[llvm-dev] [RFC] Tablegen-erated GlobalISel Combine Rules
Daniel Sanders via llvm-dev
llvm-dev at lists.llvm.org
Fri Nov 9 16:18:52 PST 2018
Thanks Nicolai!
> On Nov 9, 2018, at 02:55, Nicolai Hähnle <nhaehnle at gmail.com> wrote:
>
> Hi Daniel,
>
> Lots of good stuff in there! I especially like the design for specifying out-of-line predicates. I have a couple of small comments and one major one below.
>
>
> On 09.11.18 02:42, Daniel Sanders via llvm-dev wrote:
>> _Passing arbitrary data from match to apply_
>> _
>> _
>> As mentioned earlier, the defs block defines the interface between the match and apply steps. This can be used to arrange for arbitrary data to be passed from match to apply.
>> In the current AArch64PreLegalizeCombiner we have a rule that matches a G_LOAD and all its uses simultaneously and decides on the best way to rewrite it to minimize the sign/zero/any-extend operations. This rule passes a struct (PreferredTuple) between the current C++ equivalent for the match to the current C++ equivalent to the apply. Converting that into this tablegen syntax, we'd write:
>> def extending_load_matchdata : GIDefMatchData<"PreferredTuple">;
>> def extending_load_predicate : GIMatchPredicate<
>> (ins reg:$A, extending_load_matchdata:$B), bool, [{
>> return Helper.matchCombineExtendingLoads(${A}, ${matchinfo});
>
> I assume this was intended to be ${B} instead of ${matchinfo}?
Yes, that's right. There's always at least one of these typos when I type emails about this :-). It looks like I forgot to rename one of them when I moved the code into a predicate definition.
> I also think you should have 'ins' and 'outs' separately; after all, a predicate may have to do a combined check on two matched registers / operands, and produce one or more values for later re-use.
>
> Once you have separate 'ins' and 'outs', the "bool" in there seems a bit redundant.
I think there's three kinds values involved in the predicates. The first is the inputs like these values come from other parts of the match and it makes sense that they belong in 'ins'. The second is values that are directly related to the truthiness of the predicate such as bool, unsigned (register number), MachineInstr*, etc.. These are the result of the underlying function and there can only be one of these per predicate. The type can potentially be std::pair or std::tuple to give multiple results provided a suitable conversion to bool is provided. The third is metadata that's just carrying additional information to the apply. These are currently in the 'ins' section and I agree that they don't really make sense there. I can pull them out into a separate 'outs' section. This has a flexibility cost since the user can no longer specify the complete argument order but that's probably a good thing for readability.
Here's an example of a non-bool result:
def reg_with_shift : GIMatchPredicate<
(ins reg:$A), (outs uint64_t:$shift), MachineOperandPtr, [{
// MachineOperand *matchARMShiftedOp2(const MachineOperand &, uint64_t);
return matchARMShiftedOp2(${A}, ${imm});
}]>;
def : GICombineRule<
(defs root:$D, reg:$S1, reg_with_shift:$S2),
(match [{MIR %D = G_ADD %S1, %S2 }]),
(apply [{MIR %D = ADD %S1, %S2, %S2.shift }])>;
(I've just realized there's a gap in the ability to name metadata when used in this way. I've gone with 'S2.shift' for now)
This is equivalent to:
def reg_with_shift : GIMatchPredicate<
(ins reg:$A), (outs ptr_to_reg:$reg, imm:$shift), bool, [{
// bool matchARMShiftedOp2(const MachineOperand &, MachineOperand*&, uint64_t&);
return matchARMShiftedOp2(${A}, ${reg}, ${imm});
}]>;
def : GICombineRule<
(defs root:$D, reg:$S1, reg_with_shift:$S1),
(match [{MIR %D = G_ADD %S1, %S2.reg }]),
(apply [{MIR %D = ADD %S1, %S2, %S2.shift }])>;
but the first definition slightly more efficient for most targets since there's usually a register or two for return values which in this version we're spending solely on a redundant bool and requires fewer argument registers, the second version has to store the MachineOperand* on the stack to pass it by reference. The difference is fairly small when considered on an individual rule but should accumulate on large rulesets or large functions.
>> }]>;
>> def extending_loads : GICombineRule<
>> (defs root:$root, reg:$A, extending_load_matchdata:$matchinfo),
>> (match [{MIR %root = G_LOAD %A }],
>> (extending_load_predicate root:$A,
>> extending_load_matchdata:$matchinfo)),
>> (apply (exec [{ Helper.applyCombineExtendingLoads(${root}, ${matchinfo}); }],
>> reg:$root, extending_load_matchdata:$matchinfo)>;
>> The GIDefMatchData declares a new type of data that can be passed from the match to the apply. Tablegen is responsible for arranging for appropriate storage during the Combine algorithm. The GIMatchPredicate declares a C++ predicate that fills out the PreferredTuple (passed by reference) whenever it returns true for a successful match. We could have made the predicate return std::pair<bool, PreferredTuple> instead but that's less efficient (it would be an sret return on many targets) and would require us to define the truthiness (no examples of are in this email as I expect it to be a rare thing to need) in order to act as a predicate. Normally, you'd feed this into a (create_imm ...) or a (create_operand ...) in the apply section. However, in this particular case the data being passed determines the entirety of the replacement so we escape into arbitrary C++ instead and arrange for the variables to be injected appropriately using the 'exec' operator.
>
> Have you considered the possibility of defining custom `create` actions for the apply, analogous to custom predicates?
>
> In your description, it appears there is a fixed list of builtin actions such as `create_imm`, `create_operand` (not really described anywhere?), `exec` and the tentative debug info approach.
>
> It seems valuable to be able to define one's own operator for custom operations.
I didn't go into the mechanical definitions but create_imm is really just a specialization of create_operand and is equivalent to (create_operand [{ MachineOperand::CreateImm( <code> ) }]). The string concatenation needed for that specialization currently doesn't work on the code type but that seems easily fixable. I haven't defined one yet, but I don't see any reason we couldn't define non-operands in a similar way.
>> _Macros_
>> _
>> _
>> I simplified the previous example a bit. Rather than only matching a G_LOAD, the current rule in AArch64 can match any of G_LOAD, G_SEXTLOAD, and G_ZEXTLOAD. We need some means to match one of several alternatives as well as collect and re-use common subpatterns. I've yet to look into how this would be practically implemented and this section is a bit vague as a result but here's the current thinking on how it should look and behave:
>> def ANYLOAD : GIMacro<(defs def:$R, use:$S, uint64_t:$IDX),
>> (match (oneof [{MIR %R = G_LOAD %S}],
>> [{MIR %R = G_SEXTLOAD %S}],
>> [{MIR %R = G_ZEXTLOAD %S}]):$IDX>;
>> def extending_loads : GICombineRule<
>> (defs root:$root, reg:$A, extending_load_matchdata:$matchinfo, ANYLOAD:$ANYLOAD),
>> (match [{MIR %root = ANYLOAD %A }],
>> (extending_load_predicate root:$A,
>> extending_load_matchdata:$matchinfo)),
>> (apply (exec [{ Helper.applyCombineExtendingLoads(${root}, ${matchinfo}); }],
>> reg:$root, extending_load_matchdata:$matchinfo)>;
>> Effectively, we're declaring a fake instruction and import it into this rule, possibly renaming it using the argument name ($ANYLOAD in this case) to provide a more convenient name in the MIR block or to disambiguate multiple instances. Once we've parsed the MIR, we would recursively replace any instance of ANYLOAD with code match one of the alternatives. The variables in the 'defs' section of the macro would be available as $ANYLOAD_R, $ANYLOAD_S, and $ANYLOAD_IDX (we have to use '_' instead of a '.' to fit within tablegen's syntax) with $ANYLOAD_IDX indicating which alternative the (oneof ...) matched. When nested by including a macro in the macro's defs section and using it, the names to access the sub-macros variables would grow longer by underscore separated concatenation, for example '$ANYLOAD_ANYEXT_A'.
>
> This magic generation of variable names seems really wrong to me.
I agree. I'd have preferred to use a structure-like style ($ANYLOAD.ANYEXT.A) but while that's fine in a code block, it doesn't fit into tablegen's syntax. Thinking on it a bit more, a better means of getting access to the defs would be:
def extending_loads : GICombineRule<
(defs root:$root, reg:$A, extending_load_matchdata:$matchinfo,
(import_macro ANYLOAD:$ANYLOAD, def:$R, use:$S, uint64_t:$IDX)),
(match [{MIR %root = ANYLOAD %A }],
(extending_load_predicate root:$A,
extending_load_matchdata:$matchinfo)),
(apply (exec [{ Helper.applyCombineExtendingLoads(${root}, ${matchinfo}); }],
reg:$root, extending_load_matchdata:$matchinfo)>;
That would enable the rule to specify the names and if we require that of nested macros too we can nest them as needed. We would also want the defs/ops distinction you mention below to allow macros to have local defs.
> Let's just use a natural operands interface, like so (and I'm also doing something else here, more on that later):
>
> def ANYLOAD : GIMacro<
> (ops def:$R, use:$S, imm:$IDX),
> (defs), // optional extra defs that can be used in predicates
> (oneof (match (G_LOAD $R, $S), 0:$IDX),
> (match (G_SEXTLOAD $R, $S), 1:$IDX),
> (match (G_ZEXTLOAD $R, $S), 2:$IDX))>;
> def extending_loads : GICombineRule<
> (defs def:$root, reg:$A, extending_load_matchdata:$matchinfo,
> imm:$idx),
> (match (ANYLOAD $root, $A, $idx),
> (extending_load_predicate $A, $matchinfo)),
> (exec [{ Helper.applyCombineExtendingLoads(
> ${root}, ${matchinfo}, ${idx}); }])>;
>
> There'd be a natural and simply way to shift predicates and variables in and out of macros: the following would be equivalent:
>
> def ANYLOAD : GIMacro<
> (ops def:$R, extendling_load_matchdata:$matchinfo, imm:$IDX),
> (defs use:$S),
> (match (oneof (match (G_LOAD $R, $S), 0:$IDX),
> (match (G_SEXTLOAD $R, $S), 1:$IDX),
> (match (G_ZEXTLOAD $R, $S), 2:$IDX)),
> (extending_load_predicate $S, $matchinfo))>;
> def extending_loads : GICombineRule<
> (defs def:$root, extending_load_matchdata:$matchinfo, imm:$idx),
> (ANYLOAD $root, $A, $matchinfo, $idx),
> (exec [{ Helper.applyCombineExtendingLoads(
> ${root}, ${matchinfo}, ${idx}); }])>;
>
> ... and of course it could all be inlined:
>
> def extending_loads : GICombineRule<
> (defs def:$root, reg:$A, extending_load_matchdata:$matchinfo,
> imm:$idx),
> (match (oneof (match (G_LOAD $root, $A), 0:$idx),
> (match (G_SEXTLOAD $root, $A), 1:$idx),
> (match (G_ZEXTLOAD $root, $A), 2:$idx)),
> (extending_load_predicate $A, $matchinfo)),
> (exec [{ Helper.applyCombineExtendingLoads(
> ${root}, ${matchinfo}, ${idx}); }])>;
>
> In other words, macros would really just be macros or sub-routines.
>
> Let's talk about using DAGs. I think I understand where the temptation comes from to describe the MIR using code blocks, but I'm also pretty sure that it's a mistake to do so.
The main motivation to move away from DAGs came from limitations in DAGs and the general ugliness of the original DAG-based definitions. Once the idea of using MIR came up, we liked the fact that it matched the existing serialization format which makes it easy to turn a specific example from the compiler output into a combine rule and avoid the need for another representation for instructions. We also liked the way it dealt with the difficult cases, MachineMemoryOperands, subregs, etc. and also that it wasn't a new syntax or parser (although it make require modifications to the existing one). It also looked like it be convenient for a tool like Alive (https://www.cs.utah.edu/~regehr/papers/pldi15.pdf) although we didn't really explore that particular thought further.
Ultimately, both representations are DAG's and it comes down to the convenience of the syntax in specifying the rules we want. MIR is very convenient because of it's familiarity and flexibility.
One of the bigger problems with using tablegen's DAG type is that it doesn't deal with multi-result instructions very well. Every time this has been raised on the list w.r.t SelectionDAG the solution has boiled down to 'use C++ instead' and it would be good to fix that so that things like UADDO are representable. You can write a rule that matches something like divmod at the top-level using the 'set' operator:
(set $D1, $D2, (divmod $A, $B))
but as soon as it's not the top-level, it gets really ugly fast even using pseudo-nodes:
(set (outs $D1, $D2), (sext (result (G_DIVMOD $A, $B):$T, 0)),
(sext (result $T, 1)))
In this example, outs is a pseudo-node needed to distinguish the results from the sinks of the DAG, and result is a pseudo-node that selects one of the multiple results for use by the parent node. Meanwhile the MIR for is:
%0, %1 = G_DIVMOD %A, %B
%D1 = G_SEXT %0
%D2 = G_SEXT %1
which seems much clearer. It becomes a bigger problem when you start wanting to match target specific instructions as multi-result becomes more common later in the backend, especially post-isel. It's also a problem for upside-down matches, inside-out, and multi-root matches since tablegen's dag type requires a single common root node and can only draw edges in one direction. If you want to have to root in the middle then you have to distort the DAG to bring it to the root and mark up the reversed edges somehow
Moving the result inside the instruction operator helps with these problems but requires a list of dags and all the temporaries need to be named:
[(G_DIVMOD $T1, $T2, $A, $B),
(G_SEXT $D1, $T1),
(G_SEXT $D2, $T2)]
and at that point, we're very nearly at MIR.
Another issue we didn't like with DAG is that you also end up needing a lot of pseudo-nodes like EXTRACT_SUBREG whereas these are natural parts of the MIR syntax. For example:
(set $d, (EXTRACT_SUBREG (MYTGT_ADD $A, $B), sub_lo))
compared to:
%d:sub_lo = MYTGT_ADD %A, %B
and:
(set $d, (REG_SEQUENCE GPR32, (MYTGT_ADD $A, $B), sub_lo, (MYTGT_ADD $C, $D), sub_hi))
compared to:
%d:sub_lo = MYTGT_ADD %A, %B
%d:sub_hi = MYTGT_ADD %C, %D
Along the same lines, I also think that the integrated debug-info is only really practical in MIR. It's possible to shoe-horn DILocation in using a pseudo-node like so:
(set $d, (add (mul $a, $b):$mul, $c):$add -> (set (merged_dilocation (muladd $a, $b, $c), $mul, $add)
but there isn't really a good place to modify the DIExpression used by DEBUG_VALUE.
> One reason is that it adds yet another parser, which is more maintenance burden without buying much.
The expectation is that we can make use of the existing MIR parser and replace it's MachineInstr/MachineOperand/etc. generation code with pattern-matching generation code. I'm going to be looking into the implementation of that over the next few weeks.
> The more important reason is that DAGs compose better with the rest of TableGen. Consider combine rules defined in multiclasses, for example. That is very common all through LLVM. In order to mirror an existing selection rule in the AMDGPU backend, we'd likely want to have something like this:
>
> multiclass Arithmetic_i16_GIPats<Instruction ginst,
> Instruction inst> {
> // This is probably wishful thinking -- would be okay if we had to
> // split this into two different rules due to the different types
> // of $dst
> def : GICombineRule<
> (defs root:$dst, operand:$src0, operand:$src1),
> (oneof (ginst i16:$dst, i16:$src0, i16:$src1),
> (match (ginst i16:$tmp, i16:$src0, i16:$src1),
> (G_ZEXT i32:$dst, $tmp))),
> (inst $dst, $src0, $src1)>;
>
> // A third rule for zext to 64 bits would go here...
> }
This is the reason that macros are explicitly imported in the defs section and the imported instances are renameable. The same thing in the MIR syntax (and using the changes to variable naming from above) would be:
multiclass Arithmetic_i16_GIPats<Instruction ginst,
Instruction inst> {
def : GICombineRule<
# There's a corner case for the double definition of $dst here. I'll come back to that later in the mixing concerns comment
(defs root:$dst, (import-macro ginst:$ginst, def:$dst, use:$src0, use:$src1)),
(match [{ %dst:(s16) = ginst %src0:(s16), %src1:(s16) }]), # I've corrected the i16 to s16 here. i16 isn't a type in GlobalISel
(apply /* I'll come back to this bit */)>;
// Covers s32 and s64
def : GICombineRule<
(defs root:$dst, operand:$src0, operand:$src1, (import-macro ginst:$ginst)),
(match [{ %0:(s16) = ginst %src0:(s16), %src1:(s16)
%dst = G_ZEXT %0:(s16) }]
(isS32OrS64 type:$dst)),
(apply /* I'll come back to this bit */)>;
}
We could potentially fold the has G_ZEXT case and no G_ZEXT cases together with a macro but it seems unnecessary complexity in this case. It might be more worth it if there's a similar G_SEXT variant too.
The reason for the "I'll come back to this bit" in the apply section is because you've raised an issue I'd forgotten to deal with. The opcodes in the apply section should also be parameterizable. My first thought on that is that there should be a macro-equivalent for apply such as:
def : GIExpansion<
(defs def:$R, use:$S, imm:$IDX),
(apply (select imm:$IDX,
(0 [{MIR %R = FOO %S }]),
(1 [{MIR %R = BAR %S }]),
(2 [{MIR %0 = BAZ %S
%R = BAR %0 }])))>;
or alternatively:
def : GICombineRule<
...,
...,
(apply (create_opcode [{ ${IDX} ? MYTGT_A : MYTGT_B }] ...):$name,
[{MIR %D = name %s }])>;
The former is more powerful, but the latter is more convenient for trivial cases
> Obviously you can achieve this in your proposal as well using string concatenation, but that does seem rather backwards.
>
> Something else that falls out rather nicely from using DAGs is that it gives you a natural way to give a name to an _instruction_, to be able to pass it to a predicate for extended checks (and as a potentially more natural way of preserving debug locations!).
I don't think we need a means to name instructions since instructions can be trivially found from of their results using MachineOperand::getParent().
As noted above, merging debug locations doesn't fit very well into DAG's as you have to use a pseudo node to be able to provide both DILocations on the same result instruction, and there's no real means to include DEBUG_LOC for temporaries that are eliminated and need to be rebased from another value.
> I have a more explicit example of this later.
>
>
>> _'Upside-down' matches (i.e. roots at the top) and similar_
>> This one requires algorithm changes which I'd prefer not to discuss in this RFC. Assuming the underlying algorithm gains support for this, this is how the syntax would look:
>> def : GICombineRule<
>> (defs root:$root, reg:$A),
>> (match [{MIR %1 = G_LOAD %root
>> %A = G_SEXT %1 }]),
>> (apply [{MIR %A = G_SEXTLOAD %root }])>;
>> The only unusual thing about this rule is that the root isn't at the bottom. Instead of starting at a use and matching towards defs, we're starting at the def and matching towards uses. This has some potentially useful properties. The combine algorithm has to chose an insertion point for the replacement code and the traditional choice has been the root of the match. Assuming we keep doing the same thing, 'upside-down' matching like this is able to avoid the checks that the load is safe to move, is non-volatile, has one use, etc. that would be necessary if we moved the G_LOAD down to the G_SEXT. Also, assuming we keep the same broadly bottom-up processing order as the existing Combine algorithm this kind of rule also has relatively lower priority than conventional rules because the root is seen later. This can be useful as (algorithm-dependent) the MIR may be more settled by the time it tries to match.
>> Along the same lines, the syntax can potentially support the root being in the middle of a match. This could be used in a similar way to the upside-down match above to control the insertion point and priority. For example:
>> def : GICombineRule<
>> (defs root:$root, reg:$A, reg:$B, reg:$C),
>> (match [{MIR %1(s32) = G_TRUNC %A(s64)
>> %2(s32) = G_TRUNC %B(s64)
>> %root = G_ADD %1, %2
>> %C(s64) = G_SEXT %root }]),
>> (emit [{MIR %root = G_ADD %A, %B
>> %C = G_SEXT_INREG %root }])>;
>> Unfortunately, I don't have any concrete examples where this would be useful in comparison to a conventional or upside-down match at the moment. I'm mostly keeping the door open as I can see potential for benefits (mostly w.r.t sinking and hoisting safety around an instr that would be difficult to test for that) given an appropriate rule and also because I'm inclined to say that the tablegen syntax shouldn't be the reason it's not possible. It should be up to the Combine algorithm and and tablegen-erate pass involved in specializing the algorithm for a target.
>
> I'm concerned that the concrete syntax proposed here mixes a bunch of concerns that really should be addressed separately:
>
> 1. Defining a mapping between semantically equivalent chunks of code.
> 2. Controlling insertion points as a simple way to keep side effects under control in many cases.
> 3. Controlling additional algorithmic aspects of the combine algorithm (whether you're matching up or down, say).
I think there may be a misunderstanding with #3. There's no real concept of matching up or down in the rules, there's only the starting point for the match and from there whether it's following the uses or defs isn't something the rule needs to be concerned about. The algorithm needs to be aware of it (and historically hasn't been, DAGCombine broadly runs bottom up and expects to match towards defs and can fail to re-schedule nodes for re-visit in many common cases) but the algorithm is expected to figure that out for itself.
'root' definitions are the only place that #2 and #3 come into the proposed syntax. At the moment, the only requirement the syntax imposes on the algorithm is that it has a concept of a current MachineOperand Def* (and by extension instruction) that will be used as the starting point for the match and that it can choose a valid insertion point for each instruction produced. The insertion point is the same position in the current algorithm but that's algorithm dependent and doesn't have to be the case in the long run. There may even be more than one for rules that emit multiple instructions in the apply section. There's currently no means to specify an insertion point, it's left up to the algorithm to find a place for each instruction. That said, I'd be surprised if there was an algorithm where the current MachineOperand isn't also at least one of the insertion points.
* The current upstream code is a bit misleading about that and makes it seem that it's the MachineInstr but I made that mistake before on ISel so I'm intending to change that.
> Could we keep those separate somehow? E.g., don't distinguish between 'root' and 'reg' or 'operand' in the (defs ...) part of the rule; have TableGen figure out the roots automatically based on walking the graph implied by the matching rule, and default to using the sink(s) as roots.
operand isn't really connected to this, it's just a wildcard for an MachineOperand that doesn't care whether it's a reg, imm, MCExpr, or something else.
I'm inclined to agree that it would be nice to get 'root' out of the definition but I don't really see a good way to do it. The sinks aren't always the right choice and are a particularly bad choice on combines like the extending loads since that's trying to match all uses simultaneously and those uses could have any opcode.
> Then add some optional let-able variable in the GICombineRule to define non-default rules for the combine algorithm.
>
> Controlling the insertion point to address loads like in your example above could be done quite naturally if the MIR is expressed in a DAG:
>
> def : GICombineRule<
> (match (G_LOAD $tmp, $ptr):$load,
> (G_SEXT $dst, $tmp)),
> (apply (insertat $load),
> (G_SEXTLOAD $dst, $ptr))>;
This isn't quite the same thing as the extending loads combine in the example. In that combine, it's matching the load and all uses of its result and chosing a replacement based on the global usage of that def across the whole function, rewriting conflicting sext/zext/aext/trunc's to be as cheap as possible (a lot of the detail of that is inside the function but the code is upstream if you want to see the details). This rule is matching a single use and relying on CSE to de-dupe the N G_SEXTLOAD's that come out of it. To illustrate the difference, consider:
%0(s8) = G_LOAD %p
%d1(s16) = G_SEXT %0
%d2(s32) = G_SEXT %0
If I apply the above rule as much as possible to this (twice) with the roots at the sink, I'll get:
%d1(s16) = G_SEXTLOAD %p
%d2(s32) = G_SEXTLOAD %p
whereas if I apply the extending loads combine to it (once), I'll get:
%d2(s32) = G_SEXTLOAD %p
%d1(s16) = G_TRUNC %d2
which will normally be cheaper to execute. In combination with other rules and CSE, the first version could eventually turn into the second version but it requires more memory churn and processing time to get there.
One of the things I'm planning to look into in the algorithm is making it smarter about combines involving multiple-uses in the intermediate operands. DAGCombine is harmfully greedy in this area and will often duplicate expensive operations if one use is combinable but another isn't. This has led to the addition of hasOneUse() checks which solves that but takes things to the opposite extreme. I think it should be possible to find an algorithm that makes an intelligent decision for defs with multiple uses.
> As you can see, this discussion also makes me wonder whether the (defs) are needed at all; maybe we can rely on type inference almost everywhere?
>
>
>> _Multiple roots_
>> This one requires algorithm changes which I'd prefer not to discuss in this RFC. Assuming the underlying algorithm gains support for this, this is how the syntax would look:
>> def : GICombineRule<
>> (defs root:$root1, root:$root2, reg:$A, reg:$B),
>> (match [{MIR %root1 = G_ADD %A, %B }],
>> [{MIR %root2 = G_SUB %A, %B }]),
>> (emit [{MIR %root1, %root2 = BUTTERFLY %A, %B }])>;
>> This would match a G_ADD and G_SUB with operands in common and combine them into a BUTTERFLY operation. You can think of this as two normal rules, one with %root1 as the root and the other with %root2 as the root.
>
> Again, it seems to me that it would be preferable if we could just express this rule as:
>
> def : GICombineRule<
> (match (G_ADD $dst1, $a, $b),
> (G_SUB $dst2, $a, $b),
> reg:$a, reg:$b),
> (BUTTERFLY $dst1, $dst2, $a, $b)>;
>
> or perhaps:
>
> def : GICombineRule<
> (match (G_ADD $dst1, reg:$a, $b),
> (G_SUB $dst2, $a, reg:$b)),
> (BUTTERFLY $dst1, $dst2, $a, $b)>;
>
> and similar variations.
Both of these would be ok when inferring the roots to be the sinks.
> TableGen should be able to figure out the multiple roots by walking the dependency graph that is implied by the knowledge about which operands of G_ADD and G_SUB are defs and uses, respectively.
>
> To sum up, I really like the proposal overall, except I think it would be a **major** improvement to use DAGs for expressing the MIR, and sure, there's the odd detail here and there like about GIMacro.
>
> Cheers,
> Nicolai
Thanks for all your comments!
>> _Grouping and Ordering Rules_
>> Combine rules are ordered and grouped using definitions of the form:
>> def trivial_combines : GICombineGroup<[copy_prop]>;
>> def combines_for_extload: GICombineGroup<[extending_loads]>;
>> def all_combines : GICombineGroup<[trivial_combines, combines_for_extload]>;
>> Essentially, we create a long ordered list of all the combines. Tablegen is free to re-order rules so long as the resulting ruleset always behaves as if it were in this specific order. So for example, the list [sext_trunc_sext, zext_trunc_zext, sext_sext, zext_zexts] is free to re-order to [sext_trunc_sext, sext_sext, zext_trunc_zext, zext_zexts] because the rules involved are mutually exclusive due to the root opcode.
>> So why not make tablegen figure out the order like ISel does? ISel attempts to figure out an order using an scoring system called Complexity along with a hack (AddedComplexity) to allow the user to provide magic numbers to fix the cases it got it wrong. The simplest way to confuse it is with patterns like (add s32, complexpattern1) and (add s32, complexpattern2). These have the same Complexity score but in truth, each one has a (possibly overlapping) range of complexity depending on what C++ code is inside the complexpattern's and which path through that C++ is dynamically taken. If it matches nothing then the score should be 0 but if it matches a dozen nodes it should be 12 (or possibly higher). We don't know which until we try to match that specific pattern. Correctly figuring out an order in the presence of complexpatterns is impossible. Similarly, it's also possible to confuse it with patterns that differ but overlap and add up to the same complexity due to quirks of the scoring system.
>> _Declaring Combine Passes_
>> Combiner passes are defined by subclassing GICombiner like so:
>> def CommonPreLegalizerCombiner: GICombiner<
>> "CommonGenPreLegalizerCombiner", [all_combines]> {
>> let DisableRuleOption = "common-prelegalizercombiner-disable-rule";
>> let Modifiers = [(disable_rule copy_prop)]
>> }
>> This causes tablegen to create a class named 'CommonPreLegalizerCombiner' which you can use to perform combines. This combiner contains all the combines mentioned in the previous section because it includes the 'all_combines' group. However, it disables the copy_prop rule to prevent it from attempting to match. I'll discuss that a bit more below. It also generates a command-line option for asserts builds only which can be used to further disable rules at run-time which will be useful for tracking down bugs or for testing a rule in isolation. I'm hoping that one day tools like bugpoint will be able to search through the individual rules within a combine pass when searching for a minimal reproducer.
>> The Modifiers field is intended to allow targets to modify an existing ruleset (particularly a target independent one) with additional target specific quirks. For example, one particular rule might be doing more harm than good and should be disabled. Or maybe only a subset of the things it would normally match are safe in which case an extra predicate should be tested. Being able to make minor edits to the ruleset without taking on the whole maintenance burden of the common code or causing code bloat by duplicating tables would be useful for targets that are generally similar to the targets within LLVM but have minor quirks.
>> Larger scale changes should take an alternate approach to modifiers. It's expected that even targets that are very different from the rest of the pack still have features in common. Such targets can declare their own combiner to replace the common one but still generally make use of the improvements made by the wider community. This is where GICombinerGroup will start to shine as such a target can declare a combiner like so:
>> def MyTargetPreLegalizerCombiner: GICombiner<
>> "MyTargetGenPreLegalizerCombiner",
>> [common_extend_optimizations,
>> common_extending_loads,
>> // common_rsqrt_and_nr_iterations, // This target has a real sqrt operation
>> mytarget_special_fma,
>> common_fma,
>> common_bswap_idioms,
>> mytarget_bcd_arithmetic
>> ]> {
>> }
>> As LLVM improves on the common_* groups, the target benefits from those improvements automatically. However, it doesn't benefit from new groups being added to common_all_combines because it's no longer using that group so entirely new categories of combines added to it would not appear in the combiner. It would still be nice to find out that a new category has appeared though so that a decision can be made on it. To that end, I'm considering adding:
>> let Verify = [(has_all_of_except all_combines, common_rsqrt_and_nr_iterations)];
>> which would cause a warning if something new appeared in the original group.
>> _Conclusion_
>> There is a lot of work that needs to be done to get all this working and some of it may have to change once it runs into the reality of implementation :-). However, we think that this will prove to be a very convenient and powerful syntax with some potential a variety of tools from profilers, to bug reducers, to correctness checking tools.
>> There is a patch at https://reviews.llvm.org/D54286 makes a start on it to try out the general feel of the syntax but currently lacks the core feature of generating match and apply code from the MIR. I'm going to be looking into that over the next few weeks.
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
> --
> Lerne, wie die Welt wirklich ist,
> Aber vergiss niemals, wie sie sein sollte.
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