From d27b979d2c55b0fdcf0e5938d7f2167cc47c8791 Mon Sep 17 00:00:00 2001
From: Tim Holy <tim.holy@gmail.com>
Date: Wed, 2 Feb 2022 06:47:16 -0600
Subject: [PATCH] Add `hasbox` to check for `Core.Box` (#29)

---
 src/MethodAnalysis.jl | 24 ++++++++++++++++++++++++
 test/runtests.jl      | 24 ++++++++++++++++++++++++
 2 files changed, 48 insertions(+)

diff --git a/src/MethodAnalysis.jl b/src/MethodAnalysis.jl
index 1ba8517..ad95380 100644
--- a/src/MethodAnalysis.jl
+++ b/src/MethodAnalysis.jl
@@ -8,6 +8,7 @@ using Base.Meta: isexpr
 
 export visit, call_type, methodinstance, methodinstances, worlds  # findcallers is exported from its own file
 export visit_backedges, all_backedges, with_all_backedges, terminal_backedges, direct_backedges
+export hasbox
 
 include("visit.jl")
 include("backedges.jl")
@@ -189,6 +190,29 @@ function methodinstances(@nospecialize(types::Type))
     return methodinstances(f, types)
 end
 
+if isdefined(Base, :code_typed_by_type)
+    function hasbox(mi::MethodInstance)
+        try
+            srcs = Base.code_typed_by_type(mi.specTypes)
+            for (ci, rt) in srcs
+                (any(==(Core.Box), ci.slottypes) || any(==(Core.Box), ci.ssavaluetypes)) && return true
+            end
+            return false
+        catch
+            return false
+        end
+    end
+else
+    hasbox(mi::MethodInstance) = error("hasbox requires at least Julia 1.6")
+end
+
+"""
+    hasbox(mi::MethodInstance)
+
+Return `true` if the code for `mi` has a `Core.Box`. This often arises from a limitation in Julia's type-inference,
+see https://docs.julialang.org/en/v1/manual/performance-tips/#man-performance-captured.
+"""
+hasbox
 
 if isdefined(Core, :MethodMatch)
     include("findcallers.jl")
diff --git a/test/runtests.jl b/test/runtests.jl
index 63f4365..26f0e99 100644
--- a/test/runtests.jl
+++ b/test/runtests.jl
@@ -198,6 +198,30 @@ end
     end
 end
 
+@testset "hasbox" begin
+    function abmult(r::Int)
+        if r < 0
+            r = -r
+        end
+        f = x -> x * r
+        return f
+    end
+    function abmult2(r::Int)
+        f = x -> x * abs(r)
+        return f
+    end
+    abmult(1)
+    mi = methodinstance(abmult, (Int,))
+    abmult2(1)
+    mi2 = methodinstance(abmult2, (Int,))
+    if isdefined(Base, :code_typed_by_type)
+        @test hasbox(mi)
+        @test !hasbox(mi2)
+    else
+        @test_throws ErrorException("hasbox requires at least Julia 1.6") hasbox(mi)
+    end
+end
+
 module Callers
 
 f(x) = rand()