HW Arith Dialect Rationale

This document describes the various design points of the HWArith dialect, a dialect that is used to represent bitwidth extending arithmetic. This follows in the spirit of other MLIR Rationale docs.

• HW Arith Dialect Rationale
  • Introduction
    • Q&A
  • Bit Width Rules
  • Lowering

Introduction ¶

The hwarith dialect provides a collection of operations that define typical integer arithmetic operations which are bitwidth extending. These semantics are expressed through return type inference rules that, based on the types of an operation and its input operands, infers the type of the result operand. Importantly, for the initial implementation of this dialect, we do not allow types with uninferred width in the IR. A user is expected to iteratively build operations, expect operation return types, and use this to further construct IR. This dialect is not intended to be a “core” RTL dialect which takes part in RTL codegen, and instead mainly serves as a frontend target. The lifetime of the operations should be considered short, and lowered to their comb counterparts early in the compilation pipeline.

In general, the bit width inference rules are designed to model SystemVerilog semantics. However, unlike SystemVerilog, the aim of this dialect is to provide a strongly-typed hardware arithmetic library with the width inference rules for every operator explained in a clear and understandable way to avoid ambiguity.

To serve this goal of being strongly typed, the operations of the dialect explicitly do not accept signless types. As such, only ui#, si# types are allowed. A casting operation is provided to serve signedness casting as well as truncation and padding.

Below we try to capture some common questions on the capabilities of such a dialect, and how we’ve addressed/think about the issue.

Q&A ¶

• Q: Does this support n-ary operations?
  • n-ary operations in this dialect would have to solve the issue of how width rules in expressions such as the following are handled: %0 = hwarith.add %0, %1, %2 : si6, si5, si4
    In the short term, we envision this dialect to be considering strictly binary operations. Defining width inference rules for these is simpler and fits the immediate usecase. If, in the future, a need for n-ary operations comes up and is motivated clearly (e.g. for optimization purposes), the dialect should be adapted to support it. In this case, we expect n-ary operation rules to be a superset of those defined for binary operations.
• Q: Does this support 0-width operands?
  • 0-width values might arise from arithmetic rules which reduce the bit width of an expression wrt. the operands to that expression. One case where such rules may apply is in the implementation of modulo operations.
    We refrain from adding support at this point in time, since support for 0-width values in the remainder of the RTL dialects is, at time of writing, lacking/undefined. Once support is added in these downstream dialects, 0-width support in hwarith should be reconsidered.
• Q: Does this support width inferred types?
  • Relying on width inference is relevant when referencing results of other width-inferred value. Without this, a user/frontend must itself know and apply width inference rules while generating the IR (either explicitly, or implicitly, through op builders). Having width-inferred types will be convenient not only for generating IR, but also to leave room for width inference rules and optimizations to apply recursively. As an example:

  %1 = hwarith.add %a, %b : (ui4, ui4) -> (un) // %1 is an unsigned integer of inferred width
  %2 = hwarith.add %c, %1 : (ui3, ui) -> (un)
  

  We see merit in having inferred width types, but recognize that the work required to get this right is non-trivial and will require significant effort. Once need arises, and resources are available, we find width inferred types to be a valuable contribution to the dialect, but it will not be part of the initial implementation.
• Q: Does this support user-provided/alternative rules?
  • Initially, we want to settle on a set of common-case rules which describe the semantics required by the immediate users of the dialect, so this will not be an immediate goal of the dialect.
• Q: What about fixed point operations?
  • Fixed point operations and values may eventually be relevant in an hwarith context. However, the immediate needs of this dialect is to provide bitwidth-extending arithmetic for integer values.
• Q: What about floating point operations?
  • As above, with the additional consideration that handling floating point operations in hardware usually involves inferring external IP. Considering how float operations are added to hwarith should only be considered after the problem of operator libraries have been solved for CIRCT.

Bit Width Rules ¶

The core capability of the dialect is the bit width inference rules implemented for each operation. These define the semantics for how arithmetic overflow is to be handled both with respect to the width and signedness of operation results, as well as how this is communicated to a comb lowering.

Preliminaries ¶

The MLIR ui<w> types represents values in the interval [0, 2^w - 1]. Its signed counterpart, si<w>, represents values in the interval [-2^(w-1), 2^(w-1) - 1]. For example, ui4 covers values between 0 and 15, whereas si4 ranges from -8 to 7. For a given bit-width, the unsigned and the signed type contain the same number of values, but the represented interval is shifted by half the interval’s size.

         │                  
         ├──────────────┐   
         │     ui4      │   
 ┌───────┼──────┬───────┘   
 │     si4      │           
 └───────┼──────┘           
─────────┴─────────────────▶
-8       0      7      15   

We use the notation UI_MIN<w>, UI_MAX<w>, SI_MIN<w> and SI_MAX<w> to denote the smallest and largest values representable by an unsigned/signed type of the given bit-width. For example, UI_MAX<4> = 15, and SI_MIN<4> = -8.

The following relations for all w > 0 are handy:

UI_MIN<w>     = 0
2 * UI_MAX<w> = 2^(w+1) - 2
UI_MAX<w+1>   = 2 * UI_MAX<w> + 1 

SI_MIN<w>     < 0
2 * SI_MIN<w> = -2^w
SI_MIN<w+1>   = 2 * SI_MIN<w>
2 * SI_MAX<w> = 2^w - 2
SI_MAX<w+1>   = 2 * SI_MAX<w> + 1

The following result type inference rules are chosen to prevent a loss of precision and sign information associated with over- and underflows. The reasoning usually takes the smallest and largest possible result values into account. For example, if the result can be negative, then the result type must be signed.

Addition ¶

Overview ¶

Unsigned addition (U) ¶

Result value range: [0 + 0, UI_MAX<a> + UI_MAX<b>]

W.l.o.g., let ui<b> be the wider type. Then the following inequalities hold:

UI_MAX<b> < UI_MAX<a> + UI_MAX<b> ≤ 2 * UI_MAX<b> ≤ UI_MAX<b+1>

Example:

%0 = hwarith.add %10, %11 : (ui3, ui4) -> ui5

Signed addition (S) ¶

Result value range: [SI_MIN<a> + SI_MIN<b>, SI_MAX<a> + SI_MAX<b>]

We apply the same reasoning as in (U). W.l.o.g., let si<b> be the wider type. Then the following inequalities hold:

SI_MIN<b> > SI_MIN<a> + SI_MIN<b> ≥ 2 * SI_MIN<b> ≥ SI_MIN<b+1>
SI_MAX<b> < SI_MAX<a> + SI_MAX<b> ≤ 2 * SI_MAX<b> ≤ SI_MAX<b+1>

Example:

%1 = hwarith.add %12, %13 : (si3, si3) -> si4

Mixed addition (M) ¶

The result type must be signed, as the addition of a positive and a negative value may yield a negative result. Due to commutativity, we only consider ui<a> + si<b>. We distinguish two situations:

• a ≥ b, i.e. the unsigned operand’s width is greater or equal the signed operand’s width.

  The smallest signed type to represent all values of ui<a> is si<a+1>. We know that a + 1 > b, and derive from (S) that the result type must be si<(a+1) + 1> = si<a+2>.

• a < b, i.e. the unsigned operand’s width is strictly less than the signed operand’s width.

  As a + 1 ≤ b, the result via (S) is si<b+1>.

Examples:

%2 = hwarith.add %14, %15 : (ui3, si4) -> si5
%3 = hwarith.add %16, %17 : (si4, ui6) -> si8

Subtraction ¶

Overview ¶

Unsigned subtraction (U) ¶

Result value range: [0 - UI_MAX<b>, UI_MAX<a> - 0]

As the result can be negative, the result type must be signed. -UI_MAX<b> is covered by si<b+1>, and UI_MAX<a> is covered by si<a+1>, so the narrowest type capable of representing both extremal values is si<r> with r = max(a, b) + 1.

Example:

%0 = hwarith.sub %10, %11 : (ui3, ui4) -> si5

Signed subtraction (S) ¶

Result value range: [SI_MIN<a> - SI_MAX<b>, SI_MAX<a> - SI_MIN<b>]

We rewrite the value range and then apply the same reasoning as for the signed addition:

  [SI_MIN<a> - SI_MAX<b>, SI_MAX<a> - SI_MIN<b>]
= [SI_MIN<a> + -2^(b-1) + 1, SI_MAX<a> + 2^(b-1)]
= [SI_MIN<a> + SI_MIN<b> + 1, SI_MAX<a> + SI_MAX<b> + 1]

W.l.o.g., let si<b> be the wider type. Then the following inequalities hold:

SI_MIN<b> > SI_MIN<a> + SI_MIN<b> + 1 ≥ 2 * SI_MIN<b> + 1 ≥ SI_MIN<b+1>
SI_MAX<b> < SI_MAX<a> + SI_MAX<b> + 1 ≤ 2 * SI_MAX<b> + 1 ≤ SI_MAX<b+1>

Example:

%1 = hwarith.sub %12, %13 : (si3, si3) -> si4

Mixed subtraction (M) ¶

See mixed addition.

Examples:

%2 = hwarith.sub %14, %15 : (ui3, si4) -> si5
%3 = hwarith.sub %16, %17 : (si4, ui6) -> si8

Multiplication ¶

Overview ¶

Unsigned multiplication (U) ¶

Result value range: [0 * 0, UI_MAX<a> * UI_MAX<b>]

The result type must be at least a + *b bits wide:

UI_MAX<a> * UI_MAX<b> = (2^a - 1) * (2^b - 1)
                      = 2^(a + b) - 2^a - 2^b + 1 
                      ≤ 2^(a + b) - 1 = UI_MAX<a+b>

The result type cannot be smaller than ui<a+b>, as the following counter-example shows:

UI_MAX<3> * UI_MAX<4> = 7 * 15 = 105 > UI_MAX<3+4-1>

Example:

%0 = hwarith.mul %10, %11 : (ui3, ui4) -> ui7

Signed multiplication (S) ¶

Result value range: [min(SI_MIN<a> * SI_MAX<b>, SI_MAX<a> * SI_MIN<b>), SI_MAX<a> * SI_MAX<b>]

With the same reasoning as in (U), we know that si<a+b> is the narrowest type to accommodate SI_MAX<a> * SI_MAX<b>. This type also covers the negative extremal value (w.l.o.g. let si<b> be the wider type here):

SI_MIN<a> * SI_MAX<b> = -2^(a-1) * (2^(b-1) - 1)
                      = -2^(a + b - 2) + 2^(a-1)
                      ≥ -2^(a + b - 1) = SI_MIN<a+b>

Example:

%1 = hwarith.mul %12, %13 : (si3, si3) -> si6

Mixed multiplication (M) ¶

The result type must be signed. Let us consider ui<a> * si<b>; the flipped case si<a> * ui<b> follows analogously.

Result value range: [UI_MAX<a> * SI_MIN<b>, UI_MAX<a> * SI_MAX<b>]

Analogously to the previous cases, the result type must be at least a + b bits wide:

UI_MAX<a> * SI_MIN<b> = (2^a - 1) * -2^(b-1)
                      = -2^(a + b - 1) + 2^(b-1)
                      ≥ 2^(a + b - 1) = SI_MIN<a+b>

UI_MAX<a> * SI_MAX<b> = (2^a - 1) * (2^(b-1) - 1)
                      = 2^(a + b - 1) - 2^a - 2^(b-1) + 1
                      ≤ 2^(a + b - 1) - 1 = SI_MAX<a+b>

Again, we cannot choose a narrower type according to the following counter-example:

UI_MAX<3> * SI_MAX<4> = 7 * 7 = 49 ≥ SI_MAX<3+4-1>

Example:

%2 = hwarith.mul %14, %15 : (si3, ui5) -> si8

Division ¶

Overview ¶

Unsigned division (U) ¶

Result value range: [0 / 1, UI_MAX<a> / 1]

The result is also less or equal the dividend, so the result type is ui<a>.

Example:

%0 = hwarith.div %10, %11 : (ui3, ui4) -> ui3

Signed division (S) ¶

Result value range: [SI_MIN<a> / 1, SI_MIN<a> / -1]

- SI_MIN<a> is not covered by si<a>, so we have to widen the result type to si<a+1>.

Example:

%1 = hwarith.div %12, %13 : (si3, si3) -> si4

Division with signed divisor (M1) ¶

If at least one operand is signed, then the result type must be signed as well.

Result value range: [UI_MAX<a> / -1, UI_MAX<a> / 1]

The result type must be si<a+1> in order to accommodate UI_MAX<a>.

Example:

%2 = hwarith.div %14, %15 : (ui3, si4) -> si4

Division with signed dividend (M2) ¶

If at least one operand is signed, then the result type must be signed as well.

Result value range: [SI_MIN<a> / 1, SI_MAX<a> / 1]]

The type of the dividend, si<a>, already covers the entire result value range.

Example:

%3 = hwarith.div %16, %17 : (si4, ui6) -> si4

Casting ¶

Casting between signless and sign-aware types is essential for HWArith to coexist and interact with core dialects such as HW and Comb. Defining casting rules can be split into two sub-problems: bit-extension and truncation. Truncation to a type with identical or smaller bitwidth is simply implemented by extracting the least significant bits of the operand. Hence, it is identical for sign-aware and signless types. Bit-extension on the other hand needs to consider the signedness of the operand to preserve the operand’s value. As signless types do not express such information by design, no bit-extension can be performed. Values of type si<w> or ui<w> are sign-extended or zero-extended, respectively, regardless of the target’s signedness or lack thereof. Thus, when casting in HWArith, first the bitwidth is adjusted to the target size, then in a second step the signedness is changed in a bitcast manner.

To visualize this behavior study the following example:

%1 = hwarith.cast %10 : (ui3) -> ui5
%2 = hwarith.cast %1 : (ui5) -> si5
// ^-- both casts can be combined into one equivalent cast:
%0 = hwarith.cast %10 : (ui3) -> si5
// ^-- this would be lowered to something like this:
%zero_padding = hw.constant 0 : i2
%0_lowered = comb.concat %zero_padding, %10 : i2, i3

As an alternative to the proposed all-in-one cast op, we considered implementing a set of ops with distinct concerns, for example, an op for bitcasting between types of the same bitwidth, an op to implementing the truncation and two ops for the different bit extension types. Yet, considering that HWArith is currently intended as a short-lived dialect, being immediately lowered to comb, no obvious benefits emerge. This could also significantly increase the code redundancy during IR generation as well as when lowering to comb.

Overview ¶

†) prohibited because of the ambiguity whether a sign or a zero extension is required.

Examples ¶

%0 = hwarith.cast %10 : (ui3) -> si5 // (U)
%1 = hwarith.cast %11 : (si3) -> si4 // (S)
%2 = hwarith.cast %12 : (si7) -> ui4 // (S)
%3 = hwarith.cast %13 : (i7) -> si5  // (I)
%3 = hwarith.cast %13 : (si14) -> i4 // (S)

Comparison ¶

In order to compare two sign-aware operands, a common type must first be found that is able to preserve the values of the two operands. Once determined, the operands are casted to this comparison type as specified in the casting section. To simplify the use as well as the IR generation of the dialect, the necessary casting steps are hidden from the user and applied during the lowering. Thus, the following code:

%2 = hwarith.cast %0 : (si3) -> si6
%3 = hwarith.cast %1 : (ui5) -> si6
%4 = hwarith.icmp lt %2, %3 : si6, si6

can be simplified to:

%4 = hwarith.icmp lt %0, %1 : si3, ui5

Note that the result of the comparison is always of type i1 since the logical result is a boolean, which doesn’t have signedness semantics.

Overview ¶

Examples ¶

%0 = hwarith.icmp lt %10, %11 : ui5, ui6 // (U)
%1 = hwarith.icmp lt %12, %13 : si3, si4 // (S)
%2 = hwarith.icmp lt %12, %11 : si3, ui6 // (M)

Additional operators ¶

TODO: elaborate once operators are provided.

• hwarith.mod %0, %1 : (#l0, #l1) -> (#l2):

Lowering ¶

The general lowering style of the dialect operations consists of padding the operands of an operation based on an operation rule, and subsequently feeding these operands to the corresponding comb operator.

For instance, given %res = hwarith.add %lhs, %rhs and a rule stating w(res) = max(w(lhs), w(rhs)) + 1 the lowering would be:

%res = hwarith.add %lhs, %rhs : (ui3, ui4) -> (ui5)

// lowers to
%z2 = hw.constant 0 : i2
%z1 = hw.constant 0 : i1
%lhs_padded = comb.concat %z2, %lhs : i2, i3
%rhs_padded = comb.concat %z1, %rhs : i1, i4
%res = comb.add %lhs_padded, %rhs_padded : i5