[LLVMdev] Why can't atomic loads and stores handle floats?

[Filip Pizlo]

What is the downside of the currently generated IR?  There ain't nothin' wrong with bitcasts, IMO. 

-Filip

> On May 24, 2014, at 2:17 PM, Philip Reames <listmail at philipreames.com> wrote:
> 
> Looking through the documentation, I discovered that atomic loads and stores are only supported for integer types.  Can anyone provide some background on this?  Why is this true?
> 
> Currently, given code:
> std::atomic<float> aFloat;
> void foo() {
>   float f = atomic_load(&aFloat);
>   ..
> }
> 
> Clang generates code like:
> %"struct.std::atomic.2" = type { float }
> @aFloat = global %"struct.std::atomic.2" zeroinitializer, align 4
> 
> define void @foo() {
>   %1 = load atomic i32* bitcast (%"struct.std::atomic.2"* @aFloat to     i32*) seq_cst, align 4
>   %2 = bitcast i32 %1 to float
>   ...
> }
> 
> This seems less than ideal.  I would expect that we might have to desugar floats into integer & cast operations in the backend, but why is this imposed on the frontend? 
> 
> More generally, is there anyone who is knowledgeable and/or working on atomics and synchronization in LLVM?  I've got a number of questions w.r.t. semantics and have found a number of what I believe to be missed optimizations.  I'm happy to file the later, but I'd like to talk them over with a knowledgeable party first.
> 
> Philip
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