakar<raghu at cs.ucla.edu>  wrote:
>> Hello all,
>>
>> I am trying to modify the Sparc backend to do something for my team's project, to do the following.
>>
>> Whenever the backend encounters a call to one particular type of function names (functions like p0(), p1()..etc), I want the backend to generate a "sethi %g0,<number>", and NOT a "call p0, call p1..." instruction. However, the backend should work as usual for any other function name.
>>
>> Now I figured that there are two ways to do this :
>> 1) Modifying the "SparcISelLowering.cpp" to emit a special SDNode (with a special opcode that I have introduced into SPISD namespace) whenever it finds a "call @p0" in the LowerCall function. This node should have an entry in the "SparcInstrInfo.td" file.
>>
>> 2) No modifications to the Lowering phase. However, the pattern corresponding to the "sethi" instruction must be modified/new pattern added which corresponds to "call p0" kind of selectionDAG nodes.
>>
>> In either case, I am a bit confused about how tblgen's pattern representation scheme works. One more issue is that the destination register of "sethi %g0,<NUM>" HAS to be %g0 (which makes it a no-op on an actual sparc machine..I'm using a simulator), and<NUM>  depends on p0 and its operands. There are ways where the certain registers required can be hard-assigned during instruction selection itself, but I'm unsure as to how to do that. Am I looking at the right set of files? Is there a better way to accomplish what I'm trying to do?
>>
>> I'm relatively new to LLVM, so I have some trouble freely navigating through the source in spite of being heavily documented.
>>
>> Any suggestions is of great help.
>>
>> Thanks.
>> Raghu.
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