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author | Andreas Jaeger <aj@suse.de> | 2004-01-21 18:26:08 +0100 |
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committer | Andreas Jaeger <aj@gcc.gnu.org> | 2004-01-21 18:26:08 +0100 |
commit | bbf5a54d985232333942f49ff89d35e6debd7e8c (patch) | |
tree | 057b9f125772c207a2ba0f4d03de52e7e037ea67 /gcc | |
parent | bfccaa6f7006c6f1a56f6b8e4194583e8cddd2f4 (diff) | |
download | gcc-bbf5a54d985232333942f49ff89d35e6debd7e8c.zip gcc-bbf5a54d985232333942f49ff89d35e6debd7e8c.tar.gz gcc-bbf5a54d985232333942f49ff89d35e6debd7e8c.tar.bz2 |
extend.texi (Extended Asm): Clarify memory clobber.
2004-01-21 Andreas Jaeger <aj@suse.de>
Michael Matz <matz@suse.de>
* doc/extend.texi (Extended Asm): Clarify memory clobber.
Co-Authored-By: Michael Matz <matz@suse.de>
From-SVN: r76288
Diffstat (limited to 'gcc')
-rw-r--r-- | gcc/ChangeLog | 5 | ||||
-rw-r--r-- | gcc/doc/extend.texi | 34 |
2 files changed, 33 insertions, 6 deletions
diff --git a/gcc/ChangeLog b/gcc/ChangeLog index 67a1f8e..2cdc83e1 100644 --- a/gcc/ChangeLog +++ b/gcc/ChangeLog @@ -1,3 +1,8 @@ +2004-01-21 Andreas Jaeger <aj@suse.de> + Michael Matz <matz@suse.de> + + * doc/extend.texi (Extended Asm): Clarify memory clobber. + 2004-01-21 Jakub Jelinek <jakub@redhat.com> * crtstuff.c (frame_dummy, __do_global_ctors_1): Call diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi index 0eed679..e2bfd93 100644 --- a/gcc/doc/extend.texi +++ b/gcc/doc/extend.texi @@ -3917,13 +3917,35 @@ represents the condition codes as a specific hardware register; condition code is handled differently, and specifying @samp{cc} has no effect. But it is valid no matter what the machine. -If your assembler instruction modifies memory in an unpredictable +If your assembler instructions access memory in an unpredictable fashion, add @samp{memory} to the list of clobbered registers. This -will cause GCC to not keep memory values cached in registers across -the assembler instruction. You will also want to add the -@code{volatile} keyword if the memory affected is not listed in the -inputs or outputs of the @code{asm}, as the @samp{memory} clobber does -not count as a side-effect of the @code{asm}. +will cause GCC to not keep memory values cached in registers across the +assembler instruction and not optimize stores or loads to that memory. +You will also want to add the @code{volatile} keyword if the memory +affected is not listed in the inputs or outputs of the @code{asm}, as +the @samp{memory} clobber does not count as a side-effect of the +@code{asm}. If you know how large the accessed memory is, you can add +it as input or output but if this is not known, you should add +@samp{memory}. As an example, if you access ten bytes of a string, you +can use a memory input like: + +@example +@{"m"( (@{ struct @{ char x[10]; @} *p = (void *)ptr ; *p; @}) )@}. +@end example + +Note that in the following example the memory input is necessary, +otherwise GCC might optimize the store to @code{x} away: +@example +int foo () +@{ + int x = 42; + int *y = &x; + int result; + asm ("magic stuff accessing an 'int' pointed to by '%1'" + "=&d" (r) : "a" (y), "m" (*y)); + return result; +@} +@end example You can put multiple assembler instructions together in a single @code{asm} template, separated by the characters normally used in assembly |