[llvm-dev] returning tagged union via registers

[Csaba Hruska via llvm-dev]

Hi,

AFAIK LLVM does not have direct support for tagged unions. However I'd like
to pass tagged unions to/from functions in registers. I.e. I'd like to
represent to following data type in registers.
With Haskell syntax it looks like:

> *data AllCon = Con1Data i32 | Con2Data i32*
>

A possible LLVM in register representation could be the following including
a *create* and *consume* function as an example.

*%struct.Con1Data = type { i32 }*
> *%struct.Con2Data = type { i32 }*
> *%struct.AllCon = type { i1, %struct.Con1Data, %struct.Con2Data }*
>
> *define private fastcc %struct.AllCon @create(i1 %s) {*
> *entry:*
> *  %X = select i1 %s,*
> *    %struct.AllCon {i1 true, %struct.Con1Data { i32 4 }, %struct.Con2Data
> undef},*
> *    %struct.AllCon {i1 false, %struct.Con1Data undef, %struct.Con2Data {
> i32 5 }}*
> *  ret %struct.AllCon %X*
> *}*
>
> *define private fastcc i32 @consume(i1 %s) {*
> *grinMain.entry:*
> *  %val = call fastcc %struct.AllCon @create(i1 %s)*
> *  %tag = extractvalue %struct.AllCon %val, 0*
>
> *  %con1_int = extractvalue %struct.AllCon %val, 1, 0*
> *  %con2_int = extractvalue %struct.AllCon %val, 2, 0*
> *  %res = select i1 %s, i32 %con1_int, i32 %con2_int*
>
> *  ret i32 %res*
> *}*
>

I'd like to achieve an efficient mapping to x64 machine code. In the ideal
case the *create* function result would be stored only in 2 registers. One
i1 for the tag and one i32 for the argument value either come from Con1Data
or Con2Data. For storing the i32 values one register would be always enough
hence the constructors are mutually exclusive.

As you can see in the generated code below by default LLVM allocates
separate registers for Con1Data and Con2Data arguments (ecx and edx). But
ideally it would use only a single register for both. Of course to achieve
that the frontend have to prove somehow that these values are on mutually
exclusive data flow.

*  .text*
> *  .file "node_test_reg3.ll.1"*
> *  .p2align  4, 0x90         # -- Begin function create*
> *  .type .Lcreate, at function*
> *.Lcreate:                               # @create*
> *  .cfi_startproc*
> *# BB#0:                                 # %entry*
> *  movl  %edi, %eax*
> *  andb  $1, %al*
> *  movl  $5, %ecx*
> *  cmovnel %eax, %ecx*
> *  testb %al, %al*
> *  movl  $4, %eax*
> *  cmovnel %eax, %edx*
> *  movl  %edi, %eax*
> *  retq*
> *.Lfunc_end0:*
> *  .size .Lcreate, .Lfunc_end0-.Lcreate*
> *  .cfi_endproc*
> *                                        # -- End function*
> *  .p2align  4, 0x90         # -- Begin function consume*
> *  .type .Lconsume, at function*
> *.Lconsume:                              # @consume*
> *  .cfi_startproc*
> *# BB#0:                                 # %grinMain.entry*
> *  pushq %rbx*
> *.Lcfi0:*
> *  .cfi_def_cfa_offset 16*
> *.Lcfi1:*
> *  .cfi_offset %rbx, -16*
> *  movl  %edi, %ebx*
> *  callq .Lcreate*
> *  testb $1, %bl*
> *  cmovnel %edx, %ecx*
> *  movl  %ecx, %eax*
> *  popq  %rbx*
> *  retq*
> *.Lfunc_end1:*
> *  .size .Lconsume, .Lfunc_end1-.Lconsume*
> *  .cfi_endproc*
> *                                        # -- End function*
>
> *  .section  ".note.GNU-stack","", at progbits*
>

Does anyone have an idea how can I achieve this in LLVM? I guess I have to
extend the expressive power somehow (i.e. new metadata). What is the
simplest change to support this?

I'd appreciate if you'd help to figure out the complexity of this extension.

Best regards,
Csaba Hruska
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