> -----Original Message-----
> From: Ian Lance Taylor [mailto:i...@google.com]
> Sent: 01 May 2009 15:32
> To: Bingfeng Mei
> Cc: gcc@gcc.gnu.org; bernd.schm...@analog.com; weig...@de.ibm.com
> Subject: Re: Reload problem: asm operand requires impossible reload
>
> "Bingfeng Mei" <b...@broadcom.com> writes:
>
> > I experienced "asm operand requires impossible reload"
> error in our private porting.
> > After digging into the reloading pass, I found something a
> bit fishy.
> >
> > The error message is produced in reload_as_needed function
> (reload1.c)
> >
> > ...
> > /* If this was an ASM, make sure that all the reload insns
> > we have generated are valid. If not, give an error
> > and delete them. */
> > if (asm_noperands (PATTERN (insn)) >= 0)
> > for (p = NEXT_INSN (prev); p != next; p = NEXT_INSN (p))
> > if (p != insn && INSN_P (p)
> > && GET_CODE (PATTERN (p)) != USE
> > && (recog_memoized (p) < 0
> > || (extract_insn (p), !
> constrain_operands (1))))
> > {
> > error_for_asm (insn,
> > "%<asm%> operand requires "
> > "impossible reload");
> > delete_insn (p);
> > }
> > ...
> >
> > Here the code checks whether all generated reload insns are
> valid. The "strict"
> > argument of the constraint_oeprands is set to 1, thus any
> pseudo register operand
> > will fail the check if I understood correctly. However, at
> this stage, all the
> > generated reload instructions, including normal and asm
> insns, seems to have
> > pseudo register operands. Only later they are replaced by
> memory operand. If I
> > disable this piece of code, the code is compiled without problem.
> >
> > Did I misunderstand something here, or is it an possible
> bug? The code base is
> > GCC 4.4 branch.
>
> That code is only checking the insns which were created by
> emit_reload_insns, and after subst_reloads has run. Those
Yes, I understand that.
> instructions
> should not have any remaining references to pseudo-registers. If they
> do, it means that reload is generating instructions which refer to
> pseudo-registers, and that would be wrong. That check has been there
Actually, they do contain references to pseudo-registers. Only in following
code (reload function in reload1.c, after the reload_as_needed function
that emits error message), pseudo registers are replaced with requivalent
memory operands.
/* Now eliminate all pseudo regs by modifying them into
their equivalent memory references.
The REG-rtx's for the pseudos are modified in place,
so all insns that used to refer to them now refer to memory.
For a reg that has a reg_equiv_address, all those insns
were changed by reloading so that no insns refer to it any longer;
but the DECL_RTL of a variable decl may refer to it,
and if so this causes the debugging info to mention the variable. */
for (i = FIRST_PSEUDO_REGISTER; i < max_regno; i++)
{
rtx addr = 0;
if (reg_equiv_mem[i])
addr = XEXP (reg_equiv_mem[i], 0);
if (reg_equiv_address[i])
addr = reg_equiv_address[i];
if (addr)
{
if (reg_renumber[i] < 0)
{
rtx reg = regno_reg_rtx[i];
REG_USERVAR_P (reg) = 0;
PUT_CODE (reg, MEM);
XEXP (reg, 0) = addr;
if (reg_equiv_memory_loc[i])
MEM_COPY_ATTRIBUTES (reg, reg_equiv_memory_loc[i]);
else
{
MEM_IN_STRUCT_P (reg) = MEM_SCALAR_P (reg) = 0;
MEM_ATTRS (reg) = 0;
}
MEM_NOTRAP_P (reg) = 1;
}
else if (reg_equiv_mem[i])
XEXP (reg_equiv_mem[i], 0) = addr;
}
}
> for several releases, so I don't think it is obviously buggy.
>
> If this is still puzzling we may need to see an example.
>
I will try to produce an example for other targets.
> Ian
>
>
