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Re: asm volatile("":::"memory) uncertainty.


On 05/10/2016 10:05 AM, Andrew Haley wrote:
On 05/10/2016 04:58 PM, Tom Udale wrote:
To my mind, the only reason that these would not be considered "memory
references" is because the compiler has already decided to put the
memory into registers (true these are mostly automatic variables so they
can easily be put into registers).  But from an abstract machine
standpoint a=b is a memory read and a memory write is it not (or is that
a totally incorrect statement)?  It seems odd therefore that a register
allocation choice can fundamentally alter program behavior.

An auto variable is only considered to be "memory" if its address
is taken with & .  "memory" means that this object is reachable from
some pointer somewhere.
And note that there's a variety of analysis and transformations that will try to eliminate the address-of so that the object is no longer addressable.

Not really.  If a variable is not addressable by any means then it
can't be affected by a memory access.  Whether the compiler spills
that variable to a stack frame makes no difference.
Right.

jeff


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