[LLVMdev] Purpose of MSP430Wrapper

[Richard Osborne]

On 25/07/12 12:14, Paul Shortis wrote:
> Thanks Richard,
>
> You're correct, they are similar. In the XCoreInstrInfo.td patterns 
> what I'm struggling with is why this ....
>
> def BL_lu10 : _FLU10<
>                   (outs),
>                   (ins calltarget:$target, variable_ops),
>                   "bl $target",
>                   [(XCoreBranchLink immU20:$target)]>;
>
> def : Pat<(XCoreBranchLink tglobaladdr:$addr), (BL_lu10 
> tglobaladdr:$addr)>;
> def : Pat<(XCoreBranchLink texternalsym:$addr), (BL_lu10 
> texternalsym:$addr)>;
>
> is necessary. Are the Pat<> s just 'casting' tglobaladdr:$addr and 
> texternalsym:$addr to an immU20 to force a match ?
There is no casting going on, there are just 3 separate patterns all 
which select to the BL_lu10 instruction. You could rewrite this as:

def BL_lu10 : _FLU10<
                   (outs),
                   (ins calltarget:$target, variable_ops),
                   "bl $target",
                   []>;

def : Pat<(XCoreBranchLink immU20:$addr), (BL_lu10 immU20:$addr)>;
def : Pat<(XCoreBranchLink tglobaladdr:$addr), (BL_lu10 
tglobaladdr:$addr)>;
def : Pat<(XCoreBranchLink texternalsym:$addr), (BL_lu10 
texternalsym:$addr)>;

The advantage of specifying the pattern inline is:

* Less typing.
* Properties of the instruction are inferred from the pattern it matches 
(e.g does the instruction load or store, does it have other side 
effects). If there is no pattern you would need to specify these 
manually (let mayLoad=1 in ...)

>
> I'm guessing similar Pat<> 's aren't required for the BL_u10/immU10 
> cases because they match without any assistance ?
No, the BL_u10 instructions are not matched. At least for the XCore 
target, we always conservatively use the branch instruction with the 
largest offset. instructions can be relaxed (replaced by smaller 
versions) later.
>
> Is the XCoreBranchLink enough of a match hint that an address wrapper 
> isn't required to clarify the pattern match for these call instructions?
There's no wrapper in these patterns because the lowering code which 
creates the XCoreBranchLink SDNode 
(XCoreTargetLowering::LowerCCCCallTo()) doesn't add a wrapper around the 
callee. There is no need to add a wrapper because in the case of a call 
there is no ambiguity - a call of a global always use pc relative 
addressing on the XCore. The important thing here is that the pattern 
must match the nodes the introduced during the lowering.
> Cheers, Paul

-- 
Richard Osborne | XMOS
http://www.xmos.com