[LLVMdev] Question regarding getElementPtr/Addressing modes in backend

[Bruce Hoult]

Many CPU instruction sets have "autoincrement" / "postincrement" (pop) /
"predecrement" (push) instructions that dereference a pointer register and
also alter it by the size of the load or store.

The PowerPC has "load byte/short/word with update" instructions that add an
arbitrary positive or negative literal offset (unrelated to the load/store
size) to the pointer register BEFORE the load or store, and update the
register with the final address.

So maybe the PowerPC back end is worth looking at?

On Fri, Oct 24, 2014 at 1:30 AM, Johnny Val <johnnydval at gmail.com> wrote:

> Hi Steve,
>
> Thanks for the tip regarding MIOperandInfo, I didn't think of that part of
> the tablegen description.
>
> Sadly, I did actually mean: r1 = *(i0 += m0).
>
> So increment i0 by m0. Read memory the memory location "pointed" to by i0.
> Store in r1. Sadly I am not too familiar with compiler terminology, so I
> don't know if there is a proper term for such a load.
>
>
> On Thu, Oct 23, 2014 at 12:23 PM, Steve Montgomery <
> stephen.montgomery3 at btinternet.com> wrote:
>
>> You might need to modify selectAddr so it doesn't fold the add node but
>> instead returns two operands: one for the pointer and one for the offset.
>> You can control the register classes for the two components of the address
>> using MIOperandInfo so your base pointer can be IClass and the offset
>> MClass. See SparcInstrInfo.td (and probably others) for examples.
>>
>> I'm assuming you meant r1 = *(i0 + m0) rather than r1 = *(i0 += m0)? I
>> don't know how you'd achieve that latter.
>>
>> Steve Montgomery
>>
>> On 21 Oct 2014, at 17:15, Johnny Val <johnnydval at gmail.com> wrote:
>>
>> > Hi,
>> >
>> > I am writing a backend and having issues with properly lowering the
>> result of getElementPtr ( specifically the add node that it generates).
>> >
>> > If we take this IR:
>> >
>> > %struct.rectangle = type { i24, i24 }
>> >
>> > ; Function Attrs: nounwind readonly
>> > define i24 @area(%struct.rectangle* nocapture readonly %r) #0 {
>> > entry:
>> >  %width = getelementptr inbounds %struct.rectangle* %r, i16 0, i32 0
>> >  %0 = load i24* %width, align 4, !tbaa !1
>> >  %height = getelementptr inbounds %struct.rectangle* %r, i16 0, i32 1
>> >  %1 = load i24* %height, align 4, !tbaa !6
>> >  %mul = mul nsw i24 %1, %0
>> >  ret i24 %mul
>> > }
>> >
>> > The DAG before isel would look like isel.png.
>> >
>> > I would then pattern match the load nodes with:
>> >
>> > [(set i24:$val, (load addr:$addr))]
>> >
>> > Where addr is a complex pattern. This is fine for the 0th element in
>> the struct, as there is no offset so the resultant assembly would be (which
>> works fine):
>> >
>> > r0 = *(i0)
>> >
>> > The issue I have is with the second load. I need the result to be:
>> >
>> > m0 = 4
>> > r1 = *(i0 += m0) (offset is stored in a register(m0), and modifies the
>> original pointer)
>> >
>> > rather than
>> >
>> > r1 = *(i0 + 4) (immediate)
>> >
>> > (The specific registers numbers don't matter).
>> >
>> > I'm not sure how to achieve that. Currently addr gets matched in
>> SelectAddr in XYZISelDAGToDAG.cpp. It will match the add node and "combine"
>> (I'm not sure this is the correct terminology) it into my LDR node. The
>> result of this can be seen sched.png
>> >
>> > So instead of that TargetConst<4> I need a something that copies 4 into
>> an M register, and then LDR uses that.
>> >
>> > I'm not sure if I should put logic in SelectAddr to create a virtual
>> register and create a series of copyfromreg/copytoreg to copy the constant
>> into an M register and then pass that to the node? I feel like that is
>> fairly ugly (as no other back-end does this), but I'm not sure what a
>> better way would be.
>> >
>> > I have tried looking through other backends for an example, but
>> couldn't find something similar. That could be due to me not knowing where
>> to look.
>> >
>> > Thanks in advance for any help.
>> >
>> > Johnny
>> > <isel.png><sched.png>_______________________________________________
>> > LLVM Developers mailing list
>> > LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>> > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>>
>>
>
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