Disjoint address register problem during GCC port

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Disjoint address register problem during GCC port
\ Manish Verma (16 Apr 2008)

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Subject:	Disjoint address register problem during GCC port
Group:	Gcc
From:	Manish Verma
Date:	16 Apr 2008

Hi all,

I am trying to port GCC to a new DSP architecture (hopefully will be
made public soon) and I am having a little bit of difficulty in making
GCC to generate code. The reason is that our DSP architecture has
non-orthogonal and segmented (disjoint) address register files (e.g.
A, B and D). Also, all arithmetic operations access memory through
indirect addressing and have restrictions on which register files can
be used as which operand or destination. In addition, they support
offset addressing and post-increment and post-decrement modes.

For example, MULT <dst> <src1> <src2> operation have the restriction
that <dst> must be a register from REG_D. The operand <src1> must be a
register from REG_A and operand <src2> from REG_B and vice-versa.

I have defined three register classes (REG_D={d0, ..., d3}, REG_A,
REG_B), set up BASE_REGISTER_CLASS appropriately to make GCC generate
the code of the following form:

MULT [d0], [a0], [b0] // d[i] = a[i] * b[i]

However, I am having problems for GCC to generate the code of the
following form:

MULT [d0], [a0+1], [b0+2] // d[i] = a[i+1] * b[i+2]

I am using EXTRA_CONSTRAINT to segregate address registers according
to the address classes.
#define EXTRA_CONSTRAINT(OP, C) \
( (C) == 'Q' ? pica_A_constraint(op,c) \
: (C) == 'R' ? pica_B_constraint(op,c) \
: (C) == 'S' ? pica_D_constraint(op,c) \
: 0)

I have also defined EXTRA_MEMORY_CONSTRAINT appropriately. However, I
keep on getting "error: impossible constraint in 'asm'" error. Any
help on this aspect of GCC would be very much appreciated.

On this issue, I also would like to ask:
(a) Is it possible to have segmented (disjoint) address classes in GCC?
(b) I am using GCC 4.0.2 and I noticed that in the gcc version 4.3.0
has EXTRA_CONSTRAINT is marked as obsolete. Should I use
EXTRA_CONSTRAINT or should I use some other way of specifying an
address register class?

Regards,
Manish

PS: Thanks Andrew for directing me to the right mailing list.
---------------------------------------------------------
Manish Verma
Altera European Technology Centre,
High Wycombe, UK