[llvm] [ARM][TableGen][MC] Change the ARM mnemonic operands to be optional for ASM parsing (PR #83436)

[Simon Tatham via llvm-commits]

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@@ -1564,6 +1562,10 @@ class AsmParser {
   // method shall be called for all operands as opposed to only those
   // that have their own specified custom parsers.
   bit CallCustomParserForAllOperands = false;
+
+  // PreferSmallerInstructions - Should the assembly matcher prefer the smaller
+  // instructions.
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statham-arm wrote:

I would add to this comment

(a) that this flag is useful for Arm (so that people who only know other architectures have an idea why it needs to be here in the first place)

(b) what the preference order is if this flag is false. (Surely the only reason this flag exists, instead of just being always-true, is so that somebody can deliberately set it to false, in which case they have some reason to want a different preference order, and will need to know how to get it.)

https://github.com/llvm/llvm-project/pull/83436