[llvm-dev] [RFC] Tablegen-erated GlobalISel Combine Rules
Daniel Sanders via llvm-dev
llvm-dev at lists.llvm.org
Thu Nov 29 16:04:14 PST 2018
> On Nov 29, 2018, at 01:57, Nicolai Hähnle <nhaehnle at gmail.com> wrote:
>
> Hi Daniel,
>
>
> On 27.11.18 18:59, Daniel Sanders wrote:
>> I've more or less finished updating the examples to the DAG style we were talking about. Hopefully I haven't forgotten anything, there was a lot to keep track of :-). Overall, I think there's a couple places where things get a a little awkward (mainly debug info) but things generally look good to me.
>
> That's awesome :)
>
>
>> _A Simple Example_
>> def : GICombineRule<(defs reg:$D, reg:$S),
>> (match (G_ZEXT s32:$t1, s8:$S),
>> (G_TRUNC s16:$D, s32:$t1)),
>> (apply (G_ZEXT s16:$D, s8:$S))>;
>> This has been converted to the style that fits within tblgen's DAG type but isn't (directly) a DAG. The DAG structure is defined by the matching names instead of tblgen's DAG type. This avoids the restrictions of the DAG type and is something that's needed anyway to allow one node to be referenced in multiple places. I've kept the 'match' node at the top of the match to allow instructions and predicates to be freely combined within this section (see later examples). Without it, the stricter type checking on list<X> would require Instruction, GIMatchPredicate, and any future matching extensions to have a common base class.
>
> Makes sense.
>
> I was wondering whether it also makes sense to allow eliding the (match) if it only has one child. It seems convenient, although having it always be required may reduce the learning curve
There's no technical reason we can't for the case where there's a single child and the child is a DagInit but I'm not sure there's a good reason to make it optional. Backend writers can always define a class that tacks it on using !cons if they want.
> The other point is that I have been wondering in the past whether it wouldn't make sense to add an 'any' type to TableGen, so that heterogenous lists of type list<any> are possible. TableGen already internally has any-of-list-of-classes types -- there's no syntax to write these types down, but they were needed to consistently describe the type of a def that is derived from multiple classes (these types are printed as {Class1,Class2,...} in debug output).
>
> I haven't *really* needed an 'any' type yet, but there were times when I thought it might be convenient, so I'd be open to that.
>
> Then again, explicitly writing (match) also has the advantage that it'd be obvious which part of the rule one is looking at.
It wasn't something I consciously set out to do (I was really trying to achieve something akin to the list<any> you described above and needed it to conform to the dag type) but I did like that effect when I saw it written down :-).
> [snip]
>> _Passing arbitrary data from match to apply_
>> The main change in this section that hasn't already been discussed is that the result of extending_load_predicate has been moved to the new 'outs' section of GIMatchPredicate and the code expansion refers to a particular output of the predicate using 'matchinfo.B' similar to a struct member or multi-operand ComplexPatterns.
>> def extending_load_matchdata : GIDefMatchData<"PreferredTuple">;
>> def extending_load_predicate : GIMatchPredicate<
>> bool, (ins reg:$A), (outs extending_load_matchdata:$B), [{
>> return Helper.matchCombineExtendingLoads(${A}, ${B});
>> }]>;
>> def extending_loads : GICombineRule<
>> (defs operand:$D, reg:$A, extending_load_matchdata:$matchinfo),
>> (match (G_LOAD $D, $A),
>> (extending_load_predicate operand:$A,
>> extending_load_matchdata:$matchinfo)),
>> (apply (exec [{ Helper.applyCombineExtendingLoads(${D}, ${matchinfo.B}); }],
>> reg:$D, extending_load_matchdata:$matchinfo)>;
>> The main problem with this is that this prevents access to the contents of matchinfo from outside of C++. To overcome this, I'd suggest allowing the '${foo.bar}' syntax in tablegen's dag type. It should be a fairly simple change and there's no tblgen-internal reason to not allow dots in the names. Existing backends would have to check for it though which could be a tblgen performance issue.
>
> What does the ${matchinfo.B} mean, exactly? Why is $matchinfo alone not good enough? It appears syntactically in the place that I would expect corresponds to the out argument $B of extending_load_predicate.
${matchinfo} is the return value of the predicate which will always be true in this particular case. In other cases, the predicate could return a MachineOperand& or an unsigned (register number) and use that value.
${matchinfo.B) is the $B argument from the outs list of the predicate referred to by $matchinfo.
>> _Macros_
>> Aside from changing to the DAG style, the main change here is that the children of 'oneof' all have 'match' at the top-level. This is for the same reason 'match' was kept on the simple example: the relaxed type checking on 'dag' compared to 'list<X>' allows us to freely mix DAG-style matches and predicates.
>> def ANYLOAD : GIMacro<(defs def:$R, use:$S, uint64_t:$IDX),
>> (match (oneof (match (G_LOAD $R, $S)),
>> (match (G_SEXTLOAD $R, $S)),
>> (match (G_ZEXTLOAD $R, $S))):$IDX>;
>> def extending_loads : GICombineRule<
>> (defs reg:$D, reg:$A, extending_load_matchdata:$matchinfo, ANYLOAD:$ANYLOAD),
>> (match (ANYLOAD $D, $A),
>> (extending_load_predicate operand:$A,
>> extending_load_matchdata:$matchinfo)),
>> (apply (exec [{ Helper.applyCombineExtendingLoads(${D}, ${matchinfo.B}); }],
>> reg:$D, extending_load_matchdata:$matchinfo)>;
>
> $IDX doesn't seem to be used in the (apply) part.
The rule I based this example on doesn't actually need it (the opcode is inside the ${matchinfo} data) but if it did, the apply section would be:
(apply (exec [{ Helper.applyCombineExtendingLoads(${D}, ${matchinfo.B}, ${IDX}); }],
reg:$D, extending_load_matchdata:$matchinfo, uint64_t:$IDX)>;
>> [snip]
>
>
>> _Subregisters_
>> This is a new example based on our discussion about MIR having direct support for subregisters. We use subregister indexes to specify that it's a subregister that should be emitted by the apply step.
>> def : GICombineRule<
>> (defs reg:$D, reg:$A, reg:$B),
>> (match (G_TRUNC s32:$t1, s64:$A),
>> (G_TRUNC s32:$t2, s64:$B),
>> (G_ADD $D, $t1, $t2),
>> (apply (ADD32 $D, (sub_lo $A), (sub_lo $B)))>;
>> _Matching MachineMemOperands_
>> While re-writing these examples, I also realized I didn't have any examples for testing properties of the MachineMemOperand, so here's one:
>> def mmo_is_load_8 : GIMatchPredicate<
>> (ins instr:$A), (outs), [{
>> if (!${A}.hasOneMemOperand())
>> return false;
>> const auto &MMO = *${A}.memoperands_begin();
>> return MMO.isLoad() && MMO.getSize() == 1;
>> }]>;
>> def : GICombineRule<
>> (defs operand:$D, operand:$A),
>> (match (G_LOAD $D, $A):$MI,
>> (mmo_is_load8 instr:$MI)),
>> (apply ...)>;
>> I've made use of the naming instructions here since this was better in the debug info section. If we choose the alternative in that section then we can change it to $D or $A instead (either way the generated code would call getParent()).
>
> Looks good to me. How would getParent() work if $A also appears as a def of an address calculation?
It depends on what data needs to get to the apply. It it just needs to recognize the address calculation but doesn't need to use the value in the apply then it would be something like:
def : GICombineRule<
(defs operand:$D, operand:$A),
(match (G_LOAD $D, $A):$MI,
(mmo_is_load8 instr:$MI)
(is_addr_plus_1 operand:$A),
(apply (TGT_LOAD $D, $A, 1)>;
or if a value like an offset needs passing, it would be something like:
def : GICombineRule<
(defs operand:$D, addr_plus_simm16:$A),
(match (G_LOAD $D, $A):$MI,
(mmo_is_load8 instr:$MI)
(is_addr_plus_simm16 operand:$A),
(apply (create_imm [{ ${A}.imm }], addr_plus_simm16:$A):$IMM),
(TGT_LOAD $D, $A, $IMM))>;
> Cheers,
> Nicolai
> --
> Lerne, wie die Welt wirklich ist,
> Aber vergiss niemals, wie sie sein sollte.
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