[llvm-dev] Stuck with instruction in tablegen

Bruce Hoult via llvm-dev
Tue, 10 Jul 2018 07:33:09 -0000

It seems like not really the same case to me. Z80 "F" is similar to
the
condition code register in many other machines, and is set implicitly by
all or most instructions.

In the case of x86 EAX and EDX, I wonder if you could define the pair as a
pseudo-register, in the same way that .. well .. H and L make HL etc.


On Tue, Jul 10, 2018 at 12:17 AM, Michael Stellmann via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Thanks Craig for your quick reply and pointing me to a direction!
>
> So even when finding a syntax that tablegen would accept, it would not
> lead to the desired result.
>
> Will look at the X86 code you mentioned below and try to understand how
> it's done.
>
> Although the X86 seems to be the "closest" to the Z80 regarding
the
> instruction set (and, as far as I can see, the only officially supported
> CISC CPU), it seems also to be the most complex backend to get started
> with. But I'm commited ;-)
>
> Michael
>
> *Gesendet:* Dienstag, 10. Juli 2018 um 08:21 Uhr
> *Von:* "Craig Topper" <craig.topper at gmail.com>
> *An:* Michael.Stellmann at gmx.net
> *Cc:* llvm-dev <llvm-dev at lists.llvm.org>
> *Betreff:* Re: [llvm-dev] Stuck with instruction in tablegen
> 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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>> llvm-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
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