[llvm-dev] MC Assembler / tablegen: actually parsing variable_ops

Wouter van Oortmerssen via llvm-dev llvm-dev at lists.llvm.org
Mon Dec 3 17:32:32 PST 2018

variable_ops is used in the tablegen defs for many targets to denote
instructions that a variable number of inputs, but it seems that there
aren't any targets for which this results in variable elements in the
instruction encoding (and thus in assembler parsing), since the tablegen
generated assembly matcher ($(Target)GenAsmMatcher.inc) simply assumes that
variable_ops are not to be parsed (match table: Convert_NoOperands).

Now I understand that a variable length instruction is a weird concept for
most CPUs, but we need it for at least one instruction in WebAssembly. We
can probably hack around it in the WebAssemblyAsmParser, but I wonder if
there's something I am not understanding about variable_ops and the
generated asm matcher that might make for a cleaner solution.

The instruction in question:
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