Patch to fix emit_insn_before

[John Wehle]

> On Wed, Jan 12, 2000 at 02:27:01AM -0500, John Wehle wrote:
> > This patch fixes a problem which occurs when adding a sequence
> > to the beginning of a basic block.  Currently the last insn of
> > the sequence is returned which causes problems when updating
> > the basic block head (it causes the other insns in the sequence
> > to be stranded between basic blocks).
> 
> Who, exactly, is depending on this?

emit_block_insn_before, emit_swap_insn, scan_one_insn,
duplicate_loop_exit_test (emit_jump_insn_before calls emit_insn_before)
though it appears to assume that only one insn is emitted (based on
how reg_scan_update is used), and the code currently in insert_insn_end_bb.

I saw the failure on PA-RISC when compute_transp core dumped due to
bb = BLOCK_NUM (r->insn) referencing a NULL pointer while compiling
new1.cc.  The problem was triggered by an unrelated change which
caused compute_bb_for_insn to be called after a sequence had
been added by insert_insn_end_bb to the beginning of a block.  This
cleared the block information that insert_insn_end_bb had established
by calling set_block_num for each insn in the sequence.

For what it's worth (and I realize isn't a lot :-)  The patch
passed bootstrap and check on powerpc, sparc, and x86 in addition
to preventing the failure on the PA-RISC.  I haven't actually
completed a bootstrap and check on PA-RISC though it's in progress
(it takes a couple of days due to the machine being very slow).

> We've always returned
> the last insn of the sequence.  Changing this would mean
> auditing all existing uses of emit_insn_before.

Many of the places appear to be concerned with only one insn.  I
just looked again, and one of the places I'm not sure about is
check_dbra_loop.  I do probably need to change
delete_null_pointer_checks_1 to do emit_barrier_before (last_insn)
instead of emit_barrier_after (new_jump).

I certainly understand your concern regarding this change and am
open to suggestions.

-- John
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