[LLVMdev] How to enable use of 64bit load/store for 32bit architecture

[Pete Cooper]

Hi James, Jim

If you *really* want this to work in selection DAG then there is a solution, but its not pretty.

First make i64 not be legal.  Then, assuming the regclass you gave has some subregs, you can give load/store a custom legalisation where you change the i64 to MVT::Untyped.  So something like this for ISD::STORE:

SDValue ValueToBeStored = St.getOperand(…)

auto SeqOps[] = { 
  DAG.getTargetConstant(SP::IntPairRegClassID, MVT::i32),
  DAG.getNode(ISD::EXTRACT_ELEMENT, dl, MVT::i32, ValueToBeStored, DAG.getConstant(0, MVT::i32)),
  DAG.getTargetConstant(SP ::sub0, MVT::i32),
  DAG.getNode(ISD::EXTRACT_ELEMENT, dl, MVT::i32, ValueToBeStored, DAG.getConstant(1, MVT::i32)),
  DAG.getTargetConstant(SP ::sub1, MVT::i32)
};

SDValue NewValueToBeStored = DAG.getMachineNode(TargetOpcode::REG_SEQUENCE, dl, MVT::Untyped, SeqOps);

return DAG.getStore(…, NewValueToBeStored, …)

The legalizer won’t touch Untyped operands, so once you’ve changed the store operand to this, you won’t have the legalize complain later.  But this is very target specific and prone to error if you get sub regs or anything wrong.

Cheers,
Pete
> On Apr 2, 2015, at 1:20 PM, Jim Grosbach <grosbach at apple.com> wrote:
> 
> Hi James,
> 
> I’m not too familiar with Sparc’s design here, but you may find it useful to have a look at how the ARM target deals with this sort of thing. In particular, the load/store optimization pass which can form the LDRD/STRD instructions among other things.
> 
> -Jim
> 
>> On Apr 2, 2015, at 11:43 AM, James Y Knight <jyknight at google.com <mailto:jyknight at google.com>> wrote:
>> 
>> In http://reviews.llvm.org/D8713 <http://reviews.llvm.org/D8713>, I added the 64bit integer store ("std") and load ("ldd") instructions for 32bit sparc. But now I need codegen to know how to emit them, and am not sure the best way to go about teaching the backend that 64bit integers can be used natively, but only for loads and stores.
>> 
>> (I originally wrote an earlier draft of question in the review but it seems like it may be better on the dev list, since it's more what to do next than what was done.)
>> 
>> Basically, I'd like it if C code like:
>>   long long x,y;
>>   void f() { y = x; }
>> Or equivalently, the ll code
>>   %0 = load i64, i64* @x, align 8
>>   store i64 %0, i64* @y, align 8
>> turned into just "ldd, std" instructions, as it does in GCC, rather than loading and storing the two 32bit halves of the variables separately.
>> 
>> To allow that, I tried adding:
>>   addRegisterClass(MVT::i64, &SP::IntPairRegClass)
>> to SparcTargetLowering::SparcTargetLowering in 32bit mode.
>> 
>> Doing that then makes load/store work. But it causes llvm to try to use i64 operations for *everything*, which of course fails for all other operations, since there's no such instruction pattern for them.
>> 
>> Okay, so I then try setting all the operations to "Expand" via:
>>     for (unsigned Op = 0; Op < ISD::BUILTIN_OP_END; ++Op) {
>>       setOperationAction(Op, MVT::i64, Expand);
>>     }
>> 
>> This does not actually work properly. E.g.
>>   %res = add nsw i64 %0, 3
>> gives an error:
>>   LLVM ERROR: Cannot select: 0x43b1df0: i64 = add 0x43b1bd0, 0x43b1ce0 [ORD=3] [ID=15]
>> ...
>> Which seems odd -- it appears as if it ignored the Expand and just tried to continue onto selection for 64bit add as if it had been legal?
>> 
>> And a different example:
>>   %res = shl i64 %0, 1
>> aborts with error:
>>   llvm/lib/CodeGen/SelectionDAG/LegalizeDAG.cpp:3988: void {anonymous}::SelectionDAGLegalize::ExpandNode(llvm::SDNode*): Assertion `VT.isVector() && "Unable to legalize non-vector shift"' failed.
>> 
>> So, it seems to me that a whole bunch of other code will need to be added or modified, too, to teach LLVM how to expand i64 to i32 operations for everything other than load/store, if I continue down this path -- despite that it can do so just fine if i64 doesn't have a register class. This seems ugly...
>> 
>> Is there some other better way to notate the case that ONLY loads/stores can be done on 64bit integers, but that everything else must be done as if 64bit integers didn't exist?
>> 
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