From 2ee2956a633b795e8fdb06dc175115d5a80cfdd0 Mon Sep 17 00:00:00 2001 From: Richard Henderson Date: Wed, 6 Oct 1999 10:22:54 -0700 Subject: genconfig.c (main): Disable HAVE_conditional_arithmetic. * genconfig.c (main): Disable HAVE_conditional_arithmetic. * jump.c (jump_optimize_1): Document why. From-SVN: r29841 --- gcc/jump.c | 31 +++++++++++++++++++++++++++++++ 1 file changed, 31 insertions(+) (limited to 'gcc/jump.c') diff --git a/gcc/jump.c b/gcc/jump.c index dcd1a6a..f6e973e 100644 --- a/gcc/jump.c +++ b/gcc/jump.c @@ -943,6 +943,37 @@ jump_optimize_1 (f, cross_jump, noop_moves, after_regscan, mark_labels_only) #endif /* HAVE_cc0 */ #ifdef HAVE_conditional_arithmetic + /* ??? This is disabled in genconfig, as this simple-minded + transformation can incredibly lengthen register lifetimes. + + Consider this example from cexp.c's yyparse: + + 234 (set (pc) + (if_then_else (ne (reg:DI 149) (const_int 0 [0x0])) + (label_ref 248) (pc))) + 237 (set (reg/i:DI 0 $0) (const_int 1 [0x1])) + 239 (set (pc) (label_ref 2382)) + 248 (code_label ("yybackup")) + + This will be transformed to: + + 237 (set (reg/i:DI 0 $0) + (if_then_else:DI (eq (reg:DI 149) (const_int 0 [0x0])) + (const_int 1 [0x1]) (reg/i:DI 0 $0))) + 239 (set (pc) + (if_then_else (eq (reg:DI 149) (const_int 0 [0x0])) + (label_ref 2382) (pc))) + + which, from this narrow viewpoint looks fine. Except that + between this and 3 other ocurrences of the same pattern, $0 + is now live for basically the entire function, and we'll + get an abort in caller_save. + + Any replacement for this code should recall that a set of + a register that is not live need not, and indeed should not, + be conditionalized. Either that, or delay the transformation + until after register allocation. */ + /* See if this is a conditional jump around a small number of instructions that we can conditionalize. Don't do this before the initial CSE pass or after reload. -- cgit v1.1