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Re: Resend: [PATCH] [MIPS] Fix asm constraints for 'ins' instructions.

To: David Daney <ddaney@avtrex.com>, Ralf Baechle <ralf@linux-mips.org>, GCC Mailing List <gcc@gcc.gnu.org>, MIPS Linux List <linux-mips@linux-mips.org>, rdsandiford@googlemail.com
Subject: Re: Resend: [PATCH] [MIPS] Fix asm constraints for 'ins' instructions.
From: David Daney <ddaney@avtrex.com>
Date: Thu, 12 Jun 2008 13:10:10 -0700
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Richard Sandiford wrote:
David Daney <ddaney@avtrex.com> writes:
Richard Sandiford wrote:
David Daney <ddaney@avtrex.com> writes:
Ralf Baechle wrote:
On Wed, Jun 11, 2008 at 10:04:25AM -0700, David Daney wrote:

The third operand to 'ins' must be a constant int, not a register.

Signed-off-by: David Daney <ddaney@avtrex.com>
---
include/asm-mips/bitops.h |    6 +++---
1 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/include/asm-mips/bitops.h b/include/asm-mips/bitops.h
index 6427247..9a7274b 100644
--- a/include/asm-mips/bitops.h
+++ b/include/asm-mips/bitops.h
@@ -82,7 +82,7 @@ static inline void set_bit(unsigned long nr, volatile 
unsigned long *addr)
                "2:        b       1b                                      \n"
                "  .previous                                       \n"
                : "=&r" (temp), "=m" (*m)
-               : "ir" (bit), "m" (*m), "r" (~0));
+               : "i" (bit), "m" (*m), "r" (~0));
#endif /* CONFIG_CPU_MIPSR2 */
        } else if (cpu_has_llsc) {
                __asm__ __volatile__(
An old trick to get gcc to do the right thing.  Basically at the stage when
gcc is verifying the constraints it may not yet know that it can optimize
things into an "i" argument, so compilation may fail if "r" isn't in the
constraints.  However we happen to know that due to the way the code is
written gcc will always be able to make use of the "i" constraint so no
code using "r" should ever be created.

The trick is a bit ugly; I think it was used first in asm-i386/io.h ages ago
and I would be happy if we could get rid of it without creating new problems.
Maybe a gcc hacker here can tell more?
It is not nice to lie to GCC.

CCing GCC and Richard in hopes that a wider audience may shed some light on the 
issue.
You _might_ be able to use "i#r" instead of "ri", but I wouldn't
really recommend it.  Even if it works now, I don't think there's
any guarantee it will in future.

There are tricks you could pull to detect the problem at compile time
rather than assembly time, but that's probably not a big win.  And again,
I wouldn't recommend them.

I'm not saying anything you don't know here, but if the argument is
always a syntactic constant, the safest bet would be to apply David's
patch and also convert the function into a macro.  I notice some other
ports use macros rather than inline functions here.  I assume you've
deliberately rejected macros as being too ugly though.
I am still a little unclear on this.

To restate the question:

static inline void f(unsigned nr, unsigned *p)
{
  unsigned short bit = nr & 5;

  if (__builtin_constant_p(bit)) {
    __asm__ __volatile__ ("  foo %0, %1" : "=m" (*p) : "i" (bit));
  }
  else {
    // Do something else.
  }
}
.
.
.
  f(3, some_pointer);
.
.
.

Among the versions of GCC that can build the current kernel, will any
fail on this code because the "i" constraint cannot be matched when
expanded to RTL?

Someone will point this out if I don't, so for avoidance of doubt:
this needs to be always_inline.  It also isn't guaranteed to work
with "bit" being a separate statement.  I'm not truly sure it's
guaranteed to work even with:

    __asm__ __volatile__ ("  foo %0, %1" : "=m" (*p) : "i" (nr & 5));

but I think we'd try hard to make sure it does.

I think Maciej said that 3.2 was the minimum current version.
Even with those two issues sorted out, I don't think you can
rely on this sort of thing with compilers that used RTL inlining.
(always_inline does go back to 3.2, in case you're wondering.)


Well I withdraw the patch.  With the current kernel code we seem to always get 
good code generation.  In the event that the compiler tries to put the shift 
amount (nr) in a register, the assembler will complain.  I don't think it is 
possible to generate bad object code, so best to leave it alone.

FYI, the reason that I stumbled on this several weeks ago is that 
if(__builtin_constant_p(nr)) in the trunk compiler was generating code for the 
asm even though nr was not constant.

David Daney

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