Commit f220cc51 authored by simonmar's avatar simonmar
Browse files

[project @ 2004-09-15 11:45:24 by simonmar]

Add a couple of cases to mk_switch to catch the case when we have a
tag range that has a lot of default cases at either end, and we're not
using a single switch.  In situations like this we want to eliminate
the default cases with an if-statement, before dealing with the rest
of the branches, which might then be suitable for a switch.

Also, ignore empty tag slots at either end of the range if there is no
default case.

This might work around a gcc 2.95 bug that we tickled with the
code being generated before.
parent f19e3063
......@@ -45,7 +45,7 @@ import CLabel
import CmmUtils
import MachOp ( MachRep(..), wordRep, MachOp(..), MachHint(..),
mo_wordOr, mo_wordAnd, mo_wordNe, mo_wordEq,
mo_wordULt, machRepByteWidth )
mo_wordULt, mo_wordUGt, machRepByteWidth )
import ForeignCall ( CCallConv(..) )
import Literal ( Literal(..) )
import CLabel ( CLabel, mkStringLitLabel )
......@@ -57,6 +57,7 @@ import FastString ( LitString, FastString, unpackFS )
import Outputable
import DATA_BITS
import Maybe ( isNothing )
#include "../includes/ghcconfig.h"
-- For WORDS_BIGENDIAN
......@@ -153,7 +154,8 @@ cmmOrWord e1 e2 = CmmMachOp mo_wordOr [e1, e2]
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]
cmmULtWord e1 e2 = CmmMachOp mo_wordULt [e1, e2]
cmmUGtWord e1 e2 = CmmMachOp mo_wordUGt [e1, e2]
cmmNegate :: CmmExpr -> CmmExpr
cmmNegate (CmmLit (CmmInt n rep)) = CmmLit (CmmInt (-n) rep)
......@@ -408,6 +410,21 @@ mk_switch tag_expr branches mb_deflt lo_tag hi_tag
; return (oneCgStmt switch_stmt)
}
-- if we can knock off a bunch of default cases with one if, then do so
| Just deflt <- mb_deflt, (lowest_branch - lo_tag) >= n_branches
= do { (assign_tag, tag_expr') <- assignTemp' tag_expr
; let cond = cmmULtWord tag_expr' (CmmLit (mkIntCLit lowest_branch))
; stmtC (CmmCondBranch cond deflt)
; mk_switch tag_expr' branches mb_deflt lowest_branch hi_tag
}
| Just deflt <- mb_deflt, (hi_tag - highest_branch) >= n_branches
= do { (assign_tag, tag_expr') <- assignTemp' tag_expr
; let cond = cmmUGtWord tag_expr' (CmmLit (mkIntCLit highest_branch))
; stmtC (CmmCondBranch cond deflt)
; mk_switch tag_expr' branches mb_deflt lo_tag highest_branch
}
| otherwise -- Use an if-tree
= do { (assign_tag, tag_expr') <- assignTemp' tag_expr
-- To avoid duplication
......@@ -420,14 +437,31 @@ mk_switch tag_expr branches mb_deflt lo_tag hi_tag
}
where
use_switch = ASSERT( n_branches > 1 && n_tags > 1 )
{- pprTrace "mk_switch" (ppr tag_expr <+> text "n_tags: "
<+> int n_tags <+> text "dense: "
<+> int n_branches) $ -}
n_tags > 2 && (small || dense)
-- a 2-branch switch always turns into an if.
small = n_tags <= 4
dense = n_branches > (n_tags `div` 2)
exhaustive = n_tags == n_branches
n_tags = hi_tag - lo_tag + 1
n_branches = length branches
-- ignore default slots at each end of the range if there's
-- no default branch defined.
lowest_branch = fst (head branches)
highest_branch = fst (last branches)
real_lo_tag
| isNothing mb_deflt = lowest_branch
| otherwise = lo_tag
real_hi_tag
| isNothing mb_deflt = highest_branch
| otherwise = hi_tag
n_tags = real_hi_tag - real_lo_tag + 1
-- INVARIANT: Provided hi_tag > lo_tag (which is true)
-- lo_tag <= mid_tag < hi_tag
-- lo_branches have tags < mid_tag
......@@ -455,6 +489,9 @@ emitLitSwitch :: CmmExpr -- Tag to switch on
-- Used for general literals, whose size might not be a word,
-- where there is always a default case, and where we don't know
-- the range of values for certain. For simplicity we always generate a tree.
--
-- ToDo: for integers we could do better here, perhaps by generalising
-- mk_switch and using that. --SDM 15/09/2004
emitLitSwitch scrut [] deflt
= emitCgStmts deflt
emitLitSwitch scrut branches deflt_blk
......
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