Re: [Xen-devel] [PATCH-WIP 01/13] xen/arm: use r12 to pass the hypercall number to the hypervisor

[Nicolas Pitre <nicolas.pitre@xxxxxxxxxx>]

[ coming back from vacation and trying to catch up ]

On Wed, 29 Feb 2012, Dave Martin wrote:

> Just had a chat with some tools guys -- apparently, when passing register
> arguments to gcc inline asms there really isn't a guarantee that those
> variables will be in the expected registers on entry to the inline asm.
> 
> If gcc reorders other function calls or other code around the inline asm
> (which it can do, except under certain controlled situations), then
> intervening code can clobber any registers in general.

I'm hearing this argument about once every year or so for the last 8 
years.  I think that the tools people are getting confused between 
themselves as you may get a different interpretation of what gcc should 
do depending to whom you happen to talk to.

I did submit a bug to gcc in 2004 about this:

        http://gcc.gnu.org/bugzilla/show_bug.cgi?id=15089

You can see the confusion among gcc developers lurking there.

So let's quote the relevant gcc documentation:

-> * C Extensions::    GNU extensions to the C language family.
   -> * Explicit Reg Vars::   Defining variables residing in specified 
registers.

|GNU C allows you to put a few global variables into specified hardware 
|registers.  You can also specify the register in which an ordinary 
|register variable should be allocated.
|
|   * Global register variables reserve registers throughout the program.
|     This may be useful in programs such as programming language
|     interpreters which have a couple of global variables that are
|     accessed very often.
|
|   * Local register variables in specific registers do not reserve the
|     registers, except at the point where they are used as input or
|     output operands in an `asm' statement and the `asm' statement
|     itself is not deleted.  The compiler's data flow analysis is
|     capable of determining where the specified registers contain live
|     values, and where they are available for other uses.  Stores into
|     local register variables may be deleted when they appear to be
|     dead according to dataflow analysis.  References to local register
|     variables may be deleted or moved or simplified.
|
|     These local variables are sometimes convenient for use with the
|     extended `asm' feature (*note Extended Asm::), if you want to
|     write one output of the assembler instruction directly into a
|     particular register.  (This will work provided the register you
|     specify fits the constraints specified for that operand in the
|     `asm'.)

      -> * Local Reg Vars::

[...]

| Defining such a register variable does not reserve the register; it 
|remains available for other uses in places where flow control 
|determines the variable's value is not live.
|
| This option does not guarantee that GCC will generate code that has
|this variable in the register you specify at all times.  You may not
|code an explicit reference to this register in the _assembler
|instruction template_ part of an `asm' statement and assume it will
|always refer to this variable.  However, using the variable as an `asm'
|_operand_ guarantees that the specified register is used for the
|operand.

So, to me, the gcc documentation is perfectly clear on this topic.  
there really _is_ a guarantee that those asm marked variables will be in 
the expected registers on entry to the inline asm, given that the 
variable is _also_ listed as an operand to the asm statement.  But only 
in that case.

It is true that gcc may reorder other function calls or other code 
around the inline asm and then intervening code can clobber any 
registers.  Then it is up to gcc to preserve the variable's content 
elsewhere when its register is used for other purposes, and restore it 
when some inline asm statement is referring to it.

And if gcc does not do this then it is buggy.  Version 3.4.0 of gcc was 
buggy.  No other gcc versions in the last 7 years had such a problem or 
the __asmeq macro in the kernel would have told us.

> Or, to summarise another way, there is no way to control which register
> is used to pass something to an inline asm in general (often we get away
> with this, and there are a lot of inline asms in the kernel that assume
> it works, but the more you inline the more likely you are to get nasty
> surprises). 

This statement is therefore unfounded and wrong.  Please direct the 
tools guy who mislead you to the above gcc documentation.


Nicolas

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