[llvm-dev] Instruction selection confusion at register - chooses vector register instead of scalar one

[Alex Susu via llvm-dev]

   Hello.
     Matt, thanks for pointing me to the methods storeRegToStackSlot and 
loadRegFromStackSlot of [Target]InstrInfo class. It turned out that indeed they are the 
ones responsible for the problem with the malformed instruction with the confusion, as 
discussed in the first email.

     I have one more question: can I optimize inter-block register allocation by avoiding 
spilling (and the associated load) at the end of a basic-block with only one successor. 
More exactly, I have vector.body.preheader followed by vector.body (vector.body has as 
successor itself and an exit block, which is empty).
     Can I tell LLVM to do register allocation s.t. it avoids spilling at the end of 
vector.body.preheader and avoids performing corresponding loads at the beginning of 
vector.body?

   Thank you,
     Alex


On 10/25/2016 5:29 AM, Arsenault, Matthew wrote:
> Spills created at the end of the block (I assume you mean what fast regalloc does at -O0)
> are created long after instruction selection. In that case it sounds like your
> implementation of storeRegToStackSlot/loadRegFromStackSlot is broken
>
> -Matt
>
>
>
>
>
> On Tue, Oct 25, 2016 at 7:30 AM +0800, "Alex Susu via llvm-dev" <llvm-dev at lists.llvm.org
> <mailto:llvm-dev at lists.llvm.org>> wrote:
>
>    Hello.
>      I have extended the BPF back end with vector registers (inspiring from Mips MSA) -
> something like this:
>        def MSA128D: RegisterClass<"Connex", [v128i16], 32,
>                             (sequence "Wh%u", 0, 31)>;
>      I also added vector store and load instructions in the style of Mips MSA - see
> https://github.com/llvm-mirror/llvm/blob/master/lib/Target/Mips/MipsMSAInstrInfo.td, look
> for "def ST_D", etc.
>      Note however that my vector unit has a separate memory space. This is why I defined
> the vector store like:
>        class ST_DESC_BASE<string instr_asm, SDPatternOperator OpNode,
>                     ValueType TyNode, RegisterOperand ROWD,
>                     Operand MemOpnd = uimm4_ptr, ImmLeaf Addr = immLeafAlex,
>                     InstrItinClass itin = NoItinerary> {
>        dag OutOperandList = (outs);
>        dag InOperandList = (ins ROWD:$wd, MemOpnd:$addrdst);
>        string AsmString = !strconcat("LS[$addrdst] = $wd;",
>                                      instr_asm);
>        list<dag> Pattern = [(OpNode (TyNode ROWD:$wd), Addr:$addrdst)];
>        InstrItinClass Itinerary = itin;
>        string DecoderMethod = "DecodeMSA128Mem";
>        }
>
>       Also, BPF has its own scalar stores and loads (with the standard i64 registers), for
> example (from https://github.com/llvm-mirror/llvm/blob/master/lib/Target/BPF/BPFInstrInfo.td):
>        class STOREi64<bits<2> Opc, string OpcodeStr, PatFrag OpNode>
>          : STORE<Opc, OpcodeStr, [(OpNode i64:$src, ADDRri:$addr)]>;
>
>      However, spills and loads with vector registers, created automatically at the border
> of basic-blocks use the scalar stores and loads and NOT the vector ones that are also
> defined. For example, I obtain this ASM code when compiling with my LLVM:
>          std     -512(r10), R(0)
>          ; end of predecessor BB
>          ...
>
>          ; beginning of current BB
>          ldd     R(0), -512(r10)
>
>      As we can see STOREi64 takes i64 scalar register normally, but it confuses a v128i16
> register R(0) with an i64 scalar one (r0-r31)...
>
>      Could you please tell me if there is an easy way to fix this? I guess the problem is
> related to the fact the vector unit has its own memory space and I guess LLVM spills
> normally registers on the stack - if so can I specify a different spill region for the
> vector register?
>
>    Thank you,
>      Alex
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