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

[Arsenault, Matthew via llvm-dev]

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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