RE: [PATCH] compiler.h: Clarify comment about the need for barrier_data()

[David Laight <David.Laight@ACULAB.COM>]

From: Arvind Sankar
> Sent: 15 October 2020 23:01
,,,
> I think it's helpful to have the more detailed explanation about
> register variables -- at first glance, it's a bit mystifying as to why
> the compiler would think that the asm can't access the stack. Spilling
> registers to the stack is actually an undesirable side-effect of the
> workaround.

That is the very bit that just confuses things.
The data the memzero_explictit() is trying to clear is (probably)
on-stack already - it won't be in registers.

If it were in registers you wouldn't need the memset().

Actually I suspect that the memset() is inlined so that is
just assigns zeros to all the variables.
This will be done using 'virtual registers' that cache the
on-stack value.
You then need to do something to force the instructions to flush
the 'virtual registers' back to stack to be generated.

The fundamental thing is that the address of a local (auto!)
variable must be visible to the asm statement for the compiler
to make the contents of those variables visible.

I even suspect you may need to pass the address of the structure
(to be zeroed) to an asm block at the top of the function as well.
Otherwise the compiler could change the stack offsets where the
structure is stored.
But I don't think compilers do that.

	David

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