diff --git a/basis/compiler/cfg/stacks/clearing/clearing.factor b/basis/compiler/cfg/stacks/clearing/clearing.factor
index cb33617526..5df62202a5 100644
--- a/basis/compiler/cfg/stacks/clearing/clearing.factor
+++ b/basis/compiler/cfg/stacks/clearing/clearing.factor
@@ -1,6 +1,6 @@
 USING: accessors arrays assocs combinators.short-circuit
 compiler.cfg.instructions compiler.cfg.registers compiler.cfg.rpo
-compiler.cfg.stacks compiler.cfg.stacks.map kernel math sequences ;
+compiler.cfg.stacks compiler.cfg.stacks.padding kernel math sequences ;
 IN: compiler.cfg.stacks.clearing
 
 : state>replaces ( state -- replaces )
@@ -19,6 +19,6 @@ IN: compiler.cfg.stacks.clearing
     [ [ clearing-replaces ] keep suffix ] with map V{ } concat-as ;
 
 : clear-uninitialized ( cfg -- )
-    [ trace-stack-state ] keep [
+    [ trace-stack-state2 ] keep [
         [ visit-insns ] change-instructions drop
     ] with each-basic-block ;
diff --git a/basis/compiler/cfg/stacks/padding/padding-tests.factor b/basis/compiler/cfg/stacks/padding/padding-tests.factor
new file mode 100644
index 0000000000..23721b4a63
--- /dev/null
+++ b/basis/compiler/cfg/stacks/padding/padding-tests.factor
@@ -0,0 +1,611 @@
+USING: accessors arrays assocs compiler.cfg.instructions compiler.cfg.registers
+compiler.cfg.stacks.padding compiler.cfg.utilities kernel sequences sorting
+vectors tools.test ;
+IN: compiler.cfg.stacks.padding.tests
+
+! classify-read: vacant locations
+{ 2 2 2 } [
+    { 3 { } } 2 classify-read
+    { 0 { } } -1 classify-read
+    { 3 { } } -1 classify-read
+] unit-test
+
+! classify-read: over locations
+{ 1 1 1 1 1 } [
+    { 1 { 0 } } 1 classify-read
+    { 0 { } } 0 classify-read
+    { 3 { } } 4 classify-read
+    { 0 { } } 4 classify-read
+    { 1 { 0 } } 4 classify-read
+] unit-test
+
+! classify-read: initialized locations
+{ 0 0 0 } [
+    { 1 { 0 } } 0 classify-read
+    { 2 { 0 1 2 } } 0 classify-read
+    { 0 { 0 1 2 } } 0 classify-read
+] unit-test
+
+! fill-stack
+{
+    { 2 { 4 5 0 1 } }
+} [
+    { 2 { 4 5 } } fill-stack
+] unit-test
+
+{
+    { -1 { 3 4 } }
+} [
+    { -1 { 3 4 } } fill-stack
+] unit-test
+
+! fill-vacancies
+{
+    { { 0 { } } { 2 { 0 1 } } }
+    { { 0 { } } { 2 { 0 1 } } }
+    { { 0 { -1 -2 } } { 2 { 0 1 } } }
+} [
+    { { 0 { } } { 2 { } } } fill-vacancies
+    { { 0 { } } { 2 { 0 } } } fill-vacancies
+    { { 0 { -1 -2 } } { 2 { 0 } } } fill-vacancies
+] unit-test
+
+! combined-state
+{
+    { { 4 { } } { 2 { 0 1 } } }
+} [
+    V{ { { 4 { } } { 2 { 0 1 } } } } combine-states
+] unit-test
+
+{
+    { { 0 { } } { 0 { } } }
+} [
+    V{ } combine-states
+] unit-test
+
+! States can't be combined if their heights are different
+[
+    V{ { { 3 { } } { 0 { } } } { { 8 { } } { 0 { } } } } combine-states
+] [ height-mismatches? ] must-fail-with
+
+[
+    V{ { { 4 { } } { 2 { 0 1 } } } { { 5 { 4 3 2 } } { 0 { } } } }
+    combine-states
+] [ height-mismatches? ] must-fail-with
+
+! stack>vacant
+{
+    { 0 1 2 }
+    { }
+    { 1 }
+} [
+    { 3 { } } stack>vacant
+    { -2 { } } stack>vacant
+    { 3 { 0 2 } } stack>vacant
+] unit-test
+
+! visit-insn ##inc
+
+! We assume that overinitialized locations are always dead.
+{
+    { { 0 { } } { 0 { } } }
+} [
+    { { 3 { 0 } } { 0 { } } } T{ ##inc { loc D -3 } } visit-insn
+] unit-test
+
+! visit-insn ##call
+{
+    { { 3 { 0 1 2 } } { 0 { } } }
+} [
+    initial-state T{ ##call { height 3 } } visit-insn
+] unit-test
+
+
+{
+    { { -1 { } } { 0 { } } }
+} [
+    initial-state T{ ##call { height -1 } } visit-insn
+] unit-test
+
+
+{
+    { { 4 { 2 3 0 1 } } { 0 { } } }
+} [
+    { { 2 { 0 1 } } { 0 { } } } T{ ##call { height 2 } } visit-insn
+] unit-test
+
+! This looks weird but is right.
+{
+    { { 0 { 0 1 } } { 0 { } } }
+} [
+    { { -2 { } } { 0 { } } } T{ ##call { height 2 } } visit-insn
+] unit-test
+
+
+! if any of the stack locations are uninitialized when ##call is
+! visisted then something is wrong. ##call might gc and the
+! uninitialized locations would cause a crash.
+[
+    { { 3 { } } { 0 { } } } T{ ##call { height 3 } } visit-insn
+] [ vacant-when-calling? ] must-fail-with
+
+! ! Overinitialized locations can't be live when ##call is visited. They
+! ! could be garbage collected in the called word so they maybe wouldn't
+! ! survive.
+! [
+!     { { 0 { -1 -2 } } { 0 { -1 -2 } } } T{ ##call { height 0 } } visit-insn
+! ] [ overinitialized-when-calling? ] must-fail-with
+
+! This is tricky. Normally, there should be no overinitialized
+! locations before a ##call (I think). But if they are, we can at
+! least be sure they are dead after the call.
+{
+    { { 2 { 0 1 } } { 0 { } } }
+} [
+    { { 2 { 0 1 -1 } } { 0 { } } } T{ ##call { height 0 } } visit-insn
+] unit-test
+
+! visit-insn ##call-gc
+
+! ##call-gc ofcourse fills all uninitialized locations.
+{
+    { { 4 { 0 1 2 3 } } { 0 { } } }
+} [
+    { { 4 { } } { 0 { } } } T{ ##call-gc } visit-insn
+] unit-test
+
+! visit-insn ##peek
+{
+    { { 3 { 0 } } { 0 { } } }
+} [
+    { { 3 { 0 } } { 0 { } } } T{ ##peek { dst 1 } { loc D 0 } } visit-insn
+] unit-test
+
+! After a ##peek that can cause a stack underflow, it is certain that
+! all stack locations are initialized.
+{
+    { { 0 { } } { 2 { 0 1 2 } } }
+    { { 2 { 0 1 2 } } { 0 { } } }
+} [
+    { { 0 { } } { 2 { } } } T{ ##peek { dst 1 } { loc R 2 } } visit-insn
+    { { 2 { } } { 0 { } } } T{ ##peek { dst 1 } { loc D 2 } } visit-insn
+] unit-test
+
+{
+    { { 2 { 0 1 } } { 2 { 0 1 2 } } }
+} [
+    { { 2 { } } { 2 { } } } T{ ##peek { dst 1 } { loc R 2 } } visit-insn
+] unit-test
+
+! If the ##peek can't cause a stack underflow, then we don't have the
+! same guarantees.
+[
+    { { 3 { } } { 0 { } } } T{ ##peek { dst 1 } { loc D 0 } } visit-insn
+] [ vacant-peek? ] must-fail-with
+
+: following-stack-state ( insns -- state )
+    T{ ##branch } suffix insns>cfg trace-stack-state2
+    >alist [ first ] sort-with last second ;
+
+! trace-stack-state2
+{
+    H{
+        {
+            0
+            { { 0 { } } { 0 { } } }
+        }
+        {
+            1
+            { { 2 { } } { 0 { } } }
+        }
+        {
+            2
+            { { 2 { 0 1 2 } } { 0 { } } }
+        }
+    }
+} [
+    {
+        T{ ##inc f D 2 }
+        T{ ##peek f f D 2 }
+        T{ ##inc f D 0 }
+    } insns>cfg trace-stack-state2
+] unit-test
+
+{
+    H{
+        { 0 { { 0 { } } { 0 { } } } }
+        { 1 { { 0 { } } { 0 { } } } }
+        { 2 { { 0 { } } { 0 { } } } }
+    }
+} [
+    V{ T{ ##safepoint } T{ ##prologue } T{ ##branch } }
+    insns>cfg trace-stack-state2
+] unit-test
+
+! The peek "causes" the vacant locations to become populated.
+{
+    H{
+        { 0 { { 0 { } } { 0 { } } } }
+        { 1 { { 3 { } } { 0 { } } } }
+        { 2 { { 3 { 0 1 2 3 } } { 0 { } } } }
+    }
+} [
+    V{
+        T{ ##inc f D 3 }
+        T{ ##peek { loc D 3 } }
+        T{ ##branch }
+    }
+    insns>cfg trace-stack-state2
+] unit-test
+
+! Replace -1 then peek is ok.
+{
+    H{
+        { 0 { { 0 { } } { 0 { } } } }
+        { 1 { { 0 { -1 } } { 0 { } } } }
+        { 2 { { 0 { -1 } } { 0 { } } } }
+    }
+} [
+    V{
+        T{ ##replace { src 10 } { loc D -1 } }
+        T{ ##peek { loc D -1 } }
+        T{ ##branch }
+    }
+    insns>cfg trace-stack-state2
+] unit-test
+
+: cfg1 ( -- cfg )
+    V{
+        T{ ##inc f D 1 }
+        T{ ##replace { src 10 } { loc D 0 } }
+    } 0 insns>block
+    V{
+        T{ ##peek { dst 37 } { loc D 0 } }
+        T{ ##inc f D -1 }
+    } 1 insns>block
+    1vector >>successors block>cfg ;
+
+{
+    H{
+        { 0 { { 0 { } } { 0 { } } } }
+        { 1 { { 1 { } } { 0 { } } } }
+        { 2 { { 1 { 0 } } { 0 { } } } }
+        { 3 { { 1 { 0 } } { 0 { } } } }
+    }
+} [ cfg1 trace-stack-state2 ] unit-test
+
+! Same cfg structure as the bug1021:run-test word but with
+! non-datastack instructions mostly omitted.
+: bug1021-cfg ( -- cfg )
+    {
+        { 0 V{ T{ ##safepoint } T{ ##prologue } T{ ##branch } } }
+        {
+            1 V{
+                T{ ##inc f D 2 }
+                T{ ##replace { src 0 } { loc D 1 } }
+                T{ ##replace { src 0 } { loc D 0 } }
+            }
+        }
+        {
+            2 V{
+                T{ ##call { word <array> } { height 0 } }
+            }
+        }
+        {
+            3 V{
+                T{ ##peek { dst 0 } { loc D 0 } }
+                T{ ##peek { dst 0 } { loc D 1 } }
+                T{ ##inc f D 2 }
+                T{ ##replace { src 0 } { loc D 2 } }
+                T{ ##replace { src 0 } { loc D 3 } }
+                T{ ##replace { src 0 } { loc D 1 } }
+            }
+        }
+        {
+            8 V{
+                T{ ##peek { dst 0 } { loc D 2 } }
+                T{ ##peek { dst 0 } { loc D 1 } }
+                T{ ##inc f D 3 }
+                T{ ##replace { src 0 } { loc D 0 } }
+                T{ ##replace { src 0 } { loc D 1 } }
+                T{ ##replace { src 0 } { loc D 2 } }
+                T{ ##replace { src 0 } { loc D 3 } }
+            }
+        }
+        {
+            10 V{
+                T{ ##inc f D -3 }
+                T{ ##peek { dst 0 } { loc D 0 } }
+                T{ ##alien-invoke { gc-map T{ gc-map { scrub-d { } } } } }
+            }
+        }
+    } [ over insns>block ] assoc-map dup
+    { { 0 1 } { 1 2 } { 2 3 } { 3 8 } { 8 10 } } make-edges 0 of block>cfg ;
+
+{
+    H{
+        { 0 { { 0 { } } { 0 { } } } }
+        { 1 { { 0 { } } { 0 { } } } }
+        { 2 { { 0 { } } { 0 { } } } }
+        { 3 { { 0 { } } { 0 { } } } }
+        { 4 { { 2 { } } { 0 { } } } }
+        { 5 { { 2 { 1 } } { 0 { } } } }
+        { 6 { { 2 { 1 0 } } { 0 { } } } }
+        { 7 { { 2 { 1 0 } } { 0 { } } } }
+        { 8 { { 2 { 1 0 } } { 0 { } } } }
+        { 9 { { 2 { 1 0 } } { 0 { } } } }
+        { 10 { { 4 { 3 2 } } { 0 { } } } }
+        { 11 { { 4 { 3 2 } } { 0 { } } } }
+        { 12 { { 4 { 3 2 } } { 0 { } } } }
+        { 13 { { 4 { 3 2 1 } } { 0 { } } } }
+        { 14 { { 4 { 3 2 1 } } { 0 { } } } }
+        { 15 { { 4 { 3 2 1 } } { 0 { } } } }
+        { 16 { { 7 { 6 5 4 } } { 0 { } } } }
+        { 17 { { 7 { 6 5 4 0 } } { 0 { } } } }
+        { 18 { { 7 { 6 5 4 0 1 } } { 0 { } } } }
+        { 19 { { 7 { 6 5 4 0 1 2 } } { 0 { } } } }
+        { 20 { { 7 { 6 5 4 0 1 2 3 } } { 0 { } } } }
+        { 21 { { 4 { 3 2 1 0 } } { 0 { } } } }
+        { 22 { { 4 { 3 2 1 0 } } { 0 { } } } }
+    }
+} [
+    bug1021-cfg trace-stack-state2
+] unit-test
+
+! Same cfg structure as the bug1289:run-test word but with
+! non-datastack instructions mostly omitted.
+: bug1289-cfg ( -- cfg )
+    {
+        { 0 V{ } }
+        {
+            1 V{
+                T{ ##inc f D 3 }
+                T{ ##replace { src 0 } { loc D 2 } }
+                T{ ##replace { src 0 } { loc D 0 } }
+                T{ ##replace { src 0 } { loc D 1 } }
+            }
+        }
+        {
+            2 V{
+                T{ ##call { word <array> } { height -1 } }
+            }
+        }
+        {
+            3 V{
+                T{ ##peek { dst 0 } { loc D 1 } }
+                T{ ##peek { dst 0 } { loc D 0 } }
+                T{ ##inc f D 1 }
+                T{ ##inc f R 1 }
+                T{ ##replace { src 0 } { loc R 0 } }
+            }
+        }
+        {
+            4 V{ }
+        }
+        {
+            5 V{
+                T{ ##inc f D -2 }
+                T{ ##inc f R 5 }
+                T{ ##replace { src 0 } { loc R 3 } }
+                T{ ##replace { src 0 } { loc D 0 } }
+                T{ ##replace { src 0 } { loc R 4 } }
+                T{ ##replace { src 0 } { loc R 2 } }
+                T{ ##replace { src 0 } { loc R 1 } }
+                T{ ##replace { src 0 } { loc R 0 } }
+            }
+        }
+        {
+            6 V{
+                T{ ##call { word f } { height 0 } }
+            }
+        }
+        {
+            7 V{
+                T{ ##peek { dst 0 } { loc D 0 } }
+                T{ ##peek { dst 0 } { loc R 3 } }
+                T{ ##peek { dst 0 } { loc R 2 } }
+                T{ ##peek { dst 0 } { loc R 1 } }
+                T{ ##peek { dst 0 } { loc R 0 } }
+                T{ ##peek { dst 0 } { loc R 4 } }
+                T{ ##inc f D 2 }
+                T{ ##inc f R -5 }
+            }
+        }
+        { 8 V{ } }
+        { 9 V{ } }
+        { 10 V{ } }
+        {
+            11 V{
+                T{ ##call-gc }
+            }
+        }
+        {
+            12 V{
+                T{ ##peek { dst 0 } { loc R 0 } }
+                T{ ##inc f D -3 }
+                T{ ##inc f D 1 }
+                T{ ##inc f R -1 }
+                T{ ##replace { src 0 } { loc D 0 } }
+            }
+        }
+        {
+            13 V{ }
+        }
+    } [ over insns>block ] assoc-map dup
+    {
+        { 0 1 }
+        { 1 2 }
+        { 2 3 }
+        { 3 4 }
+        { 4 9 }
+        { 5 6 }
+        { 6 7 }
+        { 7 8 }
+        { 8 9 }
+        { 9 5 }
+        { 9 10 }
+        { 10 12 }
+        { 10 11 }
+        { 11 12 }
+        { 12 13 }
+    } make-edges 0 of block>cfg ;
+
+{
+    H{
+        { 0 { { 0 { } } { 0 { } } } }
+        { 1 { { 3 { } } { 0 { } } } }
+        { 2 { { 3 { 2 } } { 0 { } } } }
+        { 3 { { 3 { 2 0 } } { 0 { } } } }
+        { 4 { { 3 { 2 0 1 } } { 0 { } } } }
+        { 5 { { 2 { 1 0 } } { 0 { } } } }
+        { 6 { { 2 { 1 0 } } { 0 { } } } }
+        { 7 { { 2 { 1 0 } } { 0 { } } } }
+        { 8 { { 3 { 2 1 } } { 0 { } } } }
+        { 9 { { 3 { 2 1 } } { 1 { } } } }
+        { 10 { { 3 { 2 } } { 1 { 0 } } } }
+        { 11 { { 1 { 0 } } { 1 { 0 } } } }
+        { 12 { { 1 { 0 } } { 6 { 5 } } } }
+        { 13 { { 1 { 0 } } { 6 { 5 3 } } } }
+        { 14 { { 1 { 0 } } { 6 { 5 3 } } } }
+        { 15 { { 1 { 0 } } { 6 { 5 3 4 } } } }
+        { 16 { { 1 { 0 } } { 6 { 5 3 4 2 } } } }
+        { 17 { { 1 { 0 } } { 6 { 5 3 4 2 1 } } } }
+        { 18 { { 1 { 0 } } { 6 { 5 3 4 2 1 0 } } } }
+        { 19 { { 1 { 0 } } { 6 { 5 3 4 2 1 0 } } } }
+        { 20 { { 1 { 0 } } { 6 { 5 3 4 2 1 0 } } } }
+        { 21 { { 1 { 0 } } { 6 { 5 3 4 2 1 0 } } } }
+        { 22 { { 1 { 0 } } { 6 { 5 3 4 2 1 0 } } } }
+        { 23 { { 1 { 0 } } { 6 { 5 3 4 2 1 0 } } } }
+        { 24 { { 1 { 0 } } { 6 { 5 3 4 2 1 0 } } } }
+        { 25 { { 1 { 0 } } { 6 { 5 3 4 2 1 0 } } } }
+        { 26 { { 3 { 2 } } { 6 { 5 3 4 2 1 0 } } } }
+        { 27 { { 3 { 2 } } { 1 { 0 } } } }
+        { 28 { { 3 { 2 } } { 1 { 0 } } } }
+        { 29 { { 3 { 2 } } { 1 { 0 } } } }
+        { 30 { { 0 { } } { 1 { 0 } } } }
+        { 31 { { 1 { } } { 1 { 0 } } } }
+        { 32 { { 1 { } } { 0 { } } } }
+    }
+} [ bug1289-cfg trace-stack-state2 ] unit-test
+
+! following-stack-state
+{
+    { { 0 { } } { 0 { } } }
+} [ V{ } following-stack-state ] unit-test
+
+{
+    { { 1 { } } { 0 { } } }
+} [ V{ T{ ##inc f D 1 } } following-stack-state ] unit-test
+
+{
+    { { 0 { } } { 1 { } } }
+} [ V{ T{ ##inc f R 1 } } following-stack-state ] unit-test
+
+! Here the peek refers to a parameter of the word.
+{
+    { { 0 { 25 } } { 0 { } } }
+} [
+    V{
+        T{ ##peek { loc D 25 } }
+    } following-stack-state
+] unit-test
+
+! Should be ok because the value was at 0 when the gc ran.
+{
+    { { -1 { -1 } } { 0 { } } }
+} [
+    V{
+        T{ ##replace { src 10 } { loc D 0 } }
+        T{ ##alien-invoke { gc-map T{ gc-map { scrub-d { } } } } }
+        T{ ##inc f D -1 }
+        T{ ##peek { loc D -1 } }
+    } following-stack-state
+] unit-test
+
+{
+    { { 0 { 0 1 2 } } { 0 { } } }
+} [
+    V{
+        T{ ##replace { src 10 } { loc D 0 } }
+        T{ ##replace { src 10 } { loc D 1 } }
+        T{ ##replace { src 10 } { loc D 2 } }
+    } following-stack-state
+] unit-test
+
+{
+    { { 1 { 1 0 } } { 0 { } } }
+} [
+    V{
+        T{ ##replace { src 10 } { loc D 0 } }
+        T{ ##inc f D 1 }
+        T{ ##replace { src 10 } { loc D 0 } }
+    } following-stack-state
+] unit-test
+
+{
+    { { 0 { 0 } } { 0 { } } }
+} [
+    V{
+        T{ ##replace { src 10 } { loc D 0 } }
+        T{ ##inc f D 1 }
+        T{ ##replace { src 10 } { loc D 0 } }
+        T{ ##inc f D -1 }
+    } following-stack-state
+] unit-test
+
+{
+    { { 0 { } } { 0 { } } }
+} [
+    V{
+        T{ ##inc f D 1 }
+        T{ ##replace { src 10 } { loc D 0 } }
+        T{ ##inc f D -1 }
+    } following-stack-state
+] unit-test
+
+! ##call clears the overinitialized slots.
+{
+    { { -1 { } } { 0 { } } }
+} [
+    V{
+        T{ ##replace { src 10 } { loc D 0 } }
+        T{ ##inc f D -1 }
+        T{ ##call { height 0 } }
+    } following-stack-state
+] unit-test
+
+! Should not be ok because the value wasn't initialized when gc ran.
+[
+    V{
+        T{ ##inc f D 1 }
+        T{ ##alien-invoke { gc-map T{ gc-map { scrub-d { } } } } }
+        T{ ##peek { loc D 0 } }
+    } following-stack-state
+] [ vacant-peek? ] must-fail-with
+
+[
+    V{
+        T{ ##inc f D 1 }
+        T{ ##peek { loc D 0 } }
+    } following-stack-state
+] [ vacant-peek? ] must-fail-with
+
+[
+    V{
+        T{ ##inc f R 1 }
+        T{ ##peek { loc R 0 } }
+    } following-stack-state
+] [ vacant-peek? ] must-fail-with
+
+
+
+
+
+
+
+
+
+! ! (scan-c-args) run-test flip
+
+
+! seem good: (gamma-random-float>1)
diff --git a/basis/compiler/cfg/stacks/padding/padding.factor b/basis/compiler/cfg/stacks/padding/padding.factor
new file mode 100644
index 0000000000..18f86ce3e4
--- /dev/null
+++ b/basis/compiler/cfg/stacks/padding/padding.factor
@@ -0,0 +1,127 @@
+! Copyright (C) 2015 Björn Lindqvist.
+! See http://factorcode.org/license.txt for BSD license.
+USING: accessors arrays assocs compiler.cfg.dataflow-analysis
+compiler.cfg.instructions compiler.cfg.linearization compiler.cfg.predecessors
+compiler.cfg.registers compiler.cfg.stacks compiler.cfg.stacks.local
+compiler.cfg.stacks.global fry grouping kernel math math.order namespaces
+sequences ;
+QUALIFIED: sets
+IN: compiler.cfg.stacks.padding
+
+ERROR: overinitialized-when-calling seq ;
+ERROR: vacant-when-calling seq ;
+
+: safe-iota ( n -- seq )
+    0 max iota ;
+
+! !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
+! !! Stack
+! !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
+ERROR: height-mismatches seq ;
+
+: register-write ( n stack -- stack' )
+    first2 rot suffix sets:members 2array ;
+
+: adjust-stack ( n stack -- stack' )
+    first2 pick '[ _ + ] map [ + ] dip 2array ;
+
+: stack>vacant ( stack -- seq )
+    first2 [ safe-iota ] dip sets:diff ;
+
+: combine-stacks ( stacks -- stack )
+    [ [ first ] map dup all-equal? [ first ] [ height-mismatches ] if ]
+    [ [ second ] map refine ] bi 2array ;
+
+: fill-stack ( stack -- stack' )
+    first2 over safe-iota sets:union 2array ;
+
+: classify-read ( stack n -- val )
+    swap 2dup second member? [ 2drop 0 ] [ first >= [ 1 ] [ 2 ] if ] if ;
+
+: push-items ( n stack -- stack' )
+    first2 pick '[ _ + ] map pick safe-iota sets:union [ + ] dip 2array ;
+
+! !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
+! !! States
+! !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
+CONSTANT: initial-state { { 0 { } } { 0 { } } }
+
+: apply-stack-op ( state insn quote: ( n stack -- stack' ) -- state' )
+    [ [ first2 ] dip loc>> >loc< ] dip
+    [ '[ rot @ swap ] ] [ '[ swap @ ] ] bi if 2array ; inline
+
+: combine-states ( states -- state )
+    [ initial-state ] [ flip [ combine-stacks ] map ] if-empty ;
+
+: mark-location ( state insn -- state' )
+    [ register-write ] apply-stack-op ;
+
+: ensure-no-vacant ( state -- )
+    [ stack>vacant ] map dup { { } { } } =
+    [ drop ] [ vacant-when-calling ] if ;
+
+: ensure-no-overinitialized ( state -- )
+    [ second [ 0 < ] filter ] map dup { { } { } } =
+    [ drop ] [ overinitialized-when-calling ] if ;
+
+: fill-vacancies ( state -- state' )
+    [ fill-stack ] map ;
+
+GENERIC: visit-insn ( state insn -- state' )
+
+M: ##inc visit-insn ( state insn -- state' )
+    [ adjust-stack ] apply-stack-op
+    [ first2 [ 0 >= ] filter 2array ] map ;
+
+M: ##replace-imm visit-insn mark-location ;
+M: ##replace visit-insn mark-location ;
+
+M: ##call visit-insn ( state insn -- state' )
+    over ensure-no-vacant
+    height>> swap first2 [ push-items ] dip 2array
+    [ first2 [ 0 >= ] filter 2array ] map ;
+
+M: ##call-gc visit-insn ( state insn -- state' )
+    drop dup ensure-no-overinitialized fill-vacancies ;
+
+M: gc-map-insn visit-insn ( state insn -- state' )
+    drop ;
+
+ERROR: vacant-peek insn ;
+
+: underflowable-peek? ( state peek -- ? )
+    2dup loc>> >loc< swap [ 0 1 ? swap nth ] dip classify-read
+    dup 2 = [ drop vacant-peek ] [ 2nip 1 = ] if ;
+
+M: ##peek visit-insn ( state insn -- state )
+    2dup underflowable-peek? [ [ fill-vacancies ] dip ] when mark-location ;
+
+M: insn visit-insn ( state insn -- state' )
+    drop ;
+
+FORWARD-ANALYSIS: padding
+
+SYMBOL: stack-record
+
+: register-stack-state ( state insn -- )
+    insn#>> stack-record get set-at ;
+
+: visit-insns ( insns state -- state' )
+    [ [ register-stack-state ] [ visit-insn ] 2bi ] reduce ;
+
+M: padding-analysis transfer-set ( in-set bb dfa -- out-set )
+    drop instructions>> swap visit-insns ;
+
+M: padding-analysis ignore-block? ( bb dfa -- ? )
+    2drop f ;
+
+M: padding-analysis join-sets ( sets bb dfa -- set )
+    2drop combine-states ;
+
+: uniquely-number-instructions ( cfg -- )
+    cfg>insns [ swap insn#<< ] each-index ;
+
+: trace-stack-state2 ( cfg -- assoc )
+    H{ } clone stack-record set
+    [ uniquely-number-instructions ] [ compute-padding-sets ] bi
+    stack-record get ;
diff --git a/basis/compiler/cfg/stacks/vacant/vacant-docs.factor b/basis/compiler/cfg/stacks/vacant/vacant-docs.factor
index 45e8a5f634..c102118397 100644
--- a/basis/compiler/cfg/stacks/vacant/vacant-docs.factor
+++ b/basis/compiler/cfg/stacks/vacant/vacant-docs.factor
@@ -1,4 +1,4 @@
-USING: compiler.cfg compiler.cfg.instructions compiler.cfg.stacks.map
+USING: compiler.cfg compiler.cfg.instructions compiler.cfg.stacks.padding
 help.markup help.syntax sequences strings ;
 IN: compiler.cfg.stacks.vacant
 
@@ -19,7 +19,7 @@ HELP: fill-gc-maps
 
 HELP: state>gc-data
 { $values { "state" sequence } { "gc-data" sequence } }
-{ $description "Takes a stack state on the format given by " { $link trace-stack-state } " and emits an array containing two bit-patterns with locations on the data and retain stacks to scrub." } ;
+{ $description "Takes a stack state on the format given by " { $link trace-stack-state2 } " and emits an array containing two bit-patterns with locations on the data and retain stacks to scrub." } ;
 
 HELP: vacant>bits
 { $values
diff --git a/basis/compiler/cfg/stacks/vacant/vacant.factor b/basis/compiler/cfg/stacks/vacant/vacant.factor
index 2a31475bc9..4f9f26c3d5 100644
--- a/basis/compiler/cfg/stacks/vacant/vacant.factor
+++ b/basis/compiler/cfg/stacks/vacant/vacant.factor
@@ -1,26 +1,20 @@
 USING: accessors arrays assocs compiler.cfg.instructions
-compiler.cfg.linearization compiler.cfg.stacks.map fry kernel math sequences ;
+compiler.cfg.linearization compiler.cfg.stacks.padding fry kernel math
+sequences ;
 IN: compiler.cfg.stacks.vacant
 
-! ! Utils
-: write-slots ( tuple values slots -- )
-    [ execute( x y -- z ) ] 2each drop ;
-
 : vacant>bits ( vacant --  bits )
     [ { } ] [
         dup supremum 1 + 1 <array>
         [ '[ _ 0 -rot set-nth ] each ] keep
     ] if-empty ;
 
-! Operations on the analysis state
 : state>gc-data ( state -- gc-data )
     [ stack>vacant vacant>bits ] map ;
 
 : set-gc-map ( state gc-map -- )
-    swap state>gc-data { >>scrub-d >>scrub-r } write-slots ;
-    ! swap state>gc-data { { } { } } append
-    ! { >>scrub-d >>scrub-r >>check-d >>check-r } write-slots ;
+    swap state>gc-data first2 -rot >>scrub-d swap >>scrub-r drop ;
 
 : fill-gc-maps ( cfg -- )
-    [ trace-stack-state ] [ cfg>insns [ gc-map-insn? ] filter ] bi
+    [ trace-stack-state2 ] [ cfg>insns [ gc-map-insn? ] filter ] bi
     [ [ insn#>> of ] [ gc-map>> ] bi set-gc-map ] with each ;