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Re: basic asm and memory clobbers - Proposed solution

On 12/20/2015 10:26 AM, Bernd Edlinger wrote:
On 19.12.2015 19:54, David Wohlferd wrote:
mep: mep_interrupt_saved_reg looks for ASM_INPUT in the body, and
saves different registers if found.
I'm trying to follow this code.  A real challenge since I know nothing
about mep.  But what I see is:

- This routine only applies to functions marked as
- To correctly generate entry/exit, it walks thru each register (up to
FIRST_PSEUDO_REGISTER) to see if it is used by the routine. If there
is any basic asm within the routine (regardless of its contents), the
register is considered 'in use.'

The net result is that every register gets saved/restored by the
entry/exit of this routine if it contains basic asm.  The reason this
is important is that if someone just adds a colon, it would suddenly
*not* save/restore all the registers.  Depending on what the asm does,
this could be bad.

Does that sound about right?

Seems like a doc update would be appropriate here too then, if anyone
wanted to volunteer...
To confirm this, I built a cross-compiler, but It was difficult, because
of pr64402.
Yes, a function with __attribute__((interrupt)) spills lots of
registers, when it just
contains asm(""); but almost nothing, if asm("":); is used.  That is

So if you use extended and clobber a couple registers asm("":::"r0", "r1"), does it spill just those specific registers? That would be cool.

I don't write interrupt handlers, and certainly not on mep. But the little I know about them says that performance is an important (and sometimes critical) characteristic. There would be risk in changing this to extended (if you used a register but forgot to clobber it), but depending on the interrupt, it could be a nice performance 'win.'

If no one else is prepared to step up to write this, I can. I'm just uncomfortable doing so since I can't try it myself. And I feel weird writing a patch for mep given that I know nothing about it.

But since Bernd has tried it, maybe something like this added to the 'interrupt' attribute on
Be aware that if the function contains any basic @code{asm} (@pxref{Basic Asm}), all registers (whether referenced in the asm or not) will be preserved upon entry and restored upon exiting the interrupt. More efficient code can be generated by using extended @code{asm} (@pxref{Extended Asm}) and explicitly listing only the specific registers that need to be preserved (or none if your asm preserves any registers it uses).

tilegx:  They never wrap {} around inline asm. But extended asm, are
handled differently, probably automatically surrounded by braces.
I know nothing about tilegx either.  I've tried to read the code, and
it seems like basic asm does not get 'bundled' while extended might be.

Bundling for tilegx (as I understand it) is when you explicitly fill
multiple pipelines by doing something like this:

     { add r3,r4,r5 ; add r7,r8,r9 ; lw r10,r11 }

So if you have a basic asm statement, you wouldn't want it
automatically bundled by the compiler, since your asm could be more
than 3 statements (the max?).  Or your asm may do its own bundling. So
it makes sense to never output braces when outputting basic asm.

I know I'm guessing about what this means, but doesn't it seem like
those same limitations would apply to extended?  I wonder if this is a
bug.  I don't see any discussion of bundling (of this sort) in the

I wold like to build a cross compiler, but currently that target seems
to be broken. I have to check
that target anyways, because of my other patch with the memory clobbers.

I see in tilegx_asm_output_opcode, that they do somehow automatically
place braces.
An asm("pseudo":) has a special meaning, and can be replaced with ""
or "}".
However the static buf[100] means that any extended asm string > 95
characters, invokes
the gcc_assert in line 5487.  In the moment I would _not_ recommend
changing any asm
statements without very careful testing.
Yes, the handling of extended asm there is very strange.

If bundles can only be 3 instructions, then appending an entire
extended asm in a single (already in-use) bundle seems odd.  But maybe
that's just because I don't understand tilegx.

I'm not sure it's just changing basic asm to extended I would be
concerned about.  I'd worry about using extended asm at all...
I also built a tilegx cross compiler, but that was even more difficult,
because of pr68917, which hit me when building the libgcc.

I tried a while, but was unable to find any example of an extended asm
that gets
auto-braces, or which can trigger the mentioned gcc_assert. It looks
like, in all
cases the bundles are closed already before the extended asm gets scheduled.
Probably the middle end already knows that it can't parse the asm string.
I see examples of extended asm in the linux /arch/tile tree, and some have
braces, some not.  But as it seems, they are completely left alone by
the expansion.

So what do the docs need to say here? Basic asm never wraps with {}, but apparently extended doesn't either. So, there's nothing to say?

You do not have to escape the { and } for extended asm, on this target,
using %{ produces even an error.

I believe the only the only target that needs to escape {} is i386, since it's the only one that supports dialects (att & intel).

In deed asm ("pseudo":) expands to nothing, it is
probably used as a memory barrier in the .md files.

Yeah, that's what it looked like to me too.

Yes, but please add a test case with some examples where the warning is
expected to be triggered,
and where it is not.
I have one, but I haven't added the dejagnu stuff to it yet.  I'm not
really an expert there and could use some help.  Simple things like
where does the file go and what should it be called.
Look at gcc/testsuite/c-c++-common/W*.c  for examples how to do that.
I think, this would be a good place/naming convention for your test case

Given that c and cpp use different parsers, I'll want to create tests for both (well, same tests, different extensions). Should both Wonly-top-basic-asm.c and Wonly-top-basic-asm.cpp go in that directory? There aren't any other cpp files there.


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