Commit 5ba806d8 authored by simonmar's avatar simonmar
Browse files

[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).
parent d2ebcf24
......@@ -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 <+>
......
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