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Re: basic asm and memory clobbers - Proposed solution
- From: Bernd Edlinger <bernd dot edlinger at hotmail dot de>
- To: Andrew Haley <aph at redhat dot com>, David Wohlferd <dw at LimeGreenSocks dot com>, Jeff Law <law at redhat dot com>, Joseph Myers <joseph at codesourcery dot com>, "rth at redhat dot com" <rth at redhat dot com>
- Cc: Richard Earnshaw <Richard dot Earnshaw at foss dot arm dot com>, "gcc at gcc dot gnu dot org" <gcc at gcc dot gnu dot org>, "Paul_Koning at Dell dot com" <Paul_Koning at Dell dot com>, "jakub at redhat dot com" <jakub at redhat dot com>, "rth at gcc dot gnu dot org" <rth at gcc dot gnu dot org>, "pinskia at gcc dot gnu dot org" <pinskia at gcc dot gnu dot org>, Segher Boessenkool <segher at kernel dot crashing dot org>, Ian Lance Taylor <iant at google dot com>, "Sandra Loosemore" <sandra at codesourcery dot com>, Hans-Peter Nilsson <hp at bitrange dot com>
- Date: Sun, 13 Dec 2015 09:25:06 +0000
- Subject: Re: basic asm and memory clobbers - Proposed solution
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On 12.12.2015 10:51, Andrew Haley wrote:
> On 11/12/15 22:18, David Wohlferd wrote:
>> And here are the three solutions that have been proposed:
>> Solution 1:
>> Just document the current behavior of basic asm.
>> People who have written incorrect code can be pointed at the text and
>> told to fix their code. People whose code is damaged by optimizations
>> can rewrite using extended to add dependencies (possibly doc this?).
>> Solution 2:
>> Change the docs to say that basic asm clobbers everything (memory, all
>> registers, etc) or perhaps just memory (some debate here), but that due
>> to bugs that will eventually be addressed, it doesn't currently work
>> this way.
> It's not just bugs which make clobbering all registers unwise and/or
>> Eventually (presumably in a future phase 1) modify the code to
>> implement this.
>> People who have written their code incorrectly may have some
>> hard-to-find problems solved for them. This is particularly valuable
>> for projects that are no longer being maintained. And while clobbers
>> aren't the best solution to dependencies, they can help.
>> Solution 3:
>> Deprecate (and eventually disallow) the use of basic asm within
>> functions (perhaps excluding asm("") and naked functions). Do this by
>> emitting warnings (and eventually fatal errors). Doc this plan.
> You've missed the most practical solution, which meets most common
> usage: clobber memory, but not registers. That allows most of the
> effects that people intuitively want and expect, but avoids the
> breakage of register clobbers. It allows basic asm to be used in a
> sensible way by pushing and popping all used registers.
Yes, implicitly clobber memory and cc-flags, on targets where that is
also automatically added to the clobber list, even when the user does
not mention that in the extended asm syntax.
That is also my preferred solution, I prepared a patch for next stage1,
the details are still under discussion, but it is certainly do-able with
not too much effort.
The rationale for this is: there are lots of ways to write basic asm
statements, that may appear to work, if they clobber memory. Even if
they clobber CC that will also work most of the time.
And because you can use all global values simply by name, users will
expect that to be supported.
And, basic asm is a instruction scheduling barrier, that is similar to a
memory barrier. See sched-deps.c:
/* Traditional and volatile asm instructions must be considered
and clobber all hard registers, all pseudo-registers and all of
memory. So must TRAP_IF and UNSPEC_VOLATILE operations.
Consider for instance a volatile asm that changes the fpu
mode. An insn should not be moved across this even if it
pseudo-regs because it might give an incorrectly rounded
I think, this comment was written by Jeff Law in 1997, and in simple
cases (w/o loops and redundant assignments) the code below does 99%
exactly what the user expects. The problematic optimizations only
happen because of we did not have the implicit memory barriers yet,
which adds only the missing 1%.
My personal gut feeling on that warning is a bit mixed...
If we have a -Wall-enabled warning on asm("..."), people who know next
to nothing about assembler will be encouraged to "fix" this warning in a
part of the code which they probably do not understand at all. This
frightens me a bit.
Because I know they will soon find out, that adding a few colons fixes
the warning, but asm("...":::) is just more ugly, and will not be any
For me, it is just very very unlikely that any piece of assembler really
clobbers nothing and has no inputs and no outputs at the same time, even
it it looks so at first sight...
It is much more likely that someone forgot to fill in the clobber section.
So for me it would also be good to warn on asm("...":::) and require
that, if they want to fix this warning, they must at least write
something in the clobber section, like asm ("...":::"nothing"); that
would be a new clobber name which can only stand alone and, which can
get stripped after the warning processing took place in the FE.
If the warning would warn on basic asm and extended asm with zero-input,
zero-output and zero-clobbers, only then I would regard that an
improvement. And for those few examples where we really have invented
something that does not clobber anything, I would say then you should
also write "nothing" in the clobber section.