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

On Wed, Jun 12, 2013 at 7:28 PM, Guo, Xiaoyi <Xiaoyi.Guo at amd.com>
wrote:
>  So fence only forces ordering of atomic instructions.****
>
> ** **
>
> Let me change my question then.****
>
> ** **
>
> If I have a target-specific intrinsic which forces ordering of ordinary
> load/store instructions. Then should it also force ordering of load/stores
> to noalias pointers in caller functions?
>
So, something like 'asm volatile ("":::"memory")'?

LLVM can and will move noalias loads/stores across an asm statement like
this; it assumes noalias means "nothing inside this function will touch the
memory in question except accesses through this pointer".

-Eli
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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?

Xiaoyi

From: Eli Friedman [mailto:eli.friedman at gmail.com]
Sent: Wednesday, June 12, 2013 8:08 PM
To: Guo, Xiaoyi
Cc: LLVM Dev
Subject: Re: [LLVMdev] A question w.r.t fence instruction vs. noalias pointer

On Wed, Jun 12, 2013 at 7:28 PM, Guo, Xiaoyi <Xiaoyi.Guo at
amd.com<mailto:Xiaoyi.Guo at amd.com>> wrote:
So fence only forces ordering of atomic instructions.

Let me change my question then.

If I have a target-specific intrinsic which forces ordering of ordinary
load/store instructions. Then should it also force ordering of load/stores to
noalias pointers in caller functions?

So, something like 'asm volatile ("":::"memory")'?
LLVM can and will move noalias loads/stores across an asm statement like this;
it assumes noalias means "nothing inside this function will touch the
memory in question except accesses through this pointer".

-Eli
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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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