[commit: ghc] master: Generate better code for "if (3 <= x) then ..." (b482966)

Repository : ssh://darcs.haskell.org//srv/darcs/ghc

On branch  : master

http://hackage.haskell.org/trac/ghc/changeset/b48296654f83ecd6ca2aa9d823a354548058f2b5

>---------------------------------------------------------------

commit b48296654f83ecd6ca2aa9d823a354548058f2b5
Author: Simon Marlow <[email protected]>
Date:   Fri Sep 21 11:03:13 2012 +0100

    Generate better code for "if (3 <= x) then ..."

>---------------------------------------------------------------

 compiler/nativeGen/X86/CodeGen.hs |   14 +++++++++++++-
 compiler/nativeGen/X86/Cond.hs    |   19 ++++++++++++++++++-
 2 files changed, 31 insertions(+), 2 deletions(-)

diff --git a/compiler/nativeGen/X86/CodeGen.hs b/compiler/nativeGen/X86/CodeGen.hs
index b83ede8..72dd30f 100644
--- a/compiler/nativeGen/X86/CodeGen.hs
+++ b/compiler/nativeGen/X86/CodeGen.hs
@@ -1276,7 +1276,8 @@ condIntCode' _ cond x (CmmLit (CmmInt 0 pk)) = do
     return (CondCode False cond code)
 
 -- anything vs operand
-condIntCode' is32Bit cond x y | isOperand is32Bit y = do
+condIntCode' is32Bit cond x y
+ | isOperand is32Bit y = do
     dflags <- getDynFlags
     (x_reg, x_code) <- getNonClobberedReg x
     (y_op,  y_code) <- getOperand y
@@ -1284,6 +1285,17 @@ condIntCode' is32Bit cond x y | isOperand is32Bit y = do
         code = x_code `appOL` y_code `snocOL`
                   CMP (cmmTypeSize (cmmExprType dflags x)) y_op (OpReg x_reg)
     return (CondCode False cond code)
+-- operand vs. anything: invert the comparison so that we can use a
+-- single comparison instruction.
+ | isOperand is32Bit x
+ , Just revcond <- maybeFlipCond cond = do
+    dflags <- getDynFlags
+    (y_reg, y_code) <- getNonClobberedReg y
+    (x_op,  x_code) <- getOperand x
+    let
+        code = y_code `appOL` x_code `snocOL`
+                  CMP (cmmTypeSize (cmmExprType dflags x)) x_op (OpReg y_reg)
+    return (CondCode False revcond code)
 
 -- anything vs anything
 condIntCode' _ cond x y = do
diff --git a/compiler/nativeGen/X86/Cond.hs b/compiler/nativeGen/X86/Cond.hs
index ce97095..904a9ce 100644
--- a/compiler/nativeGen/X86/Cond.hs
+++ b/compiler/nativeGen/X86/Cond.hs
@@ -10,7 +10,8 @@ module X86.Cond (
 	Cond(..),
 	condUnsigned,
 	condToSigned,
-	condToUnsigned
+        condToUnsigned,
+        maybeFlipCond
 )
 
 where
@@ -57,3 +58,19 @@ condToUnsigned LTT = LU
 condToUnsigned GE  = GEU
 condToUnsigned LE  = LEU
 condToUnsigned x   = x
+
+-- | @maybeFlipCond c@ returns @Just c'@ if it is possible to flip the
+-- arguments to the conditional @c@, and the new condition should be @c'@.
+maybeFlipCond :: Cond -> Maybe Cond
+maybeFlipCond cond  = case cond of
+        EQQ   -> Just EQQ
+        NE    -> Just NE
+        LU    -> Just GU
+        GU    -> Just LU
+        LEU   -> Just GEU
+        GEU   -> Just LEU
+        LTT   -> Just GTT
+        GTT   -> Just LTT
+        LE    -> Just GE
+        GE    -> Just LE
+        _other -> Nothing