diff --git a/basis/compiler/cfg/branch-folding/branch-folding-tests.factor b/basis/compiler/cfg/branch-folding/branch-folding-tests.factor
new file mode 100644
index 0000000000..1068954f9d
--- /dev/null
+++ b/basis/compiler/cfg/branch-folding/branch-folding-tests.factor
@@ -0,0 +1,44 @@
+IN: compiler.cfg.branch-folding.tests
+USING: compiler.cfg.branch-folding compiler.cfg.instructions
+compiler.cfg compiler.cfg.registers compiler.cfg.debugger
+arrays compiler.cfg.phi-elimination
+compiler.cfg.predecessors kernel accessors
+sequences classes namespaces tools.test cpu.architecture ;
+
+V{ T{ ##branch } } 0 test-bb
+
+V{
+    T{ ##peek f V int-regs 0 D 0 }
+    T{ ##compare-branch f V int-regs 0 V int-regs 0 cc< }
+} 1 test-bb
+
+V{
+    T{ ##load-immediate f V int-regs 1 1 }
+    T{ ##branch }
+} 2 test-bb
+
+V{
+    T{ ##load-immediate f V int-regs 2 2 }
+    T{ ##branch }
+} 3 test-bb
+
+V{
+    T{ ##phi f V int-regs 3 { } }
+    T{ ##replace f V int-regs 3 D 0 }
+    T{ ##return }
+} 4 test-bb
+
+4 get instructions>> first
+2 get V int-regs 1 2array
+3 get V int-regs 2 2array 2array
+>>inputs drop
+
+test-diamond
+
+[ ] [ cfg new 0 get >>entry fold-branches compute-predecessors eliminate-phis drop ] unit-test
+
+[ 1 ] [ 1 get successors>> length ] unit-test
+[ t ] [ 1 get successors>> first 3 get eq? ] unit-test
+
+[ T{ ##copy f V int-regs 3 V int-regs 2 } ] [ 3 get instructions>> second ] unit-test
+[ 2 ] [ 4 get instructions>> length ] unit-test
\ No newline at end of file
diff --git a/basis/compiler/cfg/dce/dce.factor b/basis/compiler/cfg/dce/dce.factor
index ea4a1d22ab..fdc6601de4 100644
--- a/basis/compiler/cfg/dce/dce.factor
+++ b/basis/compiler/cfg/dce/dce.factor
@@ -99,5 +99,5 @@ M: insn live-insn? drop t ;
     dup
     [ [ instructions>> [ build-liveness-graph ] each ] each-basic-block ]
     [ [ instructions>> [ compute-live-vregs ] each ] each-basic-block ]
-    [ [ [ [ live-insn? ] filter ] change-instructions drop ] each-basic-block ]
+    [ [ instructions>> [ live-insn? ] filter-here ] each-basic-block ]
     tri ;
diff --git a/basis/compiler/cfg/def-use/def-use.factor b/basis/compiler/cfg/def-use/def-use.factor
index 4ff9814e6d..43ea89f284 100644
--- a/basis/compiler/cfg/def-use/def-use.factor
+++ b/basis/compiler/cfg/def-use/def-use.factor
@@ -1,6 +1,6 @@
 ! Copyright (C) 2008, 2009 Slava Pestov.
 ! See http://factorcode.org/license.txt for BSD license.
-USING: accessors arrays kernel compiler.cfg.instructions ;
+USING: accessors arrays kernel assocs compiler.cfg.instructions ;
 IN: compiler.cfg.def-use
 
 GENERIC: defs-vregs ( insn -- seq )
@@ -43,7 +43,7 @@ M: ##dispatch uses-vregs src>> 1array ;
 M: ##alien-getter uses-vregs src>> 1array ;
 M: ##alien-setter uses-vregs [ src>> ] [ value>> ] bi 2array ;
 M: ##fixnum-overflow uses-vregs [ src1>> ] [ src2>> ] bi 2array ;
-M: ##phi uses-vregs inputs>> ;
+M: ##phi uses-vregs inputs>> values ;
 M: _conditional-branch uses-vregs [ src1>> ] [ src2>> ] bi 2array ;
 M: _compare-imm-branch uses-vregs src1>> 1array ;
 M: _dispatch uses-vregs src>> 1array ;
diff --git a/basis/compiler/cfg/liveness/liveness-tests.factor b/basis/compiler/cfg/liveness/liveness-tests.factor
new file mode 100644
index 0000000000..271dc60d76
--- /dev/null
+++ b/basis/compiler/cfg/liveness/liveness-tests.factor
@@ -0,0 +1,15 @@
+USING: compiler.cfg compiler.cfg.instructions compiler.cfg.registers
+compiler.cfg.liveness accessors tools.test cpu.architecture ;
+IN: compiler.cfg.liveness.tests
+
+[
+    H{
+        { "A" H{ { V int-regs 1 V int-regs 1 } { V int-regs 4 V int-regs 4 } } }
+        { "B" H{ { V int-regs 3 V int-regs 3 } { V int-regs 2 V int-regs 2 } } }
+    }
+] [
+    <basic-block> V{
+        T{ ##phi f V int-regs 0 { { "A" V int-regs 1 } { "B" V int-regs 2 } } }
+        T{ ##phi f V int-regs 1 { { "B" V int-regs 3 } { "A" V int-regs 4 } } }
+    } >>instructions compute-phi-live-in
+] unit-test
\ No newline at end of file
diff --git a/basis/compiler/cfg/liveness/liveness.factor b/basis/compiler/cfg/liveness/liveness.factor
index 6c40bb3782..8a46b32070 100644
--- a/basis/compiler/cfg/liveness/liveness.factor
+++ b/basis/compiler/cfg/liveness/liveness.factor
@@ -1,7 +1,7 @@
 ! Copyright (C) 2009 Slava Pestov.
 ! See http://factorcode.org/license.txt for BSD license.
 USING: kernel namespaces deques accessors sets sequences assocs fry
-dlists compiler.cfg.def-use compiler.cfg.instructions
+hashtables dlists compiler.cfg.def-use compiler.cfg.instructions
 compiler.cfg.rpo ;
 IN: compiler.cfg.liveness
 
@@ -16,9 +16,7 @@ SYMBOL: live-ins
 ! is in conrrespondence with a predecessor
 SYMBOL: phi-live-ins
 
-: phi-live-in ( predecessor basic-block -- set )
-    [ predecessors>> index ] keep phi-live-ins get at
-    dup [ nth ] [ 2drop f ] if ;
+: phi-live-in ( predecessor basic-block -- set ) phi-live-ins get at at ;
 
 ! Assoc mapping basic blocks to sets of vregs
 SYMBOL: live-outs
@@ -45,9 +43,15 @@ SYMBOL: work-list
     [ nip kill-set ]
     2bi assoc-diff ;
 
+: conjoin-at ( value key assoc -- )
+    [ dupd ?set-at ] change-at ;
+
 : compute-phi-live-in ( basic-block -- phi-live-in )
-    instructions>> [ ##phi? ] filter
-    [ f ] [ [ inputs>> ] map flip [ unique ] map ] if-empty ;
+    instructions>> [ ##phi? ] filter [ f ] [
+        H{ } clone [
+            '[ inputs>> [ swap _ conjoin-at ] assoc-each ] each
+        ] keep
+    ] if-empty ;
 
 : update-live-in ( basic-block -- changed? )
     [ [ compute-live-in ] keep live-ins get maybe-set-at ]
diff --git a/basis/compiler/cfg/phi-elimination/phi-elimination-tests.factor b/basis/compiler/cfg/phi-elimination/phi-elimination-tests.factor
new file mode 100644
index 0000000000..4577e70997
--- /dev/null
+++ b/basis/compiler/cfg/phi-elimination/phi-elimination-tests.factor
@@ -0,0 +1,40 @@
+IN: compiler.cfg.phi-elimination.tests
+USING: compiler.cfg.instructions compiler.cfg compiler.cfg.registers
+compiler.cfg.debugger compiler.cfg.phi-elimination kernel accessors
+sequences classes namespaces tools.test cpu.architecture arrays ;
+
+V{ T{ ##branch } } 0 test-bb
+
+V{
+    T{ ##peek f V int-regs 0 D 0 }
+    T{ ##compare-branch f V int-regs 0 V int-regs 0 cc< }
+} 1 test-bb
+
+V{
+    T{ ##load-immediate f V int-regs 1 1 }
+    T{ ##branch }
+} 2 test-bb
+
+V{
+    T{ ##load-immediate f V int-regs 2 2 }
+    T{ ##branch }
+} 3 test-bb
+
+V{
+    T{ ##phi f V int-regs 3 { } }
+    T{ ##replace f V int-regs 3 D 0 }
+    T{ ##return }
+} 4 test-bb
+
+4 get instructions>> first
+2 get V int-regs 1 2array
+3 get V int-regs 2 2array 2array
+>>inputs drop
+
+test-diamond
+
+[ ] [ cfg new 0 get >>entry eliminate-phis drop ] unit-test
+
+[ T{ ##copy f V int-regs 3 V int-regs 1 } ] [ 2 get instructions>> second ] unit-test
+[ T{ ##copy f V int-regs 3 V int-regs 2 } ] [ 3 get instructions>> second ] unit-test
+[ 2 ] [ 4 get instructions>> length ] unit-test
\ No newline at end of file
diff --git a/basis/compiler/cfg/phi-elimination/phi-elimination.factor b/basis/compiler/cfg/phi-elimination/phi-elimination.factor
index 3ebf553a45..9c2f0adafd 100644
--- a/basis/compiler/cfg/phi-elimination/phi-elimination.factor
+++ b/basis/compiler/cfg/phi-elimination/phi-elimination.factor
@@ -1,21 +1,17 @@
 ! Copyright (C) 2009 Slava Pestov.
 ! See http://factorcode.org/license.txt for BSD license.
-USING: accessors compiler.cfg compiler.cfg.instructions
-compiler.cfg.rpo fry kernel sequences ;
+USING: accessors assocs fry kernel sequences
+compiler.cfg compiler.cfg.instructions compiler.cfg.rpo ;
 IN: compiler.cfg.phi-elimination
 
 : insert-copy ( predecessor input output -- )
     '[ _ _ swap ##copy ] add-instructions ;
 
-: eliminate-phi ( bb ##phi -- )
-    [ predecessors>> ] [ [ inputs>> ] [ dst>> ] bi ] bi*
-    '[ _ insert-copy ] 2each ;
+: eliminate-phi ( ##phi -- )
+    [ inputs>> ] [ dst>> ] bi '[ _ insert-copy ] assoc-each ;
 
 : eliminate-phi-step ( bb -- )
-    dup [
-        [ ##phi? ] partition
-        [ [ eliminate-phi ] with each ] dip
-    ] change-instructions drop ;
+    instructions>> [ dup ##phi? [ eliminate-phi f ] [ drop t ] if ] filter-here ;
 
 : eliminate-phis ( cfg -- cfg' )
     dup [ eliminate-phi-step ] each-basic-block ;
\ No newline at end of file
diff --git a/basis/compiler/cfg/stack-analysis/merge/merge-tests.factor b/basis/compiler/cfg/stack-analysis/merge/merge-tests.factor
index e519308974..14a81958a9 100644
--- a/basis/compiler/cfg/stack-analysis/merge/merge-tests.factor
+++ b/basis/compiler/cfg/stack-analysis/merge/merge-tests.factor
@@ -17,7 +17,7 @@ sequences kernel classes ;
     <state> H{ { D 0 V int-regs 0 } } >>locs>vregs
     <state> H{ { D 0 V int-regs 1 } } >>locs>vregs 2array
 
-    [ merge-locs locs>vregs>> keys ] { } make first inputs>>
+    [ merge-locs locs>vregs>> keys ] { } make first inputs>> values
 ] unit-test
 
 [
diff --git a/basis/compiler/cfg/stack-analysis/merge/merge.factor b/basis/compiler/cfg/stack-analysis/merge/merge.factor
index 04643a31f0..b6c443a2d3 100644
--- a/basis/compiler/cfg/stack-analysis/merge/merge.factor
+++ b/basis/compiler/cfg/stack-analysis/merge/merge.factor
@@ -48,8 +48,9 @@ IN: compiler.cfg.stack-analysis.merge
 : merge-loc ( predecessors vregs loc state -- vreg )
     ! Insert a ##phi in the current block where the input
     ! is the vreg storing loc from each predecessor block
+    [ dup ] 3dip
     '[ [ ] [ _ _ insert-peek ] ?if ] 2map
-    dup all-equal? [ first ] [ ^^phi ] if ;
+    dup all-equal? [ nip first ] [ zip ^^phi ] if ;
 
 :: merge-locs ( state predecessors states -- state )
     states [ locs>vregs>> ] map states collect-locs