llvm dev - Apr 2015 - [LLVMdev] Floating point atomic load and add

Hello,

I'm wondering how I can create an atomic load and add instruction for  
floating point values. If I use      IRBuilder::CreateAtomicRMW() I  
get the error message: "atomicrmw operand must have integer type".

I am using LLVM 3.4 and the only system I need to support is x86.

On 24 April 2015 at 13:53, Tyler Denniston <tyler at csail.mit.edu>
wrote:
> I'm wondering how I can create an atomic load and add instruction for
> floating point values. If I use      IRBuilder::CreateAtomicRMW() I get the
> error message: "atomicrmw operand must have integer type".
I don't think x86 allows you to use a lock prefix on floating point
instructions, just basic arithmetic and cmpxchg. You'll probably need
to put a loop around cmpxchg instructions to emulate it. (and might
well need to cast the pointers to integer type for the cmpxchg, I know
we talked about allowing floats there but I'm not sure it's happened
yet).

Cheers.

Tim.

Quoting Tim Northover <t.p.northover at gmail.com>:
> On 24 April 2015 at 13:53, Tyler Denniston <tyler at csail.mit.edu>
wrote:
>> I'm wondering how I can create an atomic load and add instruction
for
>> floating point values. If I use      IRBuilder::CreateAtomicRMW() I get
the
>> error message: "atomicrmw operand must have integer type".
>
> I don't think x86 allows you to use a lock prefix on floating point
> instructions, just basic arithmetic and cmpxchg. You'll probably need
> to put a loop around cmpxchg instructions to emulate it. (and might
> well need to cast the pointers to integer type for the cmpxchg, I know
> we talked about allowing floats there but I'm not sure it's
happened
> yet).
Hmm, ok. Can you briefly expand on how I should emulate this behavior?  
I don't understand what you mean by adding a loop around cmpxchg.

Thanks,

Tyler

On 04/24/2015 01:53 PM, Tyler Denniston wrote:
> Hello,
>
> I'm wondering how I can create an atomic load and add instruction for 
> floating point values. If I use IRBuilder::CreateAtomicRMW() I get the 
> error message: "atomicrmw operand must have integer type".
The AtomicRMW instruction doesn't natively support floating point 
types.  You can emulate what you're looking for with a bitcast of the 
float to an appropriately sized integer, a AtomicRMW on that, and then a 
bitcast of the result.  This would have the result of CASing in the 
bitpattern of the original float which is probably what you
intend.
>
> I am using LLVM 3.4 and the only system I need to support is x86.
>
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On 26 Apr 2015, at 20:54, Philip Reames <listmail at philipreames.com>
wrote:
> 
> The AtomicRMW instruction doesn't natively support floating point
types.  You can emulate what you're looking for with a bitcast of the float
to an appropriately sized integer, a AtomicRMW on that, and then a bitcast of
the result.  This would have the result of CASing in the bitpattern of the
original float which is probably what you intend.
AtomicRMW operations in the C11 spec are somewhat insane, as they require saving
and restoring the FP state in the compare and exchange loop[1].  It’s probably
quite difficult to define an AtomicRMW instructions on floating point values in
the IR that would have useful semantics.  Anything that does atomic operations
on floating point values is likely to have some fairly complex language-level
semantics.

David

[1] I think that this can possibly be avoided if you use Intel’s transactional
memory support to implement the atomic RMW, as the instructions that change the
floating point environment will cause a transaction abort, but I’m not 100%
sure.