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Re: reload getting the mode of (subreg:HI (reg:QI pseudo)) wrong


Bernd Schmidt <bernds_cb1@t-online.de> writes:

> Rask Ingemann Lambertsen wrote:
> > The constraints are not met because the constraint "d" is register class
> > DX_REGS consisting of register 4 and 5, and (reg:HI 5 dh) spans register 5
> > and 6. Also, HARD_REGNO_NREGS (5, HImode) returns 0.
> > The lreg and greg dumps provide the clue that reload got the mode
> > wrong:
> > (insn:HI 2485 2483 2486 39 libgcc2.c:1825 (set (subreg:HI (reg:QI
> > 178) 0)
> > 	(ashiftrt:HI (reg:HI 641)
> > 	    (const_int 15 [0xf]))) 31 {*ashrhi3_const15}
> > 		(insn_list:REG_DEP_TRUE 2480 (nil))
> >     (expr_list:REG_DEAD (reg:HI 641)
> > 	(nil)))
> 
> Probably the compiler doesn't in general like a paradoxical subreg
> that can take more hard regs than its SUBREG_REG.  I think this is
> probably something that can be worked around with a proper combination
> of MODES_TIEABLE_P, CANNOT_CHANGE_MODE_CLASS, and maybe
> REG_CANNOT_CHANGE_MODE_P.  What are your definitions of those macros?

This also suggests the possibility of writing a QImode version of the
insn which uses a scratch register.

Ian


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