llvm dev - Jun 2013 - [LLVMdev] A question w.r.t fence instruction vs. noalias pointer

On Thu, Jun 13, 2013 at 10:52 AM, Guo, Xiaoyi <Xiaoyi.Guo at amd.com>
wrote:
>  I mean something like a target-specific fence machine instruction which
> forces ordering of all loads/stores. I want to clarify the meaning of
> “noalias” in this case. Is the fence machine instruction considered
> “touching” all memory and thus breaks the “noalias” contract?
>
A fence doesn't modify memory in the sense LLVM cares about.  It's a
no-op
in a single-threaded environment, and in a multi-threaded environment it's
just a way to express ordering on memory modifications made by other
threads.

-Eli
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In a multi-threaded environment, in order to ensure the memory ordering
expressed by the memory fence, certain memory operations should not be moved
across the fence, right?


From: Eli Friedman [mailto:eli.friedman at gmail.com]
Sent: Thursday, June 13, 2013 11:18 AM
To: Guo, Xiaoyi
Cc: LLVM Dev
Subject: Re: [LLVMdev] A question w.r.t fence instruction vs. noalias pointer

On Thu, Jun 13, 2013 at 10:52 AM, Guo, Xiaoyi <Xiaoyi.Guo at
amd.com<mailto:Xiaoyi.Guo at amd.com>> wrote:
I mean something like a target-specific fence machine instruction which forces
ordering of all loads/stores. I want to clarify the meaning of
"noalias" in this case. Is the fence machine instruction considered
"touching" all memory and thus breaks the "noalias"
contract?
A fence doesn't modify memory in the sense LLVM cares about.  It's a
no-op in a single-threaded environment, and in a multi-threaded environment
it's just a way to express ordering on memory modifications made by other
threads.

-Eli
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On Thu, Jun 13, 2013 at 11:39 AM, Guo, Xiaoyi <Xiaoyi.Guo at amd.com>
wrote:
>  In a multi-threaded environment, in order to ensure the memory ordering
> expressed by the memory fence, certain memory operations should not be
> moved across the fence, right?****
>
> **
>
Yes, but operations on noalias pointers don't fall into that category.  For
noalias pointers, while the function is running, any operation on the
pointed-to memory has to be based on the noalias pointer.  This applies to
all threads in the program, not just the one the function is called from.

-Eli
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