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

[David Laight]

From: Arvind Sankar
> Sent: 15 October 2020 15:45
> 
> On Thu, Oct 15, 2020 at 08:50:05AM +0000, David Laight wrote:
> > From: Arvind Sankar
> > > Sent: 14 October 2020 22:27
> > ...
> > > +/*
> > > + * This version is i.e. to prevent dead stores elimination on @ptr
> > > + * where gcc and llvm may behave differently when otherwise using
> > > + * normal barrier(): while gcc behavior gets along with a normal
> > > + * barrier(), llvm needs an explicit input variable to be assumed
> > > + * clobbered. The issue is as follows: while the inline asm might
> > > + * access any memory it wants, the compiler could have fit all of
> > > + * @ptr into memory registers instead, and since @ptr never escaped
> > > + * from that, it proved that the inline asm wasn't touching any of
> > > + * it. This version works well with both compilers, i.e. we're telling
> > > + * the compiler that the inline asm absolutely may see the contents
> > > + * of @ptr. See also: https://llvm.org/bugs/show_bug.cgi?id=15495
> > > + */
> > > +# define barrier_data(ptr) __asm__ __volatile__("": :"r"(ptr) :"memory")
> >
> > That comment doesn't actually match the asm statement.
> > Although the asm statement probably has the desired effect.
> >
> > The "r"(ptr) constraint only passes the address of the buffer
> > into the asm - it doesn't say anything at all about the associated
> > memory.
> >
> > What the "r"(ptr) actually does is to force the address of the
> > associated data to be taken.
> > This means that on-stack space must actually be allocated.
> > The "memory" clobber will then force the registers caching
> > the variable be written out to stack.
> >
> 
> 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.

> Passing ptr to the inline asm tells clang that the asm knows the
> address, and since it also has a memory clobber, that it may use the
> data. This is somewhat suboptimal, since if the data was some small
> structure that the compiler was just holding in registers originally,
> forcing it out to memory is a bad thing to do.
> 
> > If you only want to force stores on a single data structure
> > you actually want:
> > #define barrier_data(ptr) asm volatile("" :: "m"(*ptr))
> > although it would be best then to add an explicit size
> > and associated cast.
> >
> 
> 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?

> 	}
> 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.

> But I think that might lead to even more subtle issues by dropping the
> memory clobber. For example (and this is actually done in
> sha256_transform() as well, though the zero'ing simply doesn't work with
> any compiler, as it's missing the barrier_data()'s):
> 
> 	unsigned long x, y;
> 	... do something secret with x/y ...
> 	x = y = 0;
> 	barrier_data(&x, sizeof(x));
> 	barrier_data(&y, sizeof(y));
> 	return;
> 
> Since x is not used after its barrier_data(), I think the compiler would
> be within its rights to turn that into:
> 
> 	xorl	%eax, %eax
> 	leaq	-16(%rbp), %rdx	// &x == -16(%rbp)
> 	movq	%eax, (%rdx)	// x = 0;
> 	// inline asm for barrier_data(&x, sizeof(x));
> 	movq	%eax, (%rdx)	// y = 0; (!)
> 	// inline asm for barrier_data(&y, sizeof(y));
> 
> which saves one instruction by putting y at the same location as x, once
> x is dead.
> 
> 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.
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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