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[TableGen] AsmMatcher: support for default values for optional operands
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Authored by SamWot on Mar 17 2016, 6:50 AM.

Details

Summary

This change allows to specify "DefaultMethod" for optional operand (IsOptional = 1) in AsmOperandClass that return default value for operand. This is used in convertToMCInst to set default values in MCInst.
Previously if you wanted to set default value for operand you had to create custom converter method. With this change it is possible to use standard converters even when optional operands presented.

Diff Detail

Event Timeline

SamWot updated this revision to Diff 50928.Mar 17 2016, 6:50 AM
SamWot retitled this revision from to [TableGen] AsmMatcher: support for default values for optional operands.
SamWot updated this object.
SamWot added subscribers: llvm-commits, nhaustov, arsenm.
ab edited edge metadata.Mar 28 2016, 11:13 AM

I haven't looked closely yet, but have you considered doing this entirely in tablegen, either via default operand lists (like ARM's OptionalDefOperand) or even some default value string?

lib/Target/AMDGPU/AsmParser/AMDGPUAsmParser.cpp
396–398

These look independent; feel free to commit right away (or submit another patch).

In D18242#384742, @ab wrote:

I haven't looked closely yet, but have you considered doing this entirely in tablegen, either via default operand lists (like ARM's OptionalDefOperand) or even some default value string?

I actualy thought about this but decided that DefaultMethod is better (at least for now) because usage of OptionalDefOperand or default string value would require big changes in MCTargetAsmParser’s interface and all back-ends. MCTargetAsmParser need some way to create arguments (MCParsedAsmOperand) with default values and insert them in instructions. Currently MCTargetAsmParser has no interface to create operands and it’s hard to create it because every target’s MCParsedAsmOperand implementation differs a lot. With DefaultMethod responsibility to create default arguments lies on back-end developers and no interface changes in MCTargetAsmParser required.

Another solution can be that MCTargetAsmParser would directly create MCOperands instead of MCParsedAsmOperand. But in this case it is there is no way to use RenderMethod for this operand.

ab added a comment.Apr 25 2016, 4:56 PM

I'm no fan of this, but I think you're right, there's not much better we can do right now.

What do you think of only generating the extra code only if there's at least one optional operand class? That would avoid needing a mask on other targets.

include/llvm/Target/Target.td
623

return default -> returns the default

623–624

"This method is only used if.."

625

IsOptional == 1, no?

628–630

Why pass anything? You don't use the Opcode, and the Operands are only used for the location:

  • what happens when the first and only operand is optional?
  • loc of the preceding operand is probably more accurate; but I think just using SMLoc() makes the most sense
  • does anything look at locations at this point anyway? If there's an error it would have been caught earlier, no?
utils/TableGen/AsmMatcherEmitter.cpp
206

which method?

SamWot updated this revision to Diff 54998.Apr 26 2016, 6:32 AM
SamWot edited edge metadata.

Fixes and new code to check if target has optional operands.

include/llvm/Target/Target.td
628–630

I added Opcode and Operands to arguments list because this arguments seems to be useful in defaultMethod. It is not hard to imagine situation when default value for operand differs depending on instruction opcode or other operands presented in instruction. But if you think that they are unnecessary I can remove them.

There is always first operand in OperandVector - instruction mnemonic.

ab added inline comments.Apr 27 2016, 1:35 PM
include/llvm/Target/Target.td
628–630

I added Opcode and Operands to arguments list because this arguments seems to be useful in defaultMethod. It is not hard to imagine situation when default value for operand differs depending on instruction opcode or other operands presented in instruction. But if you think that they are unnecessary I can remove them.

Eh, if you have a patch in mind that would use them, OK, but otherwise, what do you think of leaving them out, and adding them when they turn out to be necessary?

There is always first operand in OperandVector - instruction mnemonic.

You're right, forgot about that!

SamWot updated this revision to Diff 56258.May 5 2016, 3:32 AM

Removed unused arguments in defaultMethod.
Rebased on top of master.

ab accepted this revision.May 5 2016, 1:15 PM
ab edited edge metadata.

LGTM; thanks Sam!

This revision is now accepted and ready to land.May 5 2016, 1:15 PM
This revision was automatically updated to reflect the committed changes.