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Re: RFA: fix mode confusion in caller-save.c:replace_reg_with_saved_mem


On 10/07/14 15:56, Joern Rennecke wrote:
On 7 October 2014 18:38, Jeff Law <law@redhat.com> wrote:
On 10/06/14 20:57, Joern Rennecke wrote:

On 6 October 2014 19:58, Jeff Law <law@redhat.com> wrote:

What makes word_mode special here?  ie, why is special casing for
word_mode
the right thing to do?


The patch does not special-case word mode.  The if condition tests if
smode would
cover multiple hard registers.
If that would be the case, smode is replaced with word_mode.

SO I'll ask another way.  Why do you want to change smode to word_mode?

Because SImode covers four hard registers, wheras the intention is to
have a single
one.

(concatn:SI [
                     (reg:SI 18 r18)
                     (reg:SI 19 r19)
                     (mem/c:QI (plus:HI (reg/f:HI 28 r28)
                             (const_int 43 [0x2b])) [6  S1 A8])
                     (mem/c:QI (plus:HI (reg/f:HI 28 r28)
                             (const_int 44 [0x2c])) [6  S1 A8])
                 ])

(see original post) is invalid RTL, and thuis the cause of the later ICE.
Thanks.  I see the problem now.

Dont we have an ever deeper problem here, namely that registers don't necessarily have to be word_mode sized objects. More likely than not in that case the register is going to be larger than word_mode, but I could also envision a target where there's registers smaller than word_mode.

This stuff seems rather broken/fragile in that world and using word_mode isn't necessarily any better than what we're doing today.


ISTM it would be better to find the mode of the same class that corresponds to GET_MODE_SIZE (mode) / nregs. In your case that's obviously QImode :-)

I think that still breaks in various ways, but probably significantly less so than what we're doing now or with your current patch.

jeff


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