On 2/27/2014 9:35 AM, Andi Kleen wrote:
Andrew Haley <a...@redhat.com> writes:
Over the years there has been a great deal of traffic on these lists
caused by misunderstandings of GCC's inline assembler. That's partly
because it's inherently tricky, but the existing documentation needs
to be improved.
dw <limegreenso...@yahoo.com> has done a fairly thorough reworking of
the documentation. I've helped a bit.
It would be nice if you could include some discussion of the LTO
reference problems.
Something like:
It is not legal to reference a static variable or function symbol from
the assembler code, as the compiler may optimize unused symbols
away. For inline asm in functions these should be referred as "m" input
arguments. For top level asm the referenced symbol should be made global
and marked with __attribute__((externally_visible)) .
What would you say to something like this:
"Since GCC does not parse the asm, it has no visibility of any static
variables or functions it references. This may result in those symbols
getting discarded by GCC as unused. To avoid this problem, list the
symbols as inputs or outputs."
And another common problem:
For top level asm there is no guarantee the compiler outputs the
statements in order.
Well, basic asm (which is the only thing you can use at top level)
already says:
"Do not expect a sequence of |asm| statements to remain perfectly
consecutive after compilation. To ensure that assembler instructions
maintain their order, use a single |asm| statement containing multiple
instructions. Note that GCC's optimizer can move |asm| statements
relative to other code, including across jumps. "
Is something more needed?
[unless -fno-toplevel-reorder is specified, but I'm not should mention that]
-Andi
