forked from OSchip/llvm-project
848d3d0d2c
During assembly, the mask operand to an IT instruction (storing the sequence of T/E for 'Then' and 'Else') is parsed out of the mnemonic into a representation that encodes 'Then' and 'Else' in the same way regardless of the condition code. At some point during encoding it has to be converted into the instruction encoding used in the architecture, in which the mask encodes a sequence of replacement low-order bits for the condition code, so that which bit value means 'then' and which 'else' depends on whether the original condition code had its low bit set. Previously, that transformation was done by processInstruction(), half way through assembly. So an MCOperand storing an IT mask would sometimes store it in one format, and sometimes in the other, depending on where in the assembly pipeline you were. You can see this in diagnostics from `llvm-mc -debug -triple=thumbv8a -show-inst`, for example: if you give it an instruction such as `itete eq`, you'd see an `<MCOperand Imm:5>` in a diagnostic become `<MCOperand Imm:11>` in the final output. Having the same data structure store values with time-dependent semantics is confusing already, and it will get more confusing when we introduce the MVE VPT instruction which reuses the Then/Else bitmask idea in a different context. So I'm refactoring: now, all `ARMOperand` and `MCOperand` representations of an IT mask work exactly the same way, namely, 0 means 'Then' and 1 means 'Else', regardless of what original predicate is being referred to. The architectural encoding of IT that depends on the original condition is now constructed at the point when we turn the `MCOperand` into the final instruction bit pattern, and decoded similarly in the disassembler. The previous condition-independent parse-time format used 0 for Else and 1 for Then. I've taken the opportunity to flip the sense of it while I'm changing all of this anyway, because it seems to me more natural to use 0 for 'leave the starting condition unchanged' and 1 for 'invert it', as if those bits were an XOR mask. Reviewers: ostannard Subscribers: javed.absar, kristof.beyls, hiraditya, llvm-commits Tags: #llvm Differential Revision: https://reviews.llvm.org/D63219 llvm-svn: 363244 |
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README.txt
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