[PATCH] D105263: [X86] AVX512FP16 instructions enabling 1/6

[LuoYuanke via Phabricator via llvm-commits]

LuoYuanke added inline comments.


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Comment at: llvm/lib/Target/X86/X86InstrAVX512.td:4478
+  let Predicates = [HasFP16] in {
+    def VMOVSHZrr_REV: AVX512<0x11, MRMDestReg, (outs VR128X:$dst),
+        (ins VR128X:$src1, VR128X:$src2),
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pengfei wrote:
> craig.topper wrote:
> > pengfei wrote:
> > > LuoYuanke wrote:
> > > > Sorry, I forgot what REV stand for. Do you know it?
> > > > Is this just encoding difference for register operand compared with VMOVSHZrr? What is it used for?
> > > I think REV is short for revert. Which allows a different encoding when operands order are reverted.
> > > Yes. It's used for a different encoding.
> > It is short for "reverse". Meaing the operands are in the reversed order. There are two valid encodings moving from one register to another. This happens because there are separate opcodes for moving register to memory(Store) and moving memory to register(load). The memory operand for both of those opcodes can be a register as well. The assembler and isel always uses the register to register version of the load opcode. The reversed version is only used by the disassembler
> > 
> > There is an exception to that. For VEX encoded AVX/AVX2 instructions, X86MCInstLowering will use an _REV move if it allows a 2 byte VEX prefix instead of a 3 byte VEX prefix. This doesn't apply to any AVX512 instructions though. 
> Thanks Craig for the information.
> It is short for "reverse". Meaing the operands are in the reversed order. There are two valid encodings moving from one register to another. This happens because there are separate opcodes for moving register to memory(Store) and moving memory to register(load). The memory operand for both of those opcodes can be a register as well. The assembler and isel always uses the register to register version of the load opcode. The reversed version is only used by the disassembler
> 
> There is an exception to that. For VEX encoded AVX/AVX2 instructions, X86MCInstLowering will use an _REV move if it allows a 2 byte VEX prefix instead of a 3 byte VEX prefix. This doesn't apply to any AVX512 instructions though. 

I understand now. Thanks, Craig and Pengfei.




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https://reviews.llvm.org/D105263