llvm dev - Oct 2014 - [LLVMdev] assembler instruction selection vs. compiler instruction selection

Hello LLVM,

Suppose I have many instructions with AsmString "foo  $src".  The
specific foo opcode depends on the value of $src.  I arrange
AsmOperandClasses and superclasses for $src so the assembler picks the
right foo.  That seems fine as far as the assembler goes.

During compiler instruction selection, I can use a CodeGenOnly=1
pseudo with assembly "foo $src" and emit .s files from llc.  These go
to the assembler which chooses the foo opcode using the
AsmOperandClass hierarchy.  The compiler didn't have to worry about
which foo is the right foo.

However, I want to use llc's ability to produce object files directly.
That path skips assembly parsing, so my AsmOperandClass logic can't
help pick the right foo, correct?

What is the right approach?  Do I add DAG pattern matching logic to
mirror all the AsmOperandClass selection logic?  If so, is there an
easy way to detect if assembler instruction selection and compiler
instruction selection are out of sync?

Thanks,
-steve

On Fri, Oct 3, 2014 at 5:16 PM, Steve King <steve at metrokings.com>
wrote:
> Hello LLVM,
>
> Suppose I have many instructions with AsmString "foo  $src".  The
> specific foo opcode depends on the value of $src.  I arrange
> AsmOperandClasses and superclasses for $src so the assembler picks the
> right foo.  That seems fine as far as the assembler goes.
>
> During compiler instruction selection, I can use a CodeGenOnly=1
> pseudo with assembly "foo $src" and emit .s files from llc. 
These go
> to the assembler which chooses the foo opcode using the
> AsmOperandClass hierarchy.  The compiler didn't have to worry about
> which foo is the right foo.
>
> However, I want to use llc's ability to produce object files directly.
> That path skips assembly parsing, so my AsmOperandClass logic can't
> help pick the right foo, correct?
>
> What is the right approach?  Do I add DAG pattern matching logic to
> mirror all the AsmOperandClass selection logic?  If so, is there an
> easy way to detect if assembler instruction selection and compiler
> instruction selection are out of sync?
>
> Thanks,
> -steve
>
CodeGen and ASM operands matching mechanisms are different.

You can do something like this:
  def Foo_AsmOperand: AsmOperandClass {
    let Name = “Foo”;
    let PredicateMethod = "is_Foo";
  }
  def Foo_Operand : Operand<i32>, ComplexPattern<i32, 1,
"Select_Foo",
[imm], []> {
    let ParserMatchClass = Foo_AsmOperand;
  }

You can manage by having is_Foo and Select_Foo share logic by calling the
same common function.
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