Re: [PATCH] compiler.h: Fix barrier_data() on clang

[Arvind Sankar <nivedita@alum.mit.edu>]

On Thu, Oct 15, 2020 at 03:24:09PM +0000, David Laight wrote:
> > I think the comment is unclear now that you bring it up, but the problem
> > it actually addresses is not that the data is held in registers: in the
> > sha256_transform() case mentioned in the commit message, for example,
> > the data is in fact in memory even before this change (it's a 256-byte
> > array), and that together with the memory clobber is enough for gcc to
> > assume that the asm might use it. But with clang, if the address of that
> > data has never escaped -- in this case the data is a local variable
> > whose address was never passed out of the function -- the compiler
> > assumes that the asm cannot possibly depend on that memory, because it
> > has no way of getting its address.
> 
> Ok, slightly different from what i thought.
> But the current comment is just wrong.

Should I fix up the comment in the same commit, or do a second one after
moving the macro?

> > i.e. something like:
> > 	static inline void barrier_data(void *ptr, size_t size)
> > 	{
> > 		asm volatile("" : "+m"(*(char (*)[size])ptr));
> 
> I think it has to be a struct with an array member?

I don't think so, this is actually an example in gcc's documentation:

  An x86 example where the string memory argument is of unknown length.

	asm("repne scasb"
	: "=c" (count), "+D" (p)
        : "m" (*(const char (*)[]) p), "0" (-1), "a" (0));

  If you know the above will only be reading a ten byte array then you
  could instead use a memory input like: "m" (*(const char (*)[10]) p).

> 
> > 	}
> > plus some magic to disable the VLA warning, otherwise it causes a build
> > error.
> 
> It shouldn't if the size is a compile time constant.
> And given this is an instruction to the compiler it better be.

Ah right. I saw the warning when playing with something else where size
could be constant or variable depending on the call.

> > 
> > With a memory clobber, the compiler has to keep x and y at different
> > addresses, since the first barrier_data() might have saved the address
> > of x.
> 
> Maybe "+m"(*ptr) : "r"(ptr) would work.

Nothing that can only modify what ptr points to could avoid this, since
that storage is dead after the barrier.

> OTOH a "memory" clobber at the bottom of a function isn't going to
> cause bloat.
> 
> The explicit ranged memory access (without "memory") probably has its
> uses - but only if the full "memory" clobber causes grief.
> 
> 	David
> 
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