llvm dev - Aug 2012 - [LLVMdev] Load serialisation during selection DAG building

On Aug 14, 2012, at 2:05 PM, Steve Montgomery <stephen.montgomery3 at
btinternet.com> wrote:
> Further to my earlier question, I'm perhaps a bit confused about memory
serialisation. The following example, compiled using clang for the MSP430:
> 
> target datalayout =
"e-p:16:16:16-i8:8:8-i16:16:16-i32:16:32-n8:16"
> target triple = "msp430-??-??"
> 
> @y = common global i16 0, align 2
> @x = common global i16 0, align 2
> 
> define void @f() nounwind {
> entry:
>   %0 = load i16* @y, align 2, !tbaa !0
>   %1 = load volatile i16* @x, align 2, !tbaa !0
>   %add = add i16 %1, %0
>   store i16 %add, i16* @y, align 2, !tbaa !0
>   ret void
> }
> 
> has a chain store->load volatile->load. I thought this meant that the
load volatile had to occur _after_ the load but the MSP430 backend selects the
ADD16mm instruction for which I suspect the order of operand access isn't
specified. So, does the chain mean "no earlier than" rather than
"later than"?
No, a chain is supposed to mean "later than". It sounds like MSP430 is
bending
the rules here.

Dan

> No, a chain is supposed to mean "later than". It sounds like
MSP430 is bending
> the rules here.
The instruction selector for ADD16mm is autogenerated, so, this is not
MSP430 bug alone :)
This is just the single target in the tree which has mem-mem instructions.

-- 
With best regards, Anton Korobeynikov
Faculty of Mathematics and Mechanics, Saint Petersburg State University

OK, I guess the problem is in HandleMergeInputChains() or maybe
SelectCodeCommon() so should I file a bug report against SelectionDAGISel.cpp?

On 15 Aug 2012, at 00:03, Anton Korobeynikov wrote:
>> No, a chain is supposed to mean "later than". It sounds like
MSP430 is bending
>> the rules here.
> The instruction selector for ADD16mm is autogenerated, so, this is not
> MSP430 bug alone :)
> This is just the single target in the tree which has mem-mem instructions.
> 
> -- 
> With best regards, Anton Korobeynikov
> Faculty of Mathematics and Mechanics, Saint Petersburg State University