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Re: Paradoxical subreg reload issue

02/05/2012 21:36, Eric Botcazou :
>> I have an issue (gcc 4.6.3, private bacakend) when reloading operands of
>> this insn:
>> (set (subreg:SI (reg:QI 21 [ iftmp.1 ]) 0)
>>      (lshiftrt:SI (reg/v:SI 24 [ w ]) (const_int 31 [0x1f]))
>> The register 21 is reloaded into
>> (reg:QI 0 r0 [orig:21 iftmp.1 ] [21]), which is a HI-wide hw register.
>> Since it is a BIG_ENDIAN target, the SI subreg regno is then -1.
>> Note that word_mode is SImode, whereas the class r0 belongs to is
>> HI-wide. I don't know if this matters when reloading.
>> I have no idea how to debug this, if it is a backend or a reload bug.
> RA/reload is known to have issues with word-mode paradoxical subregs on 
> big-endian machines.  For example, on SPARC 64-bit, we run into similar 
> problems for FP regs, which are 32-bit.  Likewise on HP-PA 64-bit I think.
> So we have kludges in the back-end:
> /* Defines invalid mode changes.  Borrowed from the PA port.
>    SImode loads to floating-point registers are not zero-extended.
>    The definition for LOAD_EXTEND_OP specifies that integer loads
>    narrower than BITS_PER_WORD will be zero-extended.  As a result,
>    we inhibit changes from SImode unless they are to a mode that is
>    identical in size.
>    Likewise for SFmode, since word-mode paradoxical subregs are
>    problematic on big-endian architectures.  */
>   (TARGET_ARCH64						\
>    && GET_MODE_SIZE (FROM) == 4					\
>    && GET_MODE_SIZE (TO) != 4					\
>    ? reg_classes_intersect_p (CLASS, FP_REGS) : 0)

I modified CANNOT_CHANGE_MODE_CLASS as you suggested. But strange as it
may seem, it has no effect on such a reload, and I can't find a way to
make it work...

BTW, has this bug already been filed?
Do you have an idea how deep is this bug in the reload, how complex it
is and which part in the reload it is related to?


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