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Re: [PATCH][RFC] fix reload causing ICE in subreg_get_info on m68k (PR58369)


On 09/28/13 09:30, Mikael Pettersson wrote:
This patch fixes PR58369, an ICE in subreg_get_info when compiling
boost for m68k-linux.

choose_reload_regs attempts to reload a DFmode (8-byte) reg, finds
an XFmode (12-byte) reg in "last_reg", and calls subreg_regno_offset
with these two modes and a subreg offset of zero.  However, this is
not a correct lowpart subreg offset for big-endian and these two modes,
so the lowpart subreg check in subreg_get_info fails, and the code
continues to

     gcc_assert ((GET_MODE_SIZE (xmode) % GET_MODE_SIZE (ymode)) == 0);

which fails because (12 % 8) != 0.

choose_reload_regs passes the constant zero, in all cases where the reg
isn't already a subreg, as the subreg offset to subreg_regno_offset, even
though lowpart subregs on big-endian targets require an explicit offset
computation.  I think that is a bug.

I believe other big-endian targets don't see this ICE because
a) they define CANNOT_CHANGE_MODE_CLASS to reject differently-sized
    modes in floating-point registers (which prevents this path in
    choose_reload_regs), or
b) their differently-sized modes are such that the size of a larger
    mode is a whole multiple of the size of the smaller mode (which
    allows the gcc_assert above to pass).

This patch changes choose_reload_regs to call subreg_lowpart_offset
to pass an endian-correct offset to subreg_regno_offset, except where
the offset comes from a pre-existing subreg.

[Defining CANNOT_CHANGE_MODE_CLASS appropriately for m68k also fixes
the ICE, but I don't think the m68k backend really wants that, and I
think it just papers over a generic bug.]

Tested with trunk and 4.8 on {m68k,sparc64,powerpc64}-linux (big-endian),
and on x86_64-linux/armv5tel-linux-gnueabi (little-endian).  No regressions.

Comments?
Is this Ok for trunk?

gcc/

2013-09-28  Mikael Pettersson  <mikpelinux@gmail.com>

	PR rtl-optimization/58369
	* reload1.c (choose_reload_regs): Use subreg_lowpart_offset
	to pass endian-correct lowpart offset to subreg_regno_offset.
Thanks Mikael. My only concern is the lack of adjustment when the value found was already a SUBREG.

ie, let's assume rld[r].in_reg was something like
(subreg:XF (reg:DF) 0)

and our target is (reg:DF)

In this case it seems to me we still want to compute the subreg offset, right?

jeff


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