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extend.texi: Improve documentation of volatile asms.

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* extend.texi: Improve documentation of volatile asms.

	* c-tree.texi: Fix thinko.
	* gcc.texi: Likewise.

Co-Authored-By: Richard Henderson <rth@redhat.com>

From-SVN: r41712
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Mark Mitchell 2001-05-01 00:39:59 +00:00 committed by Mark Mitchell
parent d72040f508
commit e71b34aa4e
4 changed files with 44 additions and 20 deletions
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10
gcc/ChangeLog
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@ -1,3 +1,13 @@
2001-04-30 Mark Mitchell <mark@codesourcery.com>
Richard Henderson <rth@redhat.com>
* extend.texi: Improve documentation of volatile asms.
2001-04-30 Mark Mitchell <mark@codesourcery.com>
* c-tree.texi: Fix thinko.
* gcc.texi: Likewise.
2001-04-30 Richard Henderson <rth@redhat.com>
* c-common.c (c_promoting_integer_type_p): New function, from the

9
gcc/c-tree.texi
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@ -32,11 +32,10 @@ Copyright @copyright{} 1999, 2000, 2001 Free Software Foundation, Inc.
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.1 or
any later version published by the Free Software Foundation; with the
Invariant Sections being ``GNU General Public License'', the Front-Cover
texts being (a) (see below), and with the Back-Cover Texts being (b)
(see below). A copy of the license is included in the section entitled
``GNU Free Documentation License''.
any later version published by the Free Software Foundation; with no
invariant sections, the Front-Cover texts being (a) (see below), and
with the Back-Cover Texts being (b) (see below). A copy of the license
is included in the section entitled ``GNU Free Documentation License''.
(a) The FSF's Front-Cover Text is:

37
gcc/extend.texi
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@ -3052,23 +3052,38 @@ the @code{asm}. For example:
@noindent
If you write an @code{asm} instruction with no outputs, GNU CC will know
the instruction has side-effects and will not delete the instruction or
move it outside of loops. If the side-effects of your instruction are
not purely external, but will affect variables in your program in ways
other than reading the inputs and clobbering the specified registers or
memory, you should write the @code{volatile} keyword to prevent future
versions of GNU CC from moving the instruction around within a core
region.
move it outside of loops.
An @code{asm} instruction without any operands or clobbers (an ``old
style'' @code{asm}) will not be deleted or moved significantly,
regardless, unless it is unreachable, the same way as if you had
written a @code{volatile} keyword.
The @code{volatile} keyword indicates that the instruction has
important side-effects. GCC will not delete a volatile @code{asm} if
it is reachable. (The instruction can still be deleted if GCC can
prove that control-flow will never reach the location of the
instruction.) In addition, GCC will not reschedule instructions
across a volatile @code{asm} instruction. For example:
@example
(volatile int *)addr = foo;
asm volatile ("eieio" : : );
@end example
@noindent
Assume @code{addr} contains the address of a memory mapped device
register. The PowerPC @code{eieio} instruction (Enforce In-order
Execution of I/O) tells the cpu to make sure that the store to that
device register happens before it issues any other I/O.
Note that even a volatile @code{asm} instruction can be moved in ways
that appear insignificant to the compiler, such as across jump
instructions. You can't expect a sequence of volatile @code{asm}
instructions to remain perfectly consecutive. If you want consecutive
output, use a single @code{asm}.
output, use a single @code{asm}. Also, GCC will perform some
optimizations across a volatile @code{asm} instruction; GCC does not
``forget everything'' when it encounters a volatile @code{asm}
instruction the way some other compilers do.
An @code{asm} instruction without any operands or clobbers (an ``old
style'' @code{asm}) will be treated identically to a volatile
@code{asm} instruction.
It is a natural idea to look for a way to give access to the condition
code left by the assembler instruction. However, when we attempted to

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gcc/gcc.texi
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@ -125,10 +125,10 @@ Copyright (C) 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998,
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.1 or
any later version published by the Free Software Foundation; with the
Invariant Sections being ``GNU General Public License'', the Front-Cover
texts being (a) (see below), and with the Back-Cover Texts being (b)
(see below). A copy of the license is included in the section entitled
``GNU Free Documentation License''.
Invariant Sections being ``GNU General Public License'' and ``Funding
Free Software'', the Front-Cover texts being (a) (see below), and with
the Back-Cover Texts being (b) (see below). A copy of the license is
included in the section entitled ``GNU Free Documentation License''.
(a) The FSF's Front-Cover Text is: