diff --git a/doc/src/devdocs/ssair.md b/doc/src/devdocs/ssair.md index ac5e806ecf524..19e1febd53dd6 100644 --- a/doc/src/devdocs/ssair.md +++ b/doc/src/devdocs/ssair.md @@ -6,7 +6,7 @@ Beginning in Julia 0.7, parts of the compiler use a new [SSA-form](https://en.wi intermediate representation. Historically, the compiler used to directly generate LLVM IR, from a lowered form of the Julia AST. This form had most syntactic abstractions removed, but still looked a lot like an abstract syntax tree. Over time, in order to facilitate optimizations, SSA values were introduced to this IR and the IR was -linearized (i.e. a form where function arguments may only be SSA values or constants). However, non-ssa values +linearized (i.e. a form where function arguments may only be SSA values or constants). However, non-SSA values (slots) remained in the IR due to the lack of Phi nodes in the IR (necessary for back-edges and re-merging of conditional control flow), negating much of the usefulfulness of the SSA form representation to perform middle end optimizations. Some heroic effort was put into making these optimizations work without a complete SSA @@ -33,13 +33,13 @@ if edge has an entry of `15`, there must be a `goto`, `gotoifnot` or implicit fa statement `15` that targets this phi node). Values are either SSA values or constants. It is also possible for a value to be unassigned if the variable was not defined on this path. However, undefinedness checks get explicitly inserted and represented as booleans after middle end optimizations, so code generators -may assume that any use of a phi node will have an assigned value in the corresponding slot. It is also legal -for the mapping to be incomplete, i.e. for a phi node to have missing incoming edges. In that case, it must +may assume that any use of a Phi node will have an assigned value in the corresponding slot. It is also legal +for the mapping to be incomplete, i.e. for a Phi node to have missing incoming edges. In that case, it must be dynamically guaranteed that the corresponding value will not be used. PiNodes encode statically proven information that may be implicitly assumed in basic blocks dominated by a given pi node. They are conceptually equivalent to the technique introduced in the paper -"ABCD: Eliminating Array Bounds Checks on Demand" or the predicate info nodes in LLVM. To see how they work, consider, +[ABCD: Eliminating Array Bounds Checks on Demand](https://dl.acm.org/citation.cfm?id=358438.349342) or the predicate info nodes in LLVM. To see how they work, consider, e.g. ```julia @@ -51,7 +51,7 @@ else end ``` -we can perform predicate insertion and turn this into: +We can perform predicate insertion and turn this into: ```julia %x::Union{Int, Float64} # %x is some Union{Int, Float64} typed ssa value @@ -96,25 +96,29 @@ hand, every catch basic block would have `n*m` phi node arguments (`n`, the numb in the critical region, `m` the number of live values through the catch block). To work around this, we use a combination of `Upsilon` and `PhiC` (the C standing for `catch`, written `φᶜ` in the IR pretty printer, because -unicode subscript c is not available) nodes. There is several ways to think of these nodes, but +unicode subscript c is not available) nodes. There are several ways to think of these nodes, but perhaps the easiest is to think of each `PhiC` as a load from a unique store-many, read-once slot, with `Upsilon` being the corresponding store operation. The `PhiC` has an operand list of all the upsilon nodes that store to its implicit slot. The `Upsilon` nodes however, do not record which `PhiC` node they store to. This is done for more natural integration with the rest of the SSA IR. E.g. -if there are no more uses of a `PhiC` node, it is safe to delete is and the same is true of an -`Upsilon` node. In most IR passes, `PhiC` nodes can be treated similar to `Phi` nodes. One can follow -use-def chains through them, and they can be lifted to new `PhiC` nodes and new Upsilon nodes (in the +if there are no more uses of a `PhiC` node, it is safe to delete it, and the same is true of an +`Upsilon` node. In most IR passes, `PhiC` nodes can be treated like `Phi` nodes. One can follow +use-def chains through them, and they can be lifted to new `PhiC` nodes and new `Upsilon` nodes (in the same places as the original `Upsilon` nodes). The result of this scheme is that the number of -Upsilon nodes (and `PhiC` arguments) is proportional to the number of assigned values to a particular +`Upsilon` nodes (and `PhiC` arguments) is proportional to the number of assigned values to a particular variable (before SSA conversion), rather than the number of statements in the critical region. To see this scheme in action, consider the function ```julia +@noinline opaque() = invokelatest(identity, nothing) # Something opaque function foo() + local y x = 1 try y = 2 + opaque() + y = 3 error() catch end @@ -125,24 +129,26 @@ end The corresponding IR (with irrelevant types stripped) is: ``` -ir = Code -1 ─ nothing -2 ─ $(Expr(:enter, 5)) -3 ─ %3 = ϒ (#undef) -│ %4 = ϒ (1) -│ %5 = ϒ (2) -│ Main.bar() -│ %7 = ϒ (3) +1 ─ nothing::Nothing +2 ─ %2 = $(Expr(:enter, #4)) +3 ─ %3 = ϒ (false) +│ %4 = ϒ (#undef) +│ %5 = ϒ (1) +│ %6 = ϒ (true) +│ %7 = ϒ (2) +│ invoke Main.opaque()::Any +│ %9 = ϒ (true) +│ %10 = ϒ (3) +│ invoke Main.error()::Union{} +└── $(Expr(:unreachable))::Union{} +4 ┄ %13 = φᶜ (%3, %6, %9)::Bool +│ %14 = φᶜ (%4, %7, %10)::Core.Compiler.MaybeUndef(Int64) +│ %15 = φᶜ (%5)::Core.Compiler.Const(1, false) └── $(Expr(:leave, 1)) -4 ─ goto 6 -5 ─ %10 = φᶜ (%3, %5) -│ %11 = φᶜ (%4, %7) -└── $(Expr(:leave, 1)) -6 ┄ %13 = φ (4 => 2, 5 => %10)::NotInferenceDontLookHere.MaybeUndef(NotInferenceDontLookHere.Const(2, false)) -│ %14 = φ (4 => 3, 5 => %11)::Int64 -│ $(Expr(:undefcheck, :y, Core.SSAValue(13))) -│ %16 = Core.tuple(%14, %13) -└── return %17 +5 ─ $(Expr(:pop_exception, :(%2)))::Any +│ $(Expr(:throw_undef_if_not, :y, :(%13)))::Any +│ %19 = Core.tuple(%15, %14) +└── return %19 ``` Note in particular that every value live into the critical region gets @@ -155,34 +161,34 @@ catch blocks, and all incoming values have to come through a `φᶜ` node. The main `SSAIR` data structure is worthy of discussion. It draws inspiration from LLVM and Webkit's B3 IR. The core of the data structure is a flat vector of statements. Each statement is implicitly assigned -an SSA values based on its position in the vector (i.e. the result of the statement at idx 1 can be +an SSA value based on its position in the vector (i.e. the result of the statement at idx 1 can be accessed using `SSAValue(1)` etc). For each SSA value, we additionally maintain its type. Since, SSA values are definitionally assigned only once, this type is also the result type of the expression at the corresponding -index. However, while this representation is rather efficient (since the assignments don't need to be explicitly) -encoded, if of course carries the drawback that order is semantically significant, so reorderings and insertions +index. However, while this representation is rather efficient (since the assignments don't need to be explicitly +encoded), it of course carries the drawback that order is semantically significant, so reorderings and insertions change statement numbers. Additionally, we do not keep use lists (i.e. it is impossible to walk from a def to -all its uses without explicitly computing this map - def lists however are trivial since you can lookup the +all its uses without explicitly computing this map--def lists however are trivial since you can look up the corresponding statement from the index), so the LLVM-style RAUW (replace-all-uses-with) operation is unavailable. Instead, we do the following: - We keep a separate buffer of nodes to insert (including the position to insert them at, the type of the corresponding value and the node itself). These nodes are numbered by their occurrence in the insertion - buffer, allowing their values to be immediately used elesewhere in the IR (i.e. if there is 12 statements in - the original statement list, the first new statement will be accessible as `SSAValue(13)`) + buffer, allowing their values to be immediately used elesewhere in the IR (i.e. if there are 12 statements in + the original statement list, the first new statement will be accessible as `SSAValue(13)`). - RAUW style operations are performed by setting the corresponding statement index to the replacement value. - Statements are erased by setting the corresponding statement to `nothing` (this is essentially just a special-case - convention of the above -- if there are any uses of the statement being erased they will be set to `nothing`) + convention of the above. +- If there are any uses of the statement being erased, they will be set to `nothing`. -There is a `compact!` function that compacts the above data structure by performing the insertion of nodes in the appropriate place, trivial copy propagation and renaming of uses to any changed SSA values. However, the clever part +There is a `compact!` function that compacts the above data structure by performing the insertion of nodes in the appropriate place, trivial copy propagation, and renaming of uses to any changed SSA values. However, the clever part of this scheme is that this compaction can be done lazily as part of the subsequent pass. Most optimization passes need to walk over the entire list of statements, performing analysis or modifications along the way. We provide an -`IncrementalCompact` iterator that can be used to iterate over the statement list. It will perform any necessary compaction, +`IncrementalCompact` iterator that can be used to iterate over the statement list. It will perform any necessary compaction and return the new index of the node, as well as the node itself. It is legal at this point to walk def-use chains, as well as make any modifications or deletions to the IR (insertions are disallowed however). -The idea behind this arrangement is that, since the optimization passes need to touch the corresponding memory anyway, +The idea behind this arrangement is that, since the optimization passes need to touch the corresponding memory anyway and incur the corresponding memory access penalty, performing the extra housekeeping should have comparatively little overhead (and save the overhead of maintaining these data structures during IR modification).