inference: Model type propagation through exceptions

Currently the type of a caught exception is always modeled as `Any`.
This isn't a huge problem, because control flow in Julia is generally
assumed to be somewhat slow, so the extra type imprecision of not
knowing the return type does not matter all that much. However,
there are a few situations where it matters. For example:

```
maybe_getindex(A, i) =
    try; A[i]; catch e; isa(e, BoundsError) && return nothing; rethrow(); end
```

At present, we cannot infer :nothrow for this method, even if that
is the only error type that `A[i]` can throw. This is particularly
noticable, since we can now optimize away `:nothrow` exception frames
entirely (#51674). Note that this PR still does not make the above
example particularly efficient (at least interprocedurally), though
specialized codegen could be added on top of this to make that happen.
It does however improve the inference result.

A second major motivation of this change is that reasoning about
exception types is likely to be a major aspect of any future work
on interface checking (since interfaces imply the absence of
MethodErrors), so this PR lays the groundwork for appropriate modeling
of these error paths.

Note that this PR adds all the required plumbing, but does not yet have
a particularly precise model of error types for our builtins, bailing
to `Any` for any builtin not known to be `:nothrow`. This can be improved
in follow up PRs as required.

base/boot.jl

@@ -471,13 +471,13 @@ eval(Core, quote
 end)
 
 function CodeInstance(
-    mi::MethodInstance, @nospecialize(rettype), @nospecialize(inferred_const),
+    mi::MethodInstance, @nospecialize(rettype), @nospecialize(exctype), @nospecialize(inferred_const),
     @nospecialize(inferred), const_flags::Int32, min_world::UInt, max_world::UInt,
     ipo_effects::UInt32, effects::UInt32, @nospecialize(argescapes#=::Union{Nothing,Vector{ArgEscapeInfo}}=#),
     relocatability::UInt8)
     return ccall(:jl_new_codeinst, Ref{CodeInstance},
-        (Any, Any, Any, Any, Int32, UInt, UInt, UInt32, UInt32, Any, UInt8),
-        mi, rettype, inferred_const, inferred, const_flags, min_world, max_world,
+        (Any, Any, Any, Any, Any, Int32, UInt, UInt, UInt32, UInt32, Any, UInt8),
+        mi, rettype, exctype, inferred_const, inferred, const_flags, min_world, max_world,
         ipo_effects, effects, argescapes,
         relocatability)
 end

base/compiler/effects.jl

@@ -171,6 +171,60 @@ function Effects(effects::Effects = _EFFECTS_UNKNOWN;
         nonoverlayed)
 end
 
+function better_effects(new::Effects, old::Effects)
+    any_improved = false
+    if new.consistent == ALWAYS_TRUE
+        any_improved |= old.consistent != ALWAYS_TRUE
+    elseif new.consistent != old.consistent
+        return false
+    end
+    if new.effect_free == ALWAYS_TRUE
+        any_improved |= old.consistent != ALWAYS_TRUE
+    elseif new.effect_free == EFFECT_FREE_IF_INACCESSIBLEMEMONLY
+        old.effect_free == ALWAYS_TRUE && return false
+        any_improved |= old.effect_free != EFFECT_FREE_IF_INACCESSIBLEMEMONLY
+    elseif new.effect_free != old.effect_free
+        return false
+    end
+    if new.nothrow
+        any_improved |= !old.nothrow
+    elseif new.nothrow != old.nothrow
+        return false
+    end
+    if new.terminates
+        any_improved |= !old.terminates
+    elseif new.terminates != old.terminates
+        return false
+    end
+    if new.notaskstate
+        any_improved |= !old.notaskstate
+    elseif new.notaskstate != old.notaskstate
+        return false
+    end
+    if new.inaccessiblememonly == ALWAYS_TRUE
+        any_improved |= old.inaccessiblememonly != ALWAYS_TRUE
+    elseif new.inaccessiblememonly == INACCESSIBLEMEM_OR_ARGMEMONLY
+        old.inaccessiblememonly == ALWAYS_TRUE && return false
+        any_improved |= old.inaccessiblememonly != INACCESSIBLEMEM_OR_ARGMEMONLY
+    elseif new.inaccessiblememonly != old.inaccessiblememonly
+        return false
+    end
+    if new.noub == ALWAYS_TRUE
+        any_improved |= old.noub != ALWAYS_TRUE
+    elseif new.noub == NOUB_IF_NOINBOUNDS
+        old.noub == ALWAYS_TRUE && return false
+        any_improved |= old.noub != NOUB_IF_NOINBOUNDS
+    elseif new.noub != old.noub
+        return false
+    end
+    if new.nonoverlayed
+        any_improved |= !old.nonoverlayed
+    elseif new.nonoverlayed != old.nonoverlayed
+        return false
+    end
+    return any_improved
+end
+
 function merge_effects(old::Effects, new::Effects)
     return Effects(
         merge_effectbits(old.consistent, new.consistent),

base/compiler/inferencestate.jl

@@ -198,6 +198,11 @@ to enable flow-sensitive analysis.
 """
 const VarTable = Vector{VarState}
 
+mutable struct TryCatchFrame
+    exct
+    const enter_idx
+end
+
 mutable struct InferenceState
     #= information about this method instance =#
     linfo::MethodInstance
@@ -213,7 +218,8 @@ mutable struct InferenceState
     currbb::Int
     currpc::Int
     ip::BitSet#=TODO BoundedMinPrioritySet=# # current active instruction pointers
-    handler_at::Vector{Int} # current exception handler info
+    handlers::Vector{TryCatchFrame}
+    handler_at::Vector{Tuple{Int, Int}} # tuple of current (handler, excecption stack) value at the pc
     ssavalue_uses::Vector{BitSet} # ssavalue sparsity and restart info
     # TODO: Could keep this sparsely by doing structural liveness analysis ahead of time.
     bb_vartables::Vector{Union{Nothing,VarTable}} # nothing if not analyzed yet
@@ -234,6 +240,7 @@ mutable struct InferenceState
     unreachable::BitSet # statements that were found to be statically unreachable
     valid_worlds::WorldRange
     bestguess #::Type
+    exc_bestguess
     ipo_effects::Effects
 
     #= flags =#
@@ -261,7 +268,7 @@ mutable struct InferenceState
 
         currbb = currpc = 1
         ip = BitSet(1) # TODO BitSetBoundedMinPrioritySet(1)
-        handler_at = compute_trycatch(code, BitSet())
+        handler_at, handlers = compute_trycatch(code, BitSet())
         nssavalues = src.ssavaluetypes::Int
         ssavalue_uses = find_ssavalue_uses(code, nssavalues)
         nstmts = length(code)
@@ -291,6 +298,7 @@ mutable struct InferenceState
 
         valid_worlds = WorldRange(src.min_world, src.max_world == typemax(UInt) ? get_world_counter() : src.max_world)
         bestguess = Bottom
+        exc_bestguess = Bottom
         ipo_effects = EFFECTS_TOTAL
 
         insert_coverage = should_insert_coverage(mod, src)
@@ -311,9 +319,9 @@ mutable struct InferenceState
 
         return new(
             linfo, world, mod, sptypes, slottypes, src, cfg, method_info,
-            currbb, currpc, ip, handler_at, ssavalue_uses, bb_vartables, ssavaluetypes, stmt_edges, stmt_info,
+            currbb, currpc, ip, handlers, handler_at, ssavalue_uses, bb_vartables, ssavaluetypes, stmt_edges, stmt_info,
             pclimitations, limitations, cycle_backedges, callers_in_cycle, dont_work_on_me, parent,
-            result, unreachable, valid_worlds, bestguess, ipo_effects,
+            result, unreachable, valid_worlds, bestguess, exc_bestguess, ipo_effects,
             restrict_abstract_call_sites, cache_mode, insert_coverage,
             interp)
     end
@@ -343,16 +351,19 @@ function compute_trycatch(code::Vector{Any}, ip::BitSet)
     empty!(ip)
     ip.offset = 0 # for _bits_findnext
     push!(ip, n + 1)
-    handler_at = fill(0, n)
+    handler_at = fill((0, 0), n)
+    handlers = TryCatchFrame[]
 
     # start from all :enter statements and record the location of the try
     for pc = 1:n
         stmt = code[pc]
         if isexpr(stmt, :enter)
             l = stmt.args[1]::Int
-            handler_at[pc + 1] = pc
+            push!(handlers, TryCatchFrame(Bottom, pc))
+            handler_id = length(handlers)
+            handler_at[pc + 1] = (handler_id, 0)
             push!(ip, pc + 1)
-            handler_at[l] = pc
+            handler_at[l] = (handler_id, handler_id)
             push!(ip, l)
         end
     end
@@ -365,25 +376,26 @@ function compute_trycatch(code::Vector{Any}, ip::BitSet)
         while true # inner loop optimizes the common case where it can run straight from pc to pc + 1
             pc´ = pc + 1 # next program-counter (after executing instruction)
             delete!(ip, pc)
-            cur_hand = handler_at[pc]
-            @assert cur_hand != 0 "unbalanced try/catch"
+            cur_stacks = handler_at[pc]
+            @assert cur_stacks != (0, 0) "unbalanced try/catch"
             stmt = code[pc]
             if isa(stmt, GotoNode)
                 pc´ = stmt.label
             elseif isa(stmt, GotoIfNot)
                 l = stmt.dest::Int
-                if handler_at[l] != cur_hand
-                    @assert handler_at[l] == 0 "unbalanced try/catch"
-                    handler_at[l] = cur_hand
+                if handler_at[l] != cur_stacks
+                    @assert handler_at[l][1] == 0 || handler_at[l][1] == cur_stacks[1] "unbalanced try/catch"
+                    handler_at[l] = cur_stacks
                     push!(ip, l)
                 end
             elseif isa(stmt, ReturnNode)
-                @assert !isdefined(stmt, :val) "unbalanced try/catch"
+                @assert !isdefined(stmt, :val) || cur_stacks[1] == 0 "unbalanced try/catch"
                 break
             elseif isa(stmt, Expr)
                 head = stmt.head
                 if head === :enter
-                    cur_hand = pc
+                    # Already set aboves
+                    cur_stacks = (handler_at[pc´][1], cur_stacks[2])
                 elseif head === :leave
                     l = 0
                     for j = 1:length(stmt.args)
@@ -399,19 +411,20 @@ function compute_trycatch(code::Vector{Any}, ip::BitSet)
                         end
                         l += 1
                     end
+                    cur_hand = cur_stacks[1]
                     for i = 1:l
-                        cur_hand = handler_at[cur_hand]
+                        cur_hand = handler_at[handlers[cur_hand].enter_idx][1]
                     end
-                    cur_hand == 0 && break
+                    cur_stacks = (cur_hand, cur_stacks[2])
+                    cur_stacks == (0, 0) && break
+                elseif head === :pop_exception
+                    cur_stacks = (cur_stacks[1], handler_at[(stmt.args[1]::SSAValue).id][2])
                 end
             end
 
             pc´ > n && break # can't proceed with the fast-path fall-through
-            if handler_at[pc´] != cur_hand
-                if handler_at[pc´] != 0
-                    @assert false "unbalanced try/catch"
-                end
-                handler_at[pc´] = cur_hand
+            if handler_at[pc´] != cur_stacks
+                handler_at[pc´] = cur_stacks
             elseif !in(pc´, ip)
                 break  # already visited
             end
@@ -420,7 +433,7 @@ function compute_trycatch(code::Vector{Any}, ip::BitSet)
     end
 
     @assert first(ip) == n + 1
-    return handler_at
+    return handler_at, handlers
 end
 
 # check if coverage mode is enabled

base/compiler/ssair/irinterp.jl

@@ -46,9 +46,9 @@ end
 
 function abstract_call(interp::AbstractInterpreter, arginfo::ArgInfo, irsv::IRInterpretationState)
     si = StmtInfo(true) # TODO better job here?
-    (; rt, effects, info) = abstract_call(interp, arginfo, si, irsv)
+    (; rt, exct, effects, info) = abstract_call(interp, arginfo, si, irsv)
     irsv.ir.stmts[irsv.curridx][:info] = info
-    return RTEffects(rt, effects)
+    return RTEffects(rt, exct, effects)
 end
 
 function update_phi!(irsv::IRInterpretationState, from::Int, to::Int)

base/compiler/ssair/slot2ssa.jl

@@ -584,7 +584,7 @@ function construct_ssa!(ci::CodeInfo, ir::IRCode, sv::OptimizationState,
     end
 
     # Record the correct exception handler for all critical sections
-    handler_at = compute_trycatch(code, BitSet())
+    handler_at, handlers = compute_trycatch(code, BitSet())
 
     phi_slots = Vector{Int}[Int[] for _ = 1:length(ir.cfg.blocks)]
     live_slots = Vector{Int}[Int[] for _ = 1:length(ir.cfg.blocks)]
@@ -810,8 +810,8 @@ function construct_ssa!(ci::CodeInfo, ir::IRCode, sv::OptimizationState,
                         incoming_vals[id] = Pair{Any, Any}(thisval, thisdef)
                         has_pinode[id] = false
                         enter_idx = idx
-                        while handler_at[enter_idx] != 0
-                            enter_idx = handler_at[enter_idx]
+                        while handler_at[enter_idx][1] != 0
+                            (; enter_idx) = handlers[handler_at[enter_idx][1]]
                             leave_block = block_for_inst(cfg, code[enter_idx].args[1]::Int)
                             cidx = findfirst((; slot)::NewPhiCNode2->slot_id(slot)==id, new_phic_nodes[leave_block])
                             if cidx !== nothing

base/compiler/stmtinfo.jl

@@ -10,6 +10,7 @@ and any additional information (`call.info`) for a given generic call.
 """
 struct CallMeta
     rt::Any
+    exct::Any
     effects::Effects
     info::CallInfo
 end