On 2/27/2014 4:11 AM, Richard Sandiford 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.
Section 6.41 of the GCC manual has been rewritten. It has become:
6.41 How to Use Inline Assembly Language in C Code
6.41.1 Basic Asm - Assembler Instructions with No Operands
6.41.2 Extended Asm - Assembler Instructions with C Expression Operands
We could simply post the patch to GCC-patches and have at it, but I
think it's better to discuss the document here first. You can read it
at
http://www.LimeGreenSocks.com/gcc/Basic-Asm.html
http://www.LimeGreenSocks.com/gcc/Extended-Asm.html
http://www.LimeGreenSocks.com/gcc/extend04.zip (contains .texi, .patch,
and affected html pages)
All comments are very welcome.
Thanks for doing this, looks like a big improvement.
Thanks, I did my best. I appreciate you taking the time to review them.
A couple of comments:
The section on basic asms 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.
The "maintain their order" might be a bit misleading, since volatile asms
(including basic asms) must always be executed in the original order.
Maybe this was meaning placement/address order instead?
This statement is based on this text from the existing docs:
"Similarly, you can't expect a sequence of volatile |asm| instructions
to remain perfectly consecutive. If you want consecutive output, use a
single |asm|."
I do not dispute what you are saying. I just want to confirm that the
existing docs are incorrect before making a change. Also, see Andi's
response re -fno-toplevel-reorder.
It seems to me that recommending "single statement" is both the
clearest, and the safest approach here. But I'm prepared to change my
mind if there is consensus I should.
It might also be
worth mentioning that the number of instances of an asm in the output
may be different from the input. (Can it increase as well as decrease?
I'm not sure off-hand, but probably yes.)
So, in the volatile section, how about something like this for decrease:
"GCC does not delete a volatile |asm| if it is reachable, but may delete
it if it can prove that control flow never reaches the location of the
instruction."
For increase (not quite sure where to put this yet):
"Under certain circumstances, GCC may duplicate your asm code as part of
optimization. This can lead to unexpected duplicate symbol errors
during compilation if symbols or labels are being used. Using %= (see
Assembler Template) may help resolve this problem."
In the extended section:
Unless an output operand has the '&' constraint modifier (see
Modifiers), GCC may allocate it in the same register as an unrelated
input operand, [...]
It could also use it for addresses in other (memory) outputs.
Ok. But I'm not sure this really adds anything. Having warned people
that the register may be re-used unless '&' is used seems sufficient.
For:
When using asmSymbolicNames for the output operands, you may use these
names instead of digits.
it might be worth mentioning that you need the enclosing [...].
Done.
Thanks,
Richard
