Re: [Tinycc-devel] [PATCH] deprecating VT_REF

[Edmund Grimley Evans]

Currently I find that whenever I reread the descriptions, in tcc-doc
and tcc.h, of the various VT values I find myself understanding how
they work worse afterwards than before. I'm not sure how the
descriptions achieve this level of confusiveness; I think it's perhaps
because of the way the descriptions mix source and target, meaning and
use, and how a term like "lvalue" sometimes means the pointer and
sometimes the thing pointed to, and how the terms "lvalue", "pointer"
and "reference" are used without any clear distinction.

I don't know that I can do any better, but here's an attempt. Perhaps
someone can suggest improvements.


The r (and r2) fields of SValue record how and where a value is
stored. The value or r is one of the following base values possibly
bitwise ORed with some of the following flags.

# Base values

A register number, from 0 to VT_CONST - 1.
The value is in the register.

VT_CONST
The value is a constant, stored in SValue.c.

VT_LOCAL
The value is an offset on the stack, typically the address of a local
variable. The offset is stored in SValue.c.i.

VT_LLOCAL
The same as VT_LOCAL but with another level of indirection. VT_LLOCAL
is only used together with VT_LVAL to mean that the value is pointed
to by an address on the stack. This typically arises when a (register
| VT_LVAL) has to be saved to the stack, but it also can come from an
architecture-specific calling convention.

VT_CMP
The value is 0 or 1 and represented in the processor flags. The way it
is represented is indicated in SValue.c.i.

VT_JMP
VT_JMPI
...

# Flags

VT_LVAL
Add one level of indirection. So (register | VT_LVAL) means the value
is pointed to by an address in a register, and (VT_LOCAL | VT_LVAL)
means the value is pointed to by an offset on the stack, in other
words, that the value is on the stack, for example the value of a
local variable.

VT_REF
Add another level of indirection. So (VT_LOCAL | VT_LVAL | VT_REF)
means the value is pointed to by a value on the stack. VT_REF is only
used together with VT_LVAL, and currently it is only used together
with VT_LOCAL and only when the value is a structure. VT_REF must not
be used together with VT_LLOCAL. (VT_LOCAL | VT_LVAL | VT_REF) means
the same as (VT_LLOCAL | VT_LVAL) but the two are currently used in
different parts of the code.

VT_SYM
The symbol SValue.sym must be added to the value. VT_SYM is only used
together with VT_CONST.

VT_MUSTCAST
Used together with a register value to indicate that an integer stored
in a register must be cast to the value type if the value is used
(lazy casting).

VT_LVAL_BYTE
VT_LVAL_SHORT
VT_LVAL_UNSIGNED
I don't properly understand these. The current description is: "if the
lvalue has an integer type then these flags give its real type. The
type alone is not enough in case of cast optimisations." Presumably
"the lvalue" means "the value, which is stored in memory", but what
does "real type" mean? It sounds very philosophical. Perhaps the
description ought to be something like: When VT_LVAL is set, these
flags must also be set appropriately to indicate the size and sign of
the value stored in memory. Who knows?