[llvm-dev] Stuck with instruction in tablegen

[Craig Topper via llvm-dev]

To the best of my knowledge, having 2 explicit physical registers(in your
case A and F) in the output of an instruction isn't supported correctly by
tablegen and never has been. Maybe it previously appeared to work well
enough to pass build?

This is why X86 has the pattern commented out here and implements custom
handling in X86ISelDAGToDAG.cpp.

// EAX,EDX = EAX*GR32
let Defs = [EAX,EDX,EFLAGS], Uses = [EAX], hasSideEffects = 0 in
def MUL32r : I<0xF7, MRM4r, (outs),  (ins GR32:$src),
               "mul{l}\t$src",
               [/*(set EAX, EDX, EFLAGS, (X86umul_flag EAX, GR32:$src))*/]>,
               OpSize32, Sched<[WriteIMul]>;
// RAX,RDX = RAX*GR64
let Defs = [RAX,RDX,EFLAGS], Uses = [RAX], hasSideEffects = 0 indef MUL64r
: RI<0xF7, MRM4r, (outs), (ins GR64:$src),
                "mul{q}\t$src",
                [/*(set RAX, RDX, EFLAGS, (X86umul_flag RAX,
GR64:$src))*/]>,
                Sched<[WriteIMul64]>;


I vaguely recall trying to fix tablegen probably at least 5 years ago, but
didn't succeed. Looking back at the code now I can tell you the two places
in DAGISelMatcherGen.cpp that call HasOneImplicitDefWithKnownVT are
definitely unable to handle multiple implicit def destinations in a pattern.

~Craig


On Mon, Jul 9, 2018 at 9:30 PM Michael Stellmann via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hi,
>
> I'm trying to revive jacobly0's Z80 back-end (from
> https://github.com/jacobly0/llvm-z80) and build it with a current
> version of LLVM.
>
> Apart from some interface changes, I'm stuck at building the tables.
> Specifically, the generation of the DAG instruction selector causes an
> assertion in the table generator:
>
>      Assertion failed: Ops.size() >= NumSrcResults && "Didn't provide
> enough results"
>
> I tried to fix the problem by searched in the documentation and in other
> table sources, but to no avail. Even stepping through that code in the
> debugger (MatcherGen::EmitResultCode()) didn't help me to find the cause
> of the problem. The instruction tables description are quite unique in
> all the officially supported CPU models, I couldn't find any similar
> code that could help me figuring out what is going wrong.
>
> So I isolated the table sources to a minimum of two simple
> base-instructions and attached these for reproduction of the problem
> (not sure if the attachment gets through).
>
> The table generation is done with:
>
>      llvm-tblgen -gen-dag-isel -I=<path-to-llvm-include> -o=out.inc Z80.td
>
> The first instruction in Z80InstrInfo.td is a unary instruction, that
> does a rotate left of a byte in memory, resulting in the leftmost bit
> being placed in the Carry flag - and changing the memory contents at the
> pointer's address (in Z80: "RLC (HL)", I expect).
> The table generation completes without errors with just this instruction
> (by commenting the last line in Z80InstrInfo.td).
>
> The second one is a binary instruction, adding the contents of a pointer
> and a register, with results in register A and F (Z80: "ADD A,(HL)").
>
> A is an 8-bit register. The Z80 ALU can only do 8 bit arithmetic with
> that register and another param. The two instructions don't seem to
> require a specific register for the address in the table class, so the
> RegisterInfo table is not defined so far in the minimized version
>
> The problem is in Z80InstrInfo.td, in the following section of
> "BinOp8RF", which is used by the "defm" line that generates the "add"
> instruction pattern:
>
>      multiclass BinOp8RF<Prefix prefix, bits<3> opcode, string mnemonic,
>                          bit compare = 0> {
>        let isCompare = compare, Defs = [A, F], Uses = [A] in {
>          def 8ap : I8 <prefix, {0b10, opcode, 0b110}, mnemonic, "\ta,
> $src", "",
>                        (outs), (ins   ptr:$src),
>                        [(set A, F,
>                             (Z80add_flag
>                                  A, (i8 (load   iPTR:$src))))]>;
>        }
>      }
>
> and the parameters:
>
>      def SDTBinOpRF  : SDTypeProfile<2, 2, [SDTCisInt<0>,
>                                             SDTCisFlag<1>,
>                                             SDTCisSameAs<2, 0>,
>                                             SDTCisSameAs<3, 0>]>;
>
>      def Z80add_flag      : SDNode<"Z80ISD::ADD",     SDTBinOpRF,
> [SDNPCommutative]>;
>
>
> The unary instruction builds fine, which can be checked by commenting
> out the generation of the binary instruction in the last line of the
> attached Z80InstrInfo.td.
>
> The log in Github indicates that this section was present for over a
> year, so I assume that it did work at some time in the past.
>
> My attempts to fixing the instruction failed with other errors, pointing
> specifically to the specific problems I introduced with the change, e.g.
> parameter number mismatch or wrong type, so above (and attached) is the
> unmodified version.
>
> Any help is appreciated!
>
> Michael
>
>
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