Legalizer

This pass transforms the generic machine instructions such that they are legal.

A legal instruction is defined as:

  • selectable — the target will later be able to select it to a target-specific (non-generic) instruction. This doesn’t necessarily mean that InstructionSelect has to handle it though. It just means that something must handle it.
  • operating on vregs that can be loaded and stored – if necessary, the target can select a G_LOAD/G_STORE of each gvreg operand.

As opposed to SelectionDAG, there are no legalization phases. In particular, ‘type’ and ‘operation’ legalization are not separate.

Legalization is iterative, and all state is contained in GMIR. To maintain the validity of the intermediate code, instructions are introduced:

  • G_MERGE_VALUES — concatenate multiple registers of the same size into a single wider register.
  • G_UNMERGE_VALUES — extract multiple registers of the same size from a single wider register.
  • G_EXTRACT — extract a simple register (as contiguous sequences of bits) from a single wider register.

As they are expected to be temporary byproducts of the legalization process, they are combined at the end of the Legalizer pass. If any remain, they are expected to always be selectable, using loads and stores if necessary.

The legality of an instruction may only depend on the instruction itself and must not depend on any context in which the instruction is used. However, after deciding that an instruction is not legal, using the context of the instruction to decide how to legalize the instruction is permitted. As an example, if we have a G_FOO instruction of the form:

%1:_(s32) = G_CONSTANT i32 1
%2:_(s32) = G_FOO %0:_(s32), %1:_(s32)

it’s impossible to say that G_FOO is legal iff %1 is a G_CONSTANT with value 1. However, the following:

%2:_(s32) = G_FOO %0:_(s32), i32 1

can say that it’s legal iff operand 2 is an immediate with value 1 because that information is entirely contained within the single instruction.

API: LegalizerInfo

The recommended [1] API looks like this:

getActionDefinitionsBuilder({G_ADD, G_SUB, G_MUL, G_AND, G_OR, G_XOR, G_SHL})
    .legalFor({s32, s64, v2s32, v4s32, v2s64})
    .clampScalar(0, s32, s64)
    .widenScalarToNextPow2(0)
    .clampNumElements(0, v2s32, v4s32)
    .clampNumElements(0, v2s64, v2s64)
    .moreElementsToNextPow2(0);

and describes a set of rules by which we can either declare an instruction legal or decide which action to take to make it more legal.

At the core of this ruleset is the LegalityQuery which describes the instruction. We use a description rather than the instruction to both allow other passes to determine legality without having to create an instruction and also to limit the information available to the predicates to that which is safe to rely on. Currently, the information available to the predicates that determine legality contains:

  • The opcode for the instruction
  • The type of each type index (see type0, type1, etc.)
  • The size in bytes and atomic ordering for each MachineMemOperand

Note

An alternative worth investigating is to generalize the API to represent actions using std::function that implements the action, instead of explicit enum tokens (Legal, WidenScalar, …) that instruct it to call a function. This would have some benefits, most notable being that Custom could be removed.

Footnotes

[1]An API is broadly similar to SelectionDAG/TargetLowering is available but is not recommended as a more powerful API is available.

Rule Processing and Declaring Rules

The getActionDefinitionsBuilder function generates a ruleset for the given opcode(s) that rules can be added to. If multiple opcodes are given, they are all permanently bound to the same ruleset. The rules in a ruleset are executed from top to bottom and will start again from the top if an instruction is legalized as a result of the rules. If the ruleset is exhausted without satisfying any rule, then it is considered unsupported.

When it doesn’t declare the instruction legal, each pass over the rules may request that one type changes to another type. Sometimes this can cause multiple types to change but we avoid this as much as possible as making multiple changes can make it difficult to avoid infinite loops where, for example, narrowing one type causes another to be too small and widening that type causes the first one to be too big.

In general, it’s advisable to declare instructions legal as close to the top of the rule as possible and to place any expensive rules as low as possible. This helps with performance as testing for legality happens more often than legalization and legalization can require multiple passes over the rules.

As a concrete example, consider the rule:

getActionDefinitionsBuilder({G_ADD, G_SUB, G_MUL, G_AND, G_OR, G_XOR, G_SHL})
    .legalFor({s32, s64, v2s32, v4s32, v2s64})
    .clampScalar(0, s32, s64)
    .widenScalarToNextPow2(0);

and the instruction:

%2:_(s7) = G_ADD %0:_(s7), %1:_(s7)

this doesn’t meet the predicate for the .legalFor() as s7 is not one of the listed types so it falls through to the .clampScalar(). It does meet the predicate for this rule as the type is smaller than the s32 and this rule instructs the legalizer to change type 0 to s32. It then restarts from the top. This time it does satisfy .legalFor() and the resulting output is:

%3:_(s32) = G_ANYEXT %0:_(s7)
%4:_(s32) = G_ANYEXT %1:_(s7)
%5:_(s32) = G_ADD %3:_(s32), %4:_(s32)
%2:_(s7) = G_TRUNC %5:_(s32)

where the G_ADD is legal and the other instructions are scheduled for processing by the legalizer.

Rule Actions

There are various rule factories that append rules to a ruleset but they have a few actions in common:

  • legalIf(), legalFor(), etc. declare an instruction to be legal if the predicate is satisfied.
  • narrowScalarIf(), narrowScalarFor(), etc. declare an instruction to be illegal if the predicate is satisfied and indicates that narrowing the scalars in one of the types to a specific type would make it more legal. This action supports both scalars and vectors.
  • widenScalarIf(), widenScalarFor(), etc. declare an instruction to be illegal if the predicate is satisfied and indicates that widening the scalars in one of the types to a specific type would make it more legal. This action supports both scalars and vectors.
  • fewerElementsIf(), fewerElementsFor(), etc. declare an instruction to be illegal if the predicate is satisfied and indicates reducing the number of vector elements in one of the types to a specific type would make it more legal. This action supports vectors.
  • moreElementsIf(), moreElementsFor(), etc. declare an instruction to be illegal if the predicate is satisfied and indicates increasing the number of vector elements in one of the types to a specific type would make it more legal. This action supports vectors.
  • lowerIf(), lowerFor(), etc. declare an instruction to be illegal if the predicate is satisfied and indicates that replacing it with equivalent instruction(s) would make it more legal. Support for this action differs for each opcode.
  • libcallIf(), libcallFor(), etc. declare an instruction to be illegal if the predicate is satisfied and indicates that replacing it with a libcall would make it more legal. Support for this action differs for each opcode.
  • customIf(), customFor(), etc. declare an instruction to be illegal if the predicate is satisfied and indicates that the backend developer will supply a means of making it more legal.
  • unsupportedIf(), unsupportedFor(), etc. declare an instruction to be illegal if the predicate is satisfied and indicates that there is no way to make it legal and the compiler should fail.
  • fallback() falls back on an older API and should only be used while porting existing code from that API.

Rule Predicates

The rule factories also have predicates in common:

  • legal(), lower(), etc. are always satisfied.
  • legalIf(), narrowScalarIf(), etc. are satisfied if the user-supplied LegalityPredicate function returns true. This predicate has access to the information in the LegalityQuery to make its decision. User-supplied predicates can also be combined using all(P0, P1, ...).
  • legalFor(), narrowScalarFor(), etc. are satisfied if the type matches one in a given set of types. For example .legalFor({s16, s32}) declares the instruction legal if type 0 is either s16 or s32. Additional versions for two and three type indices are generally available. For these, all the type indices considered together must match all the types in one of the tuples. So .legalFor({{s16, s32}, {s32, s64}}) will only accept {s16, s32}, or {s32, s64} but will not accept {s16, s64}.
  • legalForTypesWithMemSize(), narrowScalarForTypesWithMemSize(), etc. are similar to legalFor(), narrowScalarFor(), etc. but additionally require a MachineMemOperand to have a given size in each tuple.
  • legalForCartesianProduct(), narrowScalarForCartesianProduct(), etc. are satisfied if each type index matches one element in each of the independent sets. So .legalForCartesianProduct({s16, s32}, {s32, s64}) will accept {s16, s32}, {s16, s64}, {s32, s32}, and {s32, s64}.

Composite Rules

There are some composite rules for common situations built out of the above facilities:

  • widenScalarToNextPow2() is like widenScalarIf() but is satisfied iff the type size in bits is not a power of 2 and selects a target type that is the next largest power of 2.
  • minScalar() is like widenScalarIf() but is satisfied iff the type size in bits is smaller than the given minimum and selects the minimum as the target type. Similarly, there is also a maxScalar() for the maximum and a clampScalar() to do both at once.
  • minScalarSameAs() is like minScalar() but the minimum is taken from another type index.
  • moreElementsToNextMultiple() is like moreElementsToNextPow2() but is based on multiples of X rather than powers of 2.

Minimum Rule Set

GlobalISel’s legalizer has a great deal of flexibility in how a given target shapes the GMIR that the rest of the backend must handle. However, there are a small number of requirements that all targets must meet.

Before discussing the minimum requirements, we’ll need some terminology:

Producer Type Set
The set of types which is the union of all possible types produced by at least one legal instruction.
Consumer Type Set
The set of types which is the union of all possible types consumed by at least one legal instruction.

Both sets are often identical but there’s no guarantee of that. For example, it’s not uncommon to be unable to consume s64 but still be able to produce it for a few specific instructions.

Minimum Rules For Scalars

  • G_ANYEXT must be legal for all inputs from the producer type set and all larger outputs from the consumer type set.
  • G_TRUNC must be legal for all inputs from the producer type set and all smaller outputs from the consumer type set.

G_ANYEXT, and G_TRUNC have mandatory legality since the GMIR requires a means to connect operations with different type sizes. They are usually trivial to support since G_ANYEXT doesn’t define the value of the additional bits and G_TRUNC is discarding bits. The other conversions can be lowered into G_ANYEXT/G_TRUNC with some additional operations that are subject to further legalization. For example, G_SEXT can lower to:

%1 = G_ANYEXT %0
%2 = G_CONSTANT ...
%3 = G_SHL %1, %2
%4 = G_ASHR %3, %2

and the G_CONSTANT/G_SHL/G_ASHR can further lower to other operations or target instructions. Similarly, G_FPEXT has no legality requirement since it can lower to a G_ANYEXT followed by a target instruction.

G_MERGE_VALUES and G_UNMERGE_VALUES do not have legality requirements since the former can lower to G_ANYEXT and some other legalizable instructions, while the latter can lower to some legalizable instructions followed by G_TRUNC.

Minimum Legality For Vectors

Within the vector types, there aren’t any defined conversions in LLVM IR as vectors are often converted by reinterpreting the bits or by decomposing the vector and reconstituting it as a different type. As such, G_BITCAST is the only operation to account for. We generally don’t require that it’s legal because it can usually be lowered to COPY (or to nothing using replaceAllUses()). However, there are situations where G_BITCAST is non-trivial (e.g. little-endian vectors of big-endian data such as on big-endian MIPS MSA and big-endian ARM NEON, see _i_bitcast). To account for this G_BITCAST must be legal for all type combinations that change the bit pattern in the value.

There are no legality requirements for G_BUILD_VECTOR, or G_BUILD_VECTOR_TRUNC since these can be handled by: * Declaring them legal. * Scalarizing them. * Lowering them to G_TRUNC+G_ANYEXT and some legalizable instructions. * Lowering them to target instructions which are legal by definition.

The same reasoning also allows G_UNMERGE_VALUES to lack legality requirements for vector inputs.

Minimum Legality for Pointers

There are no minimum rules for pointers since G_INTTOPTR and G_PTRTOINT can be selected to a COPY from register class to another by the legalizer.

Minimum Legality For Operations

The rules for G_ANYEXT, G_MERGE_VALUES, G_BITCAST, G_BUILD_VECTOR, G_BUILD_VECTOR_TRUNC, G_CONCAT_VECTORS, G_UNMERGE_VALUES, G_PTRTOINT, and G_INTTOPTR have already been noted above. In addition to those, the following operations have requirements:

  • At least one G_IMPLICIT_DEF must be legal. This is usually trivial as it requires no code to be selected.
  • G_PHI must be legal for all types in the producer and consumer typesets. This is usually trivial as it requires no code to be selected.
  • At least one G_FRAME_INDEX must be legal
  • At least one G_BLOCK_ADDR must be legal

There are many other operations you’d expect to have legality requirements but they can be lowered to target instructions which are legal by definition.