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Re: Fix mode-switching problem for SH SJLJ exceptions
- From: Richard Sandiford <richard at codesourcery dot com>
- To: gcc-patches at gcc dot gnu dot org
- Date: Tue, 31 Jul 2007 09:20:22 +0100
- Subject: Re: Fix mode-switching problem for SH SJLJ exceptions
- References: <87myxruwx4.fsf@firetop.home>
Ping
Richard Sandiford <richard@codesourcery.com> writes:
> Functions that return a value usually end with the sequence:
>
> A: (clobber (reg H))
> ...
> B: (set (reg H) (reg P))
> ...
> C: (use (reg H))
>
> where H is the hard return register and P is the pseudo register
> to which return values are assigned.
>
> mode-switching.c uses a specially-split exit block to enforce MODE_EXIT
> (the mode required on exit from a function). The code that creates this
> block -- create_pre_exit -- tries to find insn B and starts the new block
> immediately before (or sometimes immediately after) it.
>
> However, B can be deleted if P is never initialised. In this case,
> create_pre_exit splits the block immediately before A instead.
> The code only searches for A and B in the fallthrough predecessor
> to the exit block; it assumes that P is never initialised if the
> block contains neither A nor B.
>
> In some cases B may be made up of several individual insns.
> create_pre_exit therefore records the range of modified registers
> and stops on the first SET that doesn't assign to part of H.
> It then asserts that it has found a SET or CLOBBER for the
> whole of H (except in some corner cases).
>
> This shows up a problem on SJLJ targets, where the call to the SJLJ
> unregister function is inserted between A and B. If B has been deleted,
> the first SET we find may be setting up the argument to that call.
> We then stop the search without having found either A or B, so the
> assertion triggers.
>
> At this point we have already searched past the call itself, and this
> seems bogus. We are supposed to enforce MODE_EXIT _after_ the last call.
> I therefore just treated searches that reach a call in the same way as
> searches that reach the beginning of a block.
>
> Bootstrapped & regression-tested on x86_64-linux-gnu. Also regression-tested
> on sh-elf (multilibs {,-m2,-m3,-m4,-m4/-ml}) and sh-wrs-vxworks. OK to
> install?
>
> Richard
>
>
> gcc/
> * mode-switching.c (create_pre_exit): Don't search past calls.
>
> Index: gcc/mode-switching.c
> ===================================================================
> --- gcc/mode-switching.c (revision 126715)
> +++ gcc/mode-switching.c (working copy)
> @@ -246,6 +246,17 @@ create_pre_exit (int n_entities, int *en
>
> if (INSN_P (return_copy))
> {
> + /* When using SJLJ exceptions, the call to the
> + unregister function is inserted between the
> + clobber of the return value and the copy.
> + We do not want to split the block before this
> + or any other call; if we have not found the
> + copy yet, the copy must have been deleted. */
> + if (CALL_P (return_copy))
> + {
> + short_block = 1;
> + break;
> + }
> return_copy_pat = PATTERN (return_copy);
> switch (GET_CODE (return_copy_pat))
> {