OpenE2K
/
gcc
2
0
Fork 0
Code Issues Pull Requests Projects Releases Wiki Activity

rtl.texi: Updated subreg section. 

2008-06-19  Kenneth Zadeck <zadeck@naturalbridge.com>

	* doc/rtl.texi: Updated subreg section.

From-SVN: r136978
This commit is contained in:
Kenneth Zadeck 2008-06-20 03:26:10 +00:00 committed by Kenneth Zadeck
parent 2368a460c8
commit 1ba4e523d1
2 changed files with 67 additions and 16 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

4
gcc/ChangeLog
View File

@ -1,3 +1,7 @@
2008-06-19 Kenneth Zadeck <zadeck@naturalbridge.com>
* doc/rtl.texi: Updated subreg section.
2008-06-19 Jakub Jelinek <jakub@redhat.com>
PR c++/36523

79
gcc/doc/rtl.texi
View File

@ -1747,12 +1747,45 @@ mode other than its natural one, or to refer to one register of
a multi-part @code{reg} that actually refers to several registers.
Each pseudo register has a natural mode. If it is necessary to
operate on it in a different mode, the pseudo register must be
operate on it in a different mode, the register must be
enclosed in a @code{subreg}.
There are currently three supported types for the first operand of a
@code{subreg}:
@itemize
@item pseudo registers
This is the most common case. Most @code{subreg}s have pseudo
@code{reg}s as their first operand.
@item mem
@code{subreg}s of @code{mem} were common in earlier versions of GCC and
are still supported. During the reload pass these are replaced by plain
@code{mem}s. On machines that do not do instruction scheduling, use of
@code{subreg}s of @code{mem} are still used, but this is no longer
recommended. Such @code{subreg}s are considered to be
@code{register_operand}s rather than @code{memory_operand}s before and
during reload. Because of this, the scheduling passes cannot properly
schedule instructions with @code{subreg}s of @code{mem}, so for machines
that do scheduling, @code{subreg}s of @code{mem} should never be used.
To support this, the combine and recog passes have explicit code to
inhibit the creation of @code{subreg}s of @code{mem} when
@code{INSN_SCHEDULING} is defined.
The use of @code{subreg}s of @code{mem} after the reload pass is an area
that is not well understood and should be avoided. There is still some
code in the compiler to support this, but this code has possibly rotted.
This use of @code{subreg}s is discouraged and will most likely not be
supported in the future.
@item hard registers
It is seldom necessary to wrap hard registers in @code{subreg}s; such
registers would normally reduce to a single @code{reg} rtx. This use of
@code{subregs} is discouraged and may not be supported in the future.
@code{subreg}s is discouraged and may not be supported in the future.
@end itemize
@code{subreg}s of @code{subreg}s are not supported. Using
@code{simplify_gen_subreg} is the recommended way to avoid this problem.
@code{subreg}s come in two distinct flavors, each having its own
usage and rules:
@ -1768,10 +1801,26 @@ GET_MODE_SIZE (@var{m1}) > GET_MODE_SIZE (@var{m2})
@end smallexample
Paradoxical @code{subreg}s can be used as both lvalues and rvalues.
When used as an lvalue, the low-order bits of the source value
are stored in @var{reg} and the high-order bits are discarded.
When used as an rvalue, the low-order bits of the @code{subreg} are
taken from @var{reg} while the high-order bits are left undefined.
When used as an lvalue, the low-order bits of the source value are
stored in @var{reg} and the high-order bits are discarded.
taken from @var{reg} while the high-order bits may or may not be
defined.
The high-order bits of rvalues are in the following circumstances:
@itemize
@item @code{subreg}s of @code{mem}
When @var{m2} is smaller than a word, the macro @code{LOAD_EXTEND_OP},
can control how the high-order bits are defined.
@item @code{subreg} of @code{reg}s
The upper bits are defined when @code{SUBREG_PROMOTED_VAR_P} is true.
@code{SUBREG_PROMOTED_UNSIGNED_P} describes what the upper bits hold.
Such subregs usually represent local variables, register variables
and parameter pseudo variables that have been promoted to a wider mode.
@end itemize
@var{bytenum} is always zero for a paradoxical @code{subreg}, even on
big-endian targets.
@ -1789,18 +1838,19 @@ stores the lower 2 bytes of @var{y} in @var{x} and discards the upper
(set @var{z} (subreg:SI (reg:HI @var{x}) 0))
@end smallexample
would set the lower two bytes of @var{z} to @var{y} and set the upper two
bytes to an unknown value.
would set the lower two bytes of @var{z} to @var{y} and set the upper
two bytes to an unknown value assuming @code{SUBREG_PROMOTED_VAR_P} is
false.
@item Normal subregs
When @var{m1} is at least as narrow as @var{m2} the @code{subreg}
expression is called @dfn{normal}.
Normal @code{subreg}s restrict consideration to certain bits of @var{reg}.
There are two cases. If @var{m1} is smaller than a word, the
@code{subreg} refers to the least-significant part (or @dfn{lowpart})
of one word of @var{reg}. If @var{m1} is word-sized or greater, the
@code{subreg} refers to one or more complete words.
Normal @code{subreg}s restrict consideration to certain bits of
@var{reg}. There are two cases. If @var{m1} is smaller than a word,
the @code{subreg} refers to the least-significant part (or
@dfn{lowpart}) of one word of @var{reg}. If @var{m1} is word-sized or
greater, the @code{subreg} refers to one or more complete words.
When used as an lvalue, @code{subreg} is a word-based accessor.
Storing to a @code{subreg} modifies all the words of @var{reg} that
@ -1917,11 +1967,8 @@ The first operand of a @code{subreg} expression is customarily accessed
with the @code{SUBREG_REG} macro and the second operand is customarily
accessed with the @code{SUBREG_BYTE} macro.
@code{subreg}s of @code{subreg}s are not supported. Using
@code{simplify_gen_subreg} is the recommended way to avoid this problem.
It has been several years since a platform in which
@code{BYTES_BIG_ENDIAN} was not equal to @code{WORDS_BIG_ENDIAN} has
@code{BYTES_BIG_ENDIAN} not equal to @code{WORDS_BIG_ENDIAN} has
been tested. Anyone wishing to support such a platform in the future
may be confronted with code rot.