compiler.cfg.branch-folding: fold away branches where both inputs are the same register. This results in 'push' being compiled slightly better

basis/compiler/cfg/branch-folding/branch-folding.factor

@@ -0,0 +1,30 @@
+! Copyright (C) 2009 Slava Pestov.
+! See http://factorcode.org/license.txt for BSD license.
+USING: accessors combinators.short-circuit kernel sequences vectors
+compiler.cfg.instructions compiler.cfg.rpo ;
+IN: compiler.cfg.branch-folding
+
+! Fold comparisons where both inputs are the same. Predecessors must be
+! recomputed after this
+
+: fold-branch? ( bb -- ? )
+    instructions>> last {
+        [ ##compare-branch? ]
+        [ [ src1>> ] [ src2>> ] bi = ]
+    } 1&& ;
+
+: chosen-successor ( bb -- succ )
+    [ instructions>> last cc>> { cc= cc<= cc>= } memq? 0 1 ? ]
+    [ successors>> ]
+    bi nth ;
+
+: fold-branch ( bb -- )
+    dup chosen-successor 1vector >>successors
+    instructions>> [ pop* ] [ [ \ ##branch new-insn ] dip push ] bi ;
+
+: fold-branches ( cfg -- cfg' )
+    dup [
+        dup fold-branch?
+        [ fold-branch ] [ drop ] if
+    ] each-basic-block
+    f >>post-order ;

basis/compiler/cfg/optimizer/optimizer.factor

@@ -9,6 +9,7 @@ compiler.cfg.branch-splitting
 compiler.cfg.alias-analysis
 compiler.cfg.value-numbering
 compiler.cfg.dce
+compiler.cfg.branch-folding
 compiler.cfg.write-barrier
 compiler.cfg.liveness
 compiler.cfg.rpo
@@ -24,6 +25,8 @@ SYMBOL: check-optimizer?
     ] when ;
 
 : optimize-cfg ( cfg -- cfg' )
+    ! Note that compute-predecessors has to be called several times.
+    ! The passes that need this document it.
     [
         optimize-tail-calls
         compute-predecessors
@@ -34,6 +37,8 @@ SYMBOL: check-optimizer?
         compute-liveness
         alias-analysis
         value-numbering
+        fold-branches
+        compute-predecessors
         eliminate-dead-code
         eliminate-write-barriers
         eliminate-phis