[project @ 2005-04-11 08:52:29 by simonmar]

When generating a switch for:

  case e of
    0 -> A
    1 -> B

instead of generating

  if (e < 1) then goto A
  B

generate

  if (e >= 1) then goto B
  A

because this helps the NCG to generate better code.  In particular, if
e is a comparison, then we don't need to reverse the sense of the
comparison to eliminate the comparse against 1 (the NCG does try to
reverse the comparison, but floating-point comparisons can't be
reversed).

diff --git a/ghc/compiler/codeGen/CgUtils.hs b/ghc/compiler/codeGen/CgUtils.hs
index 535d016..67e5973 100644 (file)
--- a/ghc/compiler/codeGen/CgUtils.hs
+++ b/ghc/compiler/codeGen/CgUtils.hs
@@ -45,7 +45,7 @@ import CLabel
 import CmmUtils
 import MachOp          ( MachRep(..), wordRep, MachOp(..),  MachHint(..),
                          mo_wordOr, mo_wordAnd, mo_wordNe, mo_wordEq,
-                         mo_wordULt, mo_wordUGt, machRepByteWidth )
+                         mo_wordULt, mo_wordUGt, mo_wordUGe, machRepByteWidth )
 import ForeignCall     ( CCallConv(..) )
 import Literal         ( Literal(..) )
 import CLabel          ( CLabel, mkStringLitLabel )
@@ -153,6 +153,7 @@ cmmAndWord e1 e2 = CmmMachOp mo_wordAnd [e1, e2]
 cmmNeWord  e1 e2 = CmmMachOp mo_wordNe  [e1, e2]
 cmmEqWord  e1 e2 = CmmMachOp mo_wordEq  [e1, e2]
 cmmULtWord e1 e2 = CmmMachOp mo_wordULt [e1, e2]
+cmmUGeWord e1 e2 = CmmMachOp mo_wordUGe [e1, e2]
 cmmUGtWord e1 e2 = CmmMachOp mo_wordUGt [e1, e2]
 
 cmmNegate :: CmmExpr -> CmmExpr
@@ -456,11 +457,18 @@ mk_switch tag_expr branches mb_deflt lo_tag hi_tag via_C
                                lo_tag (mid_tag-1) via_C
        ; hi_stmts <- mk_switch tag_expr' hi_branches mb_deflt 
                                mid_tag hi_tag via_C
-       ; lo_id <- forkCgStmts lo_stmts
-       ; let cond = cmmULtWord tag_expr' (CmmLit (mkIntCLit mid_tag))
-             branch_stmt = CmmCondBranch cond lo_id
-       ; return (assign_tag `consCgStmt` (branch_stmt `consCgStmt` hi_stmts)) 
+       ; hi_id <- forkCgStmts hi_stmts
+       ; let cond = cmmUGeWord tag_expr' (CmmLit (mkIntCLit mid_tag))
+             branch_stmt = CmmCondBranch cond hi_id
+       ; return (assign_tag `consCgStmt` (branch_stmt `consCgStmt` lo_stmts)) 
        }
+       -- we test (e >= mid_tag) rather than (e < mid_tag), because
+       -- the former works better when e is a comparison, and there
+       -- are two tags 0 & 1 (mid_tag == 1).  In this case, the code
+       -- generator can reduce the condition to e itself without
+       -- having to reverse the sense of the comparison: comparisons
+       -- can't always be easily reversed (eg. floating
+       -- pt. comparisons).
   where
     use_switch          = {- pprTrace "mk_switch" (
                        ppr tag_expr <+> text "n_tags:" <+> int n_tags <+>