[LLVMdev] atomic operations for ARM

[Evan Cheng]

It would be useful if you can post some example code and what you  
think the assembly code should look like.


On Mar 26, 2009, at 5:41 PM, Robert Schuster wrote:

> Hi,
> I have reworked my previous example and got something which is  
> accepted
> by tblgen:
>
> let isCall = 1,
>  Defs = [R0, R1, R2, R3, R12, LR,
>          D0, D1, D2, D3, D4, D5, D6, D7, CPSR] in {
>
> def ARM_ATOMIC_CMP_SWAP : ABXI<0b1011, (outs GPR:$dst), (ins
> i32imm:$ptr, i32imm:$old, i32imm:$new),
>           "do_something",
>           [(set GPR:$dst,
>                 (atomic_cmp_swap_32 globaladdr:$ptr, imm:$old,
> imm:$new))]>;
> }
>
> What I want to achieve first is that llc picks this definition when it
> finds an occurance of the @atomic.cmp.swap.i32 intrinsic.
>
> Therefore I wrote a basic .ll file containing only a call to this
> function. I then let it run through "llvm-as | llc".

It would be useful if you post this.

>
>
> With the above definition I expected "do_something" to appears in the
> assembler output when -march=arm. Unfortunately atm I still get the
> error, telling me that the selection failed:
>
> llvm-as < Atomics-32.ll | llc -march=arm
> Cannot yet select: 0x1f7c540: i32,ch = AtomicCmpSwap 0x1f7cd00:1,
> 0x1f7cd00, 0x1f7c448, 0x1f7c350 <0x1f6fbe8:0> <volatile> alignment=4
> Stack dump:
> 0.	Program arguments: llc -march=arm
> 1.	Running pass 'ARM Instruction Selection' on function
> '@test_compare_and_swap'

It's hard to guess what the problem is from this. Are you able to step  
through the code in ARMGenDAGISel.inc to see why it fails to match?

Evan

>
>
> I took a look at how the atomic_compare_swap implementation is done  
> for
> PowerPC and X86 but got no clue how it gets from atomic_cmp_swap_32 to
> the target specific variant. Well, for X86 its done via a custom  
> lowering.
>
> Any idea how the DAG entry for ARM_ATOMIC_CMP_SWAP should look like so
> that it is picked up by for the @atomic.cmp.swap.i32 intrinsic?
>
>
> Regards
> Robert
>
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